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Ieke W. Ayu, S. Priyono dan Soemarno

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1 Ieke W. Ayu, S. Priyono dan Soemarno
NERACA LENGAS TANAH Oleh: Ieke W. Ayu, S. Priyono dan Soemarno psl-ppsub nopember 2012

2 LENGAS TANAH Definisi: Air yang disimpan dalam tanah.
Salah satu faktor yang sangat penting dalam proses pedologis dan pertumbuhan tanaman. Ada tiga macam bentuk lengas tanah: Water adhering in thin films by molecular attraction to the surface of soil particles and not available for plants is termed hygroscopic water. Water forming thicker films and occupying the smaller pore spaces is termed capillary water. Since it is held against the force of gravity it is permanently available for plant growth and it is this type of soil water which contains plant nutrients in solution. Water in excess of hygroscopic and capillary water is termed gravitational water, which is of a transitory nature because it flows away under the influence of gravity. When the excess has drained away the amount of water retained in the soil is termed its field capacity, when some of its pore spaces are still free of water. (Source: LANDY / DUNSTE) Diunduh dari: ….. 15/11/2012

3 SOIL WATER BALANCE Plant production involves CO2 intake through stomatal openings in the epidermis. Most water that plants take up from the soil is again lost to the atmosphere by transpiration through the same openings. The daily turnover can be considerable: transpiration from 0.4 cm of water from a crop surface on a clear sunny day corresponds with a water loss from the root zone of than kg ha-1 d-1. If soil moisture uptake by the roots is not replenished, the soil will dry out to such an extent that the plants wilt and - ultimately- die. The tenacity with which the soil retains its water is equalled by the suction which roots must exert to be able to take up soil moisture. This suction known as the soil moisture potential or 'matrix suction', can be measured. In hydrology, the potential is usually used and is expressed as energy unit per weight of soil water, with the dimension of length (van Bakel, 1981). An optimum range exists within which the plant takes up water freely. Above or below this level the plant senses stress; it reacts by actively curbing its daily water consumption through partial or complete closure of the stomata. The consequence is evident: this stomatal closure interferes with CO2 intake and reduces assimilation and dry matter production consequently. A crop growth simulation model must therefore keep track of the soil moisture potential to determine when and to what degree a crop is exposed to water stress. This is commonly done with the aid of a water balance equation, which compares for a given period of time, incoming water in the rooted soil with outgoing water and quantifies the difference between the two as a change in the soil moisture amount stored. Diunduh dari: 12/11/2012

4 Perubahan air dalam tanah = Input air – Kehilangan air
PERSAMAAN NERACA LENGAS TANAH Soil water balance, like a financial statement of income and expenditure, is an account of all quantities of water added, removed or stored in a given volume of soil during a given period of time. The soil water balance equation thus helps in making estimates of parameters, which influence the amount of soil water. Using the soil water balance equation, one can identify periods of water stress/excesses which may have adverse affect on crop performance. This identification will help in adopting appropriate management practices to alleviate the constraint and increase the crop yields. The amount of water in a soil layer is determined by those factors that add water to the soil and those factors that remove water from it. The soil water balance equation in its simplest form of expression is: Perubahan air dalam tanah = Input air – Kehilangan air Diunduh dari: 12/11/2012

Water is usually added to the soil in three measurable ways - precipitation (P), irrigation (I), and contribution from the ground-water table (C). The contribution from the ground water will be significant only if the ground-water table is near the surface. So, the inputs of water can be presented as: Input Air = P + I + C Kehilangan Air dari tanah: Water is removed from the soil through evaporation from soil surface or transpiration through plant together known as evapotranspiration (ET), and deep drainage (D). Further, a part of the rain water received at the soil surface may be lost as surface run-off (RO). The above three factors are negative factors in the equation. The losses of water from soil can then be represented by the following equation. Kehilangan Air = ET + D + RO Diunduh dari: 12/11/2012

6 NERACA LENGAS TANAH Change in Soil water = (P + I + C) - (ET + D + RO)
The change in the soil water content which is the difference between the water added and water withdrawn will now read: Change in Soil water = (P + I + C) - (ET + D + RO) Soil water refers to the amount of water held in the root zone at a given time. This amount can be measured. The change in soil water from one measurement to another depends on the contribution of components in the equation. Suppose the amount of water in the root zone at the beginning is M1 mm and at the end of a given period is M2 mm, thus the equation is expressed as : M1 - M2 = P + I + C - ET - D - RO or M1 + P + I + C = ET + D + RO + M2 With the help of this equation one can compute any one unknown parameter in the equation if all others are known. The quantitative data on rainfall (P) evapotranspiration (ET), deep drainage (D) and soil moisture at a given time (M1 or M2) for different locations and for different practices are useful for selecting appropriate water-management strategies. Diunduh dari: 12/11/2012

Let us work a few examples using the Soil Water Balance Equation to appreciate the usefulness of this model. Contoh 1: Soil = Vertisol Crop = Sorghum Period = 01 to 31 Aug Area = 2 ha Given: Soil moisture in the profile # on Aug 01 (M1) = 300 mm Precipitation or Rainfall (P) = 70 mm Irrigation (I) = Nil Contribution from ground water (C) = Nil Run-off of 200 cubic m from 2 ha field (R) = 10 mm Deep drainage (D) = Nil Soil moisture in the profile on Aug 31 (M2) = 250 mm Estimate evapotranspiration (ET) from the field during 01 to 31 Aug. Equation: M1 + P + I + C = ET + D + RO +M = ET ET = 370 mm mm = 110 mm Thus, evapotranspiration which is difficult to be measured could be estimated using the Soil Water Balance Equation. Diunduh dari: 12/11/2012

Contoh 2: Soil = Alfisol Crop = Millet Area=1 ha Period = 10 June (sowing date) to 30 Sept (harvesting) given: Soil moisture in the profile on Jun 10 (M1) = 150 mm Precipitation or Rainfall (P) = 600 mm Irrigation (I) = Nil Contribution from ground water (C) = Nil Evapotranspiration (estimated) (ET) = 530 mm Run-off of 200 cubic m from 1 ha field (RO) = 70 mm Soil moisture in the profile on Sep 30 (M2) = 60mm Estimate: Deep drainage (D) losses from the field during crop period. Equation: M1 + P + I + C = ET + D + RO + M2 = D D = 750 mm mm = 90 mm Thus, deep drainage (D) losses in the field which is not easy to measure could be estimated using the Soil Water Balance Equation. We hope that this lesson and the examples have helped you in understanding and computing the various components of the Soil Water Balance Equation. For more detailed treatment please refer any standard textbook on soil physics. Diunduh dari: 13/11/2012

9 ET = I + P - RO - DP + CR ± D SF ± D SW
NERACA LENGAS TANAH Evapotranspiration can also be determined by measuring the various components of the soil water balance. The method consists of assessing the incoming and outgoing water flux into the crop root zone over some time period (Figure 6). Irrigation (I) and rainfall (P) add water to the root zone. Part of I and P might be lost by surface runoff (RO) and by deep percolation (DP) that will eventually recharge the water table. Water might also be transported upward by capillary rise (CR) from a shallow water table towards the root zone or even transferred horizontally by subsurface flow in (SFin) or out of (SFout) the root zone. In many situations, however, except under conditions with large slopes, SFin and SFout are minor and can be ignored. Soil evaporation and crop transpiration deplete water from the root zone. If all fluxes other than evapotranspiration (ET) can be assessed, the evapotranspiration can be deduced from the change in soil water content (D SW) over the time period: ET = I + P - RO - DP + CR ± D SF ± D SW Diunduh dari: 13/11/2012

10 Dr, i = Dr, i-1 - (P - RO)i - Ii - CRi + ETc, i + DPi
NERACA LENGAS TANAH The estimation of Ks requires a daily water balance computation for the root zone. The root zone can be presented by means of a container in which the water content may fluctuate. The daily water balance, expressed in terms of depletion at the end of the day is: Dr, i = Dr, i-1 - (P - RO)i - Ii - CRi + ETc, i + DPi where : Dr, i root zone depletion at the end of day i [mm], Dr, i-1 water content in the root zone at the end of the previous day, i-1 [mm], Pi precipitation on day i [mm], ROi runoff from the soil surface on day i [mm], Ii net irrigation depth on day i that infiltrates the soil [mm], CRi capillary rise from the groundwater table on day i [mm], ETc, i crop evapotranspiration on day i [mm], DPi water loss out of the root zone by deep percolation on day i [mm]. Diunduh dari: 13/11/2012

11 Transpor air dalam profil tanah
…NERACA LENGAS TANAH Transpor air dalam profil tanah Diunduh dari: 13/11/2012

Soil-water balance is an accounting procedure for near-surface soil-moisture. It is included in a class of models known as “bucket” models : Properties are averaged for each drainage basin , for example, a watershed has a single water holding capacity Changes in soil moisture are calculated using a mass balance Historical precipitation is known Evaporation can be estimated using the temperature-based (Hargreaves) equation . Runoff is dependent upon soil moisture and precipitation Diunduh dari: ….. 13/11/2012

13 MODEL GROUNDWATER The groundwater model is a lumped-parameter type
Portions of the formation fed by different catchment areas are assumed to have uniform storage and transmissivity parameters Properties are also averaged over depth This is different from finite element models like MODFLOW or GMS where there is an analysis grid consisting of a large number of cells Recharge functions : Simulate interaction between surface water and groundwater Recharge to aquifer is mostly from channel losses in streams that flow over outcrop Empirical functions were developed by plotting inflows against known recharge determined by USGS Inflow is the stream flow at a gauge above the recharge zone plus intervening runoff; intervening runoff is calculated using an areal scaling ratio assuming that conditions are the same for catchment and intervening regions Used historical pumping data Diunduh dari: ….. 13/11/2012

Diunduh dari: ….. 13/11/2012

The components of the soil water balance (SWB) are depicted in Figure . The water balance can be summarised as in equation : ÄSm = Sm + P – T – E – I – R – D The change in the soil water content (ÄSm) over a period of time depends on the original water content (Sm) plus precipitation (rain and irrigation, P); minus transpiration (loss of water by plants, T); minus evaporation (loss of water from the soil surface, E); minus interception (water held in the plant canopy, I); minus runoff (surface water not penetrating the soil and running away, R) ; minus drainage (water draining away below root zone, D). Diunduh dari: ….. 13/11/2012

The combined results from the water balances have been used to assess how much water systems have; how much is stored; what the variability factors are; and what the connections between resources are. A discussion of the relative contribution of the main components of the balances is provided. Diunduh dari: 13/11/2012

In large canal irrigation project areas, integrated management of surface and groundwater resources can improve water use efficiencies and agricultural productivity and also control water logging. Such integrated management requires an estimation of spatial distribution of recharge and ground water flow in the underlying aquifer. Recharge occurs both as percolation losses from fields and seepage losses from the water distribution network. Diunduh dari: 13/11/2012

18 NERACA LENGAS TANAH A soil water balance model, if designed to adequately represent the physical processes involved, and if carried out with a short enough (daily) time step, can provide realistic estimates of deep drainage (potential recharge) over long periods. The single store (single layer) mass water balance model applicable to semi-arid areas, which recognises the wetting of the near surface during rainfall, with subsequent availability of water for evaporation and transpiration in the days following rainfall. The model allows for the major hydrological processes taking place at or near the soil-vegetation surface including runoff. Diunduh dari: 13/11/2012

19 Schematization of the soil profile in the AWC calculation of the CERU32 program (Le Bas et al., 1997) The soil profile is schematized in three layers (Le Bas et al., 1997): A worked surface layer, defined by ploughing depth. The available water for this layer corresponds to the total available water (water volume between –1500 kPa suction (wilting point) and –5 kPa (field capacity)) estimated by rules using the topsoil input variables. A subsurface layer, between the ploughing depth and the EAW (Easily Available Water) depth. The latter is user defined. The available water for this layer corresponds to the total available water (water volume between –1500 kPa suction (wilting point) and –5 kPa (field capacity)) estimated by rules valid for the subsoil, using the subsoil input variables. But if the soil layer is less than the depth to textural change the rules for subsoil uses topsoil input variables. A deeper layer, between the EAW depth and the maximum rooting depth. The available water for this layer corresponds to the easily available water (water volume between –200 kPa suction and –5 kPa (field capacity)) estimated by rules valid for the subsoil, using the subsoil input variables. But if the soil layer is less than the depth to textural change the rules for subsoil uses topsoil input variables. Variabel Topsoil Variabel Subsoil Kedalaman Akar Diunduh dari: 13/11/2012

Therefore the purpose of soil water balance calculations is to estimate daily value of the actual soil moisture content, which influences soil moisture uptake and crop transpiration. Schematic representation of the different components of a soil water balance Diunduh dari: 12/11/2012

Actual soil moisture content can be established according to (Driessen, 1986): Where: where : qt : Actual moisture content of the root zone at time step t [cm3 cm-3] Nup : Rate of net influx through the upper root zone boundary [cm d-1] INlow : Rate of net influx through the lower root zone boundary [cm d-1] Ta : Actual transpiration rate of crop [cm d-1] RD : Actual rooting depth [cm] P : Precipitation intensity [cm d-1] Ie : Effective daily irrigation [cm d-1] Es : Soil evaporation rate [cm d-1] SSt : Surface storage [cm] SR : Rate of surface runoff [cm d-1] CR : Rate of capillary rise [cm d-1] Perc : Percolation rate [cm d-1] Dt : Time step [d] Zt : Depth of groundwater table [cm] Diunduh dari: 12/11/2012

The processes directly affecting the root zone soil moisture content can be defined as: Infiltration: i.e. transport from the soil surface into the root zone; Evaporation: i.e. the loss of soil moisture to the atmosphere; Plant transpiration: i.e. loss of water from the interior root zone; Percolation: i.e. downward transport of water from the root zone to the layer below the root zone; Capillary rise: i.e. upward transport into the rooted zone. The textural profile of the soil is conceived homogeneous. Initially the soil profile consists of three layers (zones): The rooted zone between soil surface and actual rooting depth The lower zone between actual rooting depth and maximum rooting depth The subsoil below maximum rooting depth Diunduh dari: 12/11/2012

The variables of the soil water balance in the actual water-limited production situation are calculated for freely draining soil. No influence from groundwater is assumed and crop water requirements for continuous growth with either drought stress or water excess and a possible reduction of the crop transpiration rate, leading to a reduced growth are quantified. Submodel Lengas Tanah For the rooted zone the water balance equation is solved every daily time step. The water balance is driven by rainfall, possibly buffered as surface storage, and evapotranspiration. The processes considered are infiltration, soil water retention, percolation and the loss of water beyond the maximum root zone. At the upper boundary, processes comprise infiltration of water from precipitation or irrigation, evaporation from the soil surface and crop transpiration. If the rainfall intensity exceeds the infiltration and surface storage capacity of the soil, water runs off. Water can be stored in the soil till the field capacity is reached. Additional water percolates beyond the lower boundary of the rooting zone. Flow rates are limited by the maximum percolation rate of the root zone and the maximum percolation rate to the subsoil. Diunduh dari: 12/11/2012

The variables of the soil water balance in the actual water-limited production situation are calculated for freely draining soil. No influence from groundwater is assumed and crop water requirements for continuous growth with either drought stress or water excess and a possible reduction of the crop transpiration rate, leading to a reduced growth are quantified. Submodel Lengas Tanah The textural profile of the soil is conceived homogeneous. Initially the soil profile consists of three layers (zones): the rooted zone between soil surface and actual rooting depth the lower zone between actual rooting depth and maximum rooting depth the subsoil below maximum rooting depth Root zone extension from initial rooting depth to maximum rooting depth. Its effect on the soil moisture content is accounted for in this soil water balance calculation. From the moment that the maximum rooting depth is reached the soil profile is described as a two layer system (Driessen, 1986). The lower zone no longer exists. As mentioned earlier, no groundwater influence is assumed and capillary rise is not accounted for. Only downward flow, evaporation from the soil surface and transpiration are estimated. Diunduh dari: 12/11/2012

25 …Kandungan Lengas Tanah Initial
The initial value of the actual soil moisture content in the root zone can be calculated as: Where qt : Actual soil moisture content in rooted zone [cm3 cm-3] qwp : Soil moisture content at wilting point [cm3 cm-3] Wav : Initial available soil moisture amount in excess of qwp [cm] RD : Actual rooting depth (see Section 5.6.) [cm] The initial actual soil moisture content, qt, cannot be lower than the soil moisture content at wilting point. In case the crop cannot develop airducts, the initial soil moisture content cannot be higher than the soil moisture content at field capacity. If the crop can develop airducts the initial soil moisture content cannot exceed the soil porosity. Wav, the initial available soil moisture amount in excess of qwp should be provided by the user. Multiplying the actual soil moisture content with the rooting depth yields the initial water amount in the rooted zone. Diunduh dari: 12/11/2012

. The initial amount of soil moisture in the zone between the actual rooting depth and the maximum rooting depth (i.e. lower zone), can be calculated as: Where Wlz : Soil moisture amount in the lower zone [cm] Wav : Initial available soil moisture amount in excess of qwp [cm] RDmax : Maximum rooting depth [cm] RD : Actual rooting depth [cm] qt : Actual soil moisture content in rooted zone [cm3 cm-3] qwp : Soil moisture content at wilting point [cm3 cm-3] The soil moisture content of the lower zone is also limited by the field capacity in case the crop cannot develop airducts, else the soil moisture content is limited by the soil porosity. Diunduh dari: 12/11/2012

27 The evaporation can be calculated as:
…EVAPORASI . Evaporation depends on the available soil water and the infiltration capacity of the soil. If the water layer on the surface, the so called surface storage, exceeds 1 cm, the actual evaporation rate from the soil is set to zero and the actual evaporation rate from the surface water is equal to the maximum evaporation from a shaded water surface. If the surface storage is less than 1 cm and the infiltration rate of the previous day exceeds 1 cm d-1, the actual evaporation rate from the surface water is set to zero and the actual evaporation rate from the soil is equal to the maximum evaporation from a shaded soil surface. All water on the surface can infiltrate within one day. The value of the variable days since last rain, Dslr, is reset to unity. If the infiltration rate is less than 1 cm d-1, the amount of infiltrated water is considered too small to justify a reset of the parameter Dslr and the evaporation rate decreases as the top soil starts drying. The reduction of the evaporation is thought to be proportional to the square root of time (Stroosnijder, 1987, 1982). The evaporation can be calculated as: where Es : Evaporation rate from a shaded soil surface [cm d-1] Es,max : Maximum evaporation rate from a shaded soil surface (see eq. 6.7) [cm d-1] Dslr : Days since last rain [d] When a small water amount has infiltrated, or rather wetted the soil surface, this amount can be evaporated the same day, irrespective of Dslr. Therefore, the actual evaporation from the soil surface, as calculated according to equation 6.20, should be corrected for this water amount infiltrating the soil. This amount should be added to the actual evaporation rate. However, it should be noted that the actual evaporation can never exceed the maximum evaporation rate. Diunduh dari: 12/11/2012

28 …PRESIPITASI Not all precipitation will reach the surface. A fraction will be intercepted by leaves, stems, etc. From the amount of precipitation which reaches the soil surface, a part runs off. Runoff from a field can be 0-20 percent, and even higher on unfavorable surfaces (Stroosnijder & Koné, 1982). It can be assumed that a fixed fraction of the precipitation will not infiltrate during that particular day. This fraction can be reduced in situations with relatively small amounts of rainfall. The reduction factor is defined as function of the rainfall amount (van Diepen et al., 1988). Note that the non infiltrating fraction refers to rainfall only. Irrigation water is assumed to infiltrate freely. Reduction factor of the non infiltrating fraction as a function of rainfall. Diunduh dari: 12/11/2012

29 …PERKOLASI If the root zone soil moisture content is above field capacity, water percolates to the lower part of the potentially rootable zone and the subsoil. A clear distinction is made between percolation from the actual rootzone to the so-called lower zone, and percolation from the lower zone to the subsoil. The former is called Perc and the latter is called Loss. The percolation rate from the rooted zone can be calculated as: where Perc : Percolation rate from the root zone to the lower zone [cm d-1] Wrz : Soil moisture amount in the root zone [cm] Wrz,fc : Equilibrium soil moisture amount in the root zone [cm] Dt : Time step [d] Ta : Actual transpiration rate [cm d-1] Es : Evaporation rate from a shaded soil surface [cm d-1] The equilibrium soil moisture amount in the root zone can be calculated as the soil moisture content at field capacity times the depth of the rooting zone: Wrz,fc : Equilibrium soil moisture amount in the root zone [cm] qfc : Soil moisture content at field capacity [cm3 cm-3] RD : Actual rooting depth [cm] Diunduh dari: 12/11/2012

30 …LAJU PERKOLASI . The percolation rate and infiltration rate are limited by the conductivity of the wet soil, which is soil specific and should be given by the user. Note that the percolation from the root zone to the lower zone can be limited by the uptake capacity of the lower zone. Therefore, the value calculated with equation 6.21 is preliminary and the uptake capacity should first be checked. The percolation from the lower zone to the subsoil, the so-called Loss, should take the water amount in the lower zone into account. If the water amount in the lower zone is less than the equilibrium soil moisture amount, a part of the percolating water will be retained and the percolation rate will be reduced. Water loss from the lower end of the maximum root zone can be calculated as: where Loss : Percolation rate from the lower zone to the subsoil [cm d-1] Perc : Percolation rate from root zone to lower zone [cm d-1] Wlz : Soil moisture amount in the lower zone [cm] Wlz,fc : Equilibrium soil moisture amount in the lower zone [cm] Dt : Time step Water loss from the potentially rootable zone, is also limited by the maximum percolation rate of the subsoil, which is soil specific and should be provided by the user. The equilibrium soil moisture amount in the lower zone can be calculated as the soil moisture content at field capacity times the root zone depth: where Wrz,fc : Equilibrium soil moisture amount in the lower zone [cm] qfc : Soil moisture content at field capacity [cm3 cm-3] RDmax : Maximum rooting depth [cm] RD : Actual rooting depth [cm] Diunduh dari: 12/11/2012

. For rice an additional limit of five percent of the saturated soil conductivity is set to account for puddling (a rather arbitrary value, which may be easily changed in the program). The saturated soil conductivity and is calculated using equating with pF= -1.0 (i.e. a hydraulic head of 0.1 cm). The percolation rate from the lower zone to the sub soil is not to exceed this value (van Diepen et al., 1988). As mentioned before, the value calculated with equation, should be regarded as preliminary; the storage capacity of the receiving layer may become limiting. The storage capacity of the lower zone, also called the uptake capacity, is the amount of air plus the loss (van Diepen et al., 1988). It can de defined as: where UP : Uptake capacity of lower zone [cm d-1] RDmax : Maximum rooting depth [cm] RD : Actual rooting depth [cm] Wlz : Soil moisture amount in lower zone [cm] qmax : Soil porosity (maximum soil moisture) [cm3 cm-3] Dt : Time step [d] Loss : Percolation rate from the lower zone to the subsoil [cm d-1] Percolation to the lower part of the potentially rootable zone can not exceed the uptake capacity of the lower zone. Therefore the percolation rate is set equal to the minimum of the calculated percolation rate and the uptake. Diunduh dari: 12/11/2012

32 …INFILTRASI AWAL The infiltration rate depends on the available water and the infiltration capacity of the soil. If the actual surface storage is less then or equal to 0.1 cm, the preliminary infiltration capacity is simply described as: where INp : Preliminary infiltration rate [cm d-1] FI : Maximum fraction of rain not infiltrating during time step t [-] CI : Reduction factor applied to FI as a function of the precipitation intensity [-] P : Precipitation intensity [cm d-1] Ie : Effective irrigation [cm d-1] SSt : Surface storage at time step t [cm] Dt : Time step [d] Diunduh dari: 12/11/2012

33 LAJU INFILTRASI AWAL . The maximum fraction of rain not infiltrating during time step t, FI can be either set to a fixed value or assumed to be variable by multiplying FI with a precipitation dependent reduction factor CI which is maximum for high rainfall and will be reduced for low rainfall. The user should provide FI. The CI table is included in the model and is assumed to be fixed. The calculated infiltration rate is preliminary, as the storage capacity of the soil is not yet taken into account. If the actual surface storage is more than 0.1 cm, the available water which can potentially infiltrate, is equal to the water amount on the surface (i.e. supplied via rainfall/irrigation and depleted via evaporation): where INp : Preliminary infiltration rate [cm d-1] P : Precipitation intensity [cm d-1] Ie : Effective irrigation [cm d-1] Ew : Evaporation rate from a shaded water surface [cm d-1] SSt : Surface storage at time step t [cm] Dt : Time step [d] However, the infiltration rate is hampered by the soil conductivity and cannot exceed it. Soil conductivity is soil specific and should be given by the user. Diunduh dari: 12/11/2012

Total water loss from the root zone can now be calculated as the sum of transpiration, evaporation and percolation. The sum of total water loss and available pore space in the root zone define the maximum infiltration rate. The preliminary infiltration rate cannot exceed this value. The maximum possible infiltration rate is given by: where INmax : Maximum infiltration rate [cm d-1] qmax : Soil porosity (maximum soil moisture) [cm3 cm-3] qt : Actual soil moisture content [cm3 cm-3] RD : Actual rooting depth [cm] Dt : Time step [d] Ta : Actual transpiration rate [cm d-1] Es : Evaporation rate from a shaded soil surface  [cm d-1] Perc : Percolation rate from root zone to lower zone [cm d-1] Diunduh dari: 12/11/2012

35 INFILTRASI Infiltrasi mencerminkan kecepatan meresapnya air ke dalam tanah melalui permukaan tanah. Semakin tinggi infiltrasi, semakin banyak air yang akan tersedia bagi tanaman dan semakin sedikit air runoff di permukaan tanah, semakin sedikit pula erosi dan pencucian unsur hara. Seresah tanaman, tumbuhan hidup, atau permuakan yang kasar, akan menghambat aliran air di permukaan tanah, sehingga air mempunyai kesempatan untuk meresap ke dalam tanah. Kerak tanah dapat mereduksi infiltrasi dan dapat diminimumkan dengan jalan membiarkan seresah tumbuhan tetap di permukaan tanah, memperbaiki kandungan bahan organik tanah, dan memacu aktivitas biologis. Diunduh dari: ….. 13/11/2012

.. Beberapa praktek pengelolaan tanah yang dapat meningkatkan kapasitas lapang dan memperbaiki infiltrasi: Pengelolaan bahan organik. Bahan organik dapat meningkatkan kemampuan tanah menyimpan air (water-holding capacity, WHC) melalui dua cara. Bahan organik mampu menyimpan dan menahan banyak air, dan dapat memperbaiki struktur tanah – meningkatkan total volume dan ukuran pori yang dapat menyimpan air dan mencegah pembentukan kerak tanah di permukaan. Praktek pengolahan tanah. Membiarkan seresah sisa panen di permukaan atanah dapat memperlambat runoff dan mencegah pembentukan kerak tanah di permukaan. Seresah ini dapat mendorong perkembangan populasi cacing tanah dan organisme lain yang membuat liang dalam tanah, dan air hujan dapat dengan cepat meresap ke dalam tanah melalui lubang-lubang tersebut. Pencegahan pemadatan. Pemadatan tanah dapat mereduksi WHC, karena berkurangnya jumlah dan ukuran pori tanah. Pengendalian erosi. Erosi tanah dapat mereduksi kedalaman tanah (solum tanah menjadi tipis) dan menurunkan WHC. Diunduh dari: ….. 13/11/2012

Praktek konservasi yang dapat memperburuk infiltrasi: Pembakaran dan pengangkutan sisa-sisa panen, membiarkan tanah bera dan peka terhadap erosi Metode pengolahan tanah yang merusak koneksi pori dengan permukaan tanah, dan mencegah akumulasi bahan organik tanah Lalu lintas peralatan dan ternak, terutama pada saat tabnah dalam kondisi bawah, yang menyebabkan pemadatan dan reduksi porositas tanah. Beberapa praktek konservasi membantu mempertahankan atau memperbaiki infiltrasi air ke dalam tanah : Meningkatkan tutupan vegetatif di permukaan tanah, Mengelola residu vegetatif, dan Meningkatkan bahan organik tanah. Biasanya, praktek-praktek ini meminimumkan gangguan tanah dan pemadatan tanah, melindungi tanah dari erosi, dan mendorong perkembangan struktur tanah yang baik dan ruang pori yang kontinyus. Sebagai solusi jangka pendek mengatasi buruknya infiltrasi adalah membongkar kerak permukaan dengan membajak tanah, dan lapisan tanah yang kompak dapat dibongkar dengan pengolahan tanah secara dalam. Diunduh dari: ….. 13/11/2012

Praktek konservasi yang memperbaiki laju infiltrasi: Pergiliran tanaman Tanaman penutup tanah Grazing terkendali Pengelolaan residu dan seresah tanaman, serta Pengolahan tanah Pemanfaatan sisa panen. Nilai Maksimum Laju Infiltrasi berbagai tipe tanah Tipe Tanah Laju Infiltrasi (inch/hr)* Pasir = Sand 2 Pasir berlempung =Loamy sand 1.8 Lempung berpasir = loam 1.5 Lempung = Loam 1 Debu = Silt dan Lempung Liat 0.5 Liat = Clay 0.2 . (*) Asumsi tanaman penutup tanah penuh. Laju pada Tanah bera sebesar ½ dari laju pada tanah dengan tumbuhan penutup tanah penuh. Diunduh dari: ….. 13/11/2012

39 Surface runoff can be calculated as:
…LIMPASAN PERMUKAAN Surface runoff is also taken into account by defining a maximum value for surface storage. If the surface storage exceeds this value the exceeding water amount will run off. Surface storage at time step t can be calculated as: where SSt : Surface storage at time step t [cm d-1] P : Precipitation intensity [cm d-1] Ie : Effective irrigation rate [cm d-1] Ew : Evaporation rate from a shaded water surface [cm d-1] IN : Infiltration rate (adjusted) [cm d-1] Surface runoff can be calculated as: SRt : Surface runoff at time step t [cm] SSt : Surface storage at time step t [cm] SSmax : Maximum surface storage [cm] SSmax is an environmental specific variable and should be provided by the user. Diunduh dari: 12/11/2012

40 SURFACE RUNOFF Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the hydrologic cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source. If a nonpoint source contains man-made contaminants, the runoff is called nonpoint source pollution. A land area which produces runoff that drains to a common point is called a watershed. When runoff flows along the ground, it can pick up soil contaminants such as petroleum, pesticides, or fertilizers that become discharge or nonpoint source pollution. Diunduh dari: ….. 13/11/2012

41 Karakteristik Fisik yang mempengaruhi runoff:
LIMPASAN PERMUKAAN . Faktor Meteorologi yang mempengaruhi runoff: Tipe presipitasi (rain, snow, sleet, etc.) Intensitas hujan Jumlah hujan Lamanya hujan Distribution of rainfall over the watershedS Direction of storm movement Antecedent precipitation and resulting soil moisture Other meteorological and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity, and season. Karakteristik Fisik yang mempengaruhi runoff: Land use Vegetasi Tipe Tanah Drainage area Bentuk daerah tangkapoan air Elevation Kemiringan Topography Direction of orientation Pola aliran Drainage Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or alter runoff from continuing downstream. Diunduh dari: ….. 13/11/2012

42 Kurva kapasitas infiltrasi untuk berbagai tipe tanah yang berbeda.
RUNOFF & TIPE TANAH Kapasitas infiltrasi suatu tanah dipengaruhi oleh porositas tanah, yang menentukan kapasitas simpanan air dan mempengaruhi resistensi air untuk mengalir ke lapisan tanah yang lebih dalam. Porositas suatu tanah berbeda dengan tanah lainnya. Kapasitas infiltrasdi tertinggi dijumpai pada tanah-tanah yang gembur, tekstur berpasir; sedangkan tanah-tanah liat dan berliat biasanya mempunyai kapasitas infiltrasi lebih rendah. Bagan-bagan berikut menyajikan beragam kapasitas infiltrasi yang diukur pada berbagai tipe tanah. Kapasitas infiltrasi juga tergantung pada kadar lengas tanah pada akhir periode hujan sebelumnya. Kapasitas infiltrasi aweal yang tinggi dapat menurun dengan waktu (asalkan hujan tidak berhenti) hingga mencapai suatu nilai konstan pada saat profil tanah telah jenuh air. Kurva kapasitas infiltrasi untuk berbagai tipe tanah yang berbeda. Diunduh dari: ….. 13/11/2012

43 Runoff [mm] = K x Rainfall depth [mm]
Koefisien Runoff Selain faktor-faktor yang bersifat spesifik-lokasi, perlu diperhatikan juga adalah homogenitas kondisi fisik daerah tangkapan air. Meskipun pada sekala mikro, ternyata juga ada variasi kemiringan, tipe tanah, vegetasi penutup dll. Oleh karena itu setiap daerah-tangkapan air mempunyai respon-runoff yang spesifik, dan respon ini juga akan tergantung pada ragam kejadian hujan. Disain sarana pemanenan air memerlukan pengetahuan tentang jumlah runoff yang akan dihasilkan oleh hujan dalam suatu daerah tangkapan. Biasanya diasumsikan bahwa volume runoff sebanding dengan kedalaman (jumlah) hujan. Runoff [mm] = K x Rainfall depth [mm] Dalam kondisi daerah-tangkapan di pedesaan yang tidak ada bagian kedap air, koefficien K, yang mencerminkan persentase runoff dari suatu kejadian hujan, bukanlah merupakan faktor yang konstan. Nilai koefisien ini sangat beragam dan tergantung pada faktor-faktor spesifik lokasi dan karakteristik hujannya. Misalnya, dalam suatu daerah tangkapan tertentu, dengan kondisi initial yang sama (misalnya kadar lengas tanah awal), kejadian hujan selama 40 menit dengan intensitas rataan 30 mm/jam akan menghasilkan persentase runoff lebih kecil dibandingkan dengan kejadian hujan selama 20 menit tetapi dengan rataan intensitas 60 mm/jam, walaupun total hujan keduanya sama. Diunduh dari: ….. 13/11/2012

The rates of change in the water amount in the root and lower zone are calculated straightforward from the flows found above: where DWrz : Change of the soil moisture amount in the root zone [cm] DWlz : Change of the soil moisture amount in the lower zone [cm] Ta : Actual transpiration rate [cm d-1] Es : Evaporation rate from a shaded soil surface [cm d-1] IN : Infiltration rate [cm d-1] Perc : Percolation rate from root zone to lower zone [cm d-1] Loss : Percolation rate from lower zone to sub soil [cm d-1] Dt : Time step [d] Due to extension of the roots into the lower zone, extra soil moisture becomes available, which can be calculated as: RDt : Rooting depth at time step t [cm] RDt-1 : Rooting depth at time step t-1 [cm] RDmax : Maximum rooting depth [cm] Wlz : Soil moisture amount in the lower zone [cm] DWlz : Change of the soil moisture amount in the lower zone [cm] Diunduh dari: 12/11/2012

The actual water amount in the root zone and in the lower zone can be calculated according to: where Wrz,t : Soil moisture amount in the root zone at time step t [cm] Wlz,t : Soil moisture amount in the lower zone at time step t [cm] Wrz,t-1 : Soil moisture amount in the root zone at time step t-1 [cm] Wlz,t-1 : Soil moisture amount in the lower zone at time step t-1 [cm] DWrz : Rate of change of the soil moisture amount in the root zone [cm] DWlz : Rate of change of the soil moisture amount in the lower zone [cm]. Diunduh dari: 12/11/2012

The actual soil moisture content can now be calculated according to: where qt : Actual soil moisture content at time step t [cm3 cm-3] Wrz,t : Soil moisture amount in the root zone at time step t [cm] RD : Actual rooting depth [cm] Diunduh dari: 12/11/2012

Modeled soil water content over the 10-m profile on 30 Mar (maximum soil water storage), 22 Aug (beginning dry season), and 8 Jan (minimum soil water storage). Diunduh dari: ….. 13/11/2012

48 SOIL WATER CONTENT  The distribution of soil water content in the 0–200 cm soil profile for two shrubs. (a) The driest profile for C. korshinkii, (b) The wettest profile for C. korshinkii, (c) The driest profile for S. psammophila, (d) The wettest profile for S. psammophila. The vertical dashed lines represent soil water content at the permanent wilting point of 0.064 cm3 cm− 3. Diunduh dari: ….. 13/11/2012

49 POROSITAS TANAH Patterns of vertical distribution of bulk density (a), porosity (b), and organic matter content (c) within the soil profile above the landslide. The different position of the black layer zone in c reflects an excavation slightly upslope of the pit where bulk density/porosity cores were collected (a and b). Diunduh dari: ….. 13/11/2012

50 POROSITAS TANAH Air-filled porosity for the switch plow treatment of the bare fallow and residue covered field as a function of the soil profile. Values are means, n = 4. Values in each profile layer followed by the same letter are not significantly different at the P < 0.05 level. Diunduh dari: 13/11/2012

51 POROSITAS TANAH Subsoil denitrification is a potential sink for leached nitrate (NO3–) that may otherwise contaminate ground water. Following rainfall and irrigation, subsoil N2O concentrations increased rapidly. Within days of NO3– leaching below 1 m, high concentrations of NO3–, Br–, and N2O were observed at 7-m depth. Diunduh dari: 13/11/2012

52 Soil profile and physical and chemical characteristics
Grain-size distribution and soil horizons Diunduh dari: ….. 12/11/2012

53 Soil profile and physical and chemical characteristics
Organic carbon content and cation-exchange capacity profiles Diunduh dari: ….. 12/11/2012

54 Soil profile and physical and chemical characteristics
Soil bulk density distributions determined by the core and clod methods. The bulk density of the soil profile, determined by the clod and core methods, shows relatively high compaction at the bottom of the upper 30 cm (1 ft). The soil becomes less compact below this level until the silty clay soil is reached, at which point compaction increases again. Diunduh dari: ….. 12/11/2012

55 Soil profile and physical and chemical characteristics
. Soil profile and physical and chemical characteristics Available water-holding capacity of the soil profile. The available water capacity of the soil profile WAS RELATED TO THE SOIL TEXTURE, it is generally lower because of the sandier nature of the soil profile, except where the silty clay soil is encountered at depth [below 180 cm (6 ft)]. The nature of the silty clay soil causes the available water capacity to increase significantly. Diunduh dari: ….. 12/11/2012

56 Simpanan Lengas Tanah = SOIL WATER STORAGE
Dalam kaitannya dengan irigasi dan pengairan, kapasitas simpanan air tanah (SWS) didefinisikan sebagai jumlah total air yang disimpan dalam tanah pada zone perakaran tanaman. Tekstur dan struktur tanah, serta kedalaman perakaran tanaman akan menentukan besarnya SWS ini. Semakin dalam perakaran tanaman, berarti semakin banyak air yang dapat disimpan dalam tanah dan semakin besar pula cadangan air tersedia bagi tanaman selama periode tidak ada penambahan air. Forms of Soil Water Storage (sumber:

57 Kedalaman Efektif Perakaran tanaman Dewasa.
. Bagaimana menentukan SWS dan Defisit maksimum lengas tanah (MSWD) Tahap 1. Menentukan kedalaman perakaran tanaman, RD (m). Tahap 2. Menentukan kapasitas simpanan air tersedia, AWSC (mm/m), Table 2 Tahap 3. Menghitung total simpanan lengas tanah, SWS (mm) SWS (mm) = RD (m) x AWSC (mm/m) …………….. (1) 4. Tahap 4. Menentukan koefisien eketersediaan air bagi tanaman, AC (%), Table 3 5. Tahap 5. Menghitung Defisit maksimum lengas tanah, MSWD (mm) MSWD = SWS (mm) x AC (%) ………………. (2) Kedalaman Efektif Perakaran tanaman Dewasa. Shallow 0.45 m (1.5 feet) Medium Shallow 0.60 m (2 feet) Medium Deep 0.90 m (3 feet) Deep 1.20 m (4 feet) Kubis, Timun, Sawi, Bawang merah,Lobak, dll Kentang, Wortel, Tomat, Kacang kapri, Buncis, dll Jagung, Terong, Cabe, dll Asparagus, Anggur, Beet, dll

58 Koefisien ketersediaan air tanah
Kapasitas simpanan air tersedia dari beberapa tipe tanah Tekstur Tanah Available Water Storage Capacity (AWSC) (in. water / in. soil) (in. water / ft. soil) (mm water / m soil) Clay = Liat 0.21 2.5 200 Clay Loam Silt loam 208 Clay loam 0.20 2.4 Loam 0.18 2.1 175 Fine sandy loam 0.14 1.7 142 loam 0.12 1.5 125 Loamy sand 0.10 1.2 100 Sand 0.08 1.0 83 Koefisien ketersediaan air tanah Tanaman Maximum Percent (%) Kacang Kapri 35 Kentang Pohon buah-buahan 40 Anggur Tomat Other crops 50


Air terdapat dalam tanah karena ditahan (diserap) oleh massa tanah, tertahan oleh lapisan kedap air, atau karena keadaan drainase yang kurang baik. Air dapat meresap atau ditahan oleh tanah karena adanya gaya-gaya adhesi, kohesi, dan gravitasi. Karena adanya gaya-gaya tersebut maka air dalam tanah dapat dibedakan menjadi: Air hidroskopik, adalah air yang diserap tanah sangat kuat sehingga tidak dapat digunakan tanaman, kondisi ini terjadi karena adanya gaya adhesi antara tanah dengan air. Air hidroskopik merupakan selimut air pada permukaan butir-butir tanah. Air kapiler, adalah air dalam tanah dimana daya kohesi (gaya tarik menarik antara sesama butir-butir air) dan daya adhesi (antara air dan tanah) lebih kuat dari gravitasi. Air ini dapat bergerak secara horisontal (ke samping) atau vertikal (ke atas) karena gaya-gaya kapiler. Sebagian besar dari air kapiler merupakan air yang tersedia (dapat diserap) bagi tanaman. Diunduh dari: ….. 12/11/2012

61 Kandungan Air Tanah (KAT)
KAT menyatakan banyaknya air yang ada dalam tanah. KAT dapat dinyatakan sebanyak banyaknya air ( mm kedalaman air) yang ada dalam satu meter kedalaman tanah. Misalnya: kalau sejumlah air (mm kedalaman air) 150 mm ada dalam satu meter kedalaman tanah, maka kandungan air tanah sebesar 150 mm/m. KAT dapat juga dinyatakan sebagai persen volume. Dalam hal contoh perhitungan di atas, 1 m3 volume tanah (misalnya kedalaman 1 m, dan luas permukaan 1 m2) mengandung m3 air (missal dengan kedalaman tanah of 150 mm = m dan luas permukaannya 1 m2). Hal ini menghasilkan nilai KAT dalam persen volume: Dengan demikian, kandungan air 100 mm/m setara dengan kadar air tanah 10 % v/v. Diunduh dari: ….. 12/11/2012

62 AIR DALAM TANAH Kadar (Kandungan) air tanah Mass water content (m)
m = mass water content (fraction) Mw = mass of water evaporated, g (24 105oC) Ms = mass of dry soil, g

63 Kadar air volumetrik (v)
V = volumetric water content (fraction) Vw = volume of water Vb = volume of soil sample At saturation, V =  V = As m As = apparent soil specific gravity = b/w (w = density of water = 1 g/cm3) As = b numerically when units of g/cm3 are used Ekuivalen kedalmana air (d) d = volume of water per unit land area = (v A L) / A = v L d = equivalent depth of water in a soil layer L = depth (thickness) of the soil layer

64 Volumetric Water Content & Equivalent Depth
(cm3) Equivalent Depth (cm3) (g) (g)

65 Volumetric Water Content & Equivalent Depth Typical Values for Agricultural Soils
Soil Solids (Particles): 50% 0.50 in. 1 in. Very Large Pores: % (Gravitational Water) 0.15 in. Total Pore Space: % Medium-sized Pores: 20% (Plant Available Water) 0.20 in. Very Small Pores: % (Unavailable Water) 0.15 in.

66 Water-Holding Capacity of Soil Effect of Soil Texture
Coarse Sand Silty Clay Loam Dry Soil Gravitational Water Water Holding Capacity Available Water Unavailable Water

The water holding capacity of a soil is a very important agronomic characteristic. Soils that hold generous amounts of water are less subject to leaching losses of nutrients or soil applied pesticides. This is true because a soil with a limited water holding capacity (i.e. a sandy loam) reaches the saturation point much sooner than a soil with a higher water holding capacity (i.e. a clay loam). After a soil is saturated with water, all of the excess water and some of the nutrients and pesticides that are in the soil solution are leached downward in the soil profile. Soil water holding capacity is controlled primarily by the soil texture and the soil organic matter content. Soil texture is a reflection of the particle size distribution of a soil. An example is a silt loam soil that has 30% sand, 60% silt and 10% clay sized particles. In general, the higher the percentage of silt and clay sized particles, the higher the water holding capacity. The small particles (clay and silt) have a much larger surface area than the larger sand particles. This large surface area allows the soil to hold a greater quantity of water. The amount of organic material in a soil also influences the water holding capacity. As the level of organic matter increases in a soil, the water holding capacity also increases, due to the affinity of organic matter for water.

68 PENENTUAN WHC The water holding capacity of the soil is determined by the amount of water held in the soil sample vs. the dry weight of the sample. The amount of pressure applied in these different methods can be as low as 1/3 atmosphere of pressure (about 5 psi) up to 15 atmospheres of pressure (about 225 psi).

69 . Soil Water Holding Capacity
One of the main functions of soil is to store moisture and supply it to plants between rainfalls or irrigations. Evaporation from the soil surface, transpiration by plants and deep percolation combine to reduce soil moisture status between water applications. If the water content becomes too low, plants become stressed. The plant available moisture storage capacity of a soil provides a buffer which determines a plant’s capacity to withstand dry spells. Forms of Soil Water Storage Water is held in soil in various ways and not all of it is available to plants.  Chemical water is an integral part of the molecular structure of soil minerals. It can be held tightly by electrostatic forces to the surfaces of clay crystals and other minerals and is unavailable to plants. The rest of the water in the soil is held in pores, the spaces between the soil particles. The amount of moisture that a soil can store and the amount it can supply to plants are dependent on the number and size of its pore spaces  Gravitational water is held in large soil pores and rapidly drains out under the action of gravity within a day or so after rain. Plants can only make use of gravitational water for a few days after rain. Capillary water is held in pores that are small enough to hold water against gravity, but not so tightly that roots cannot absorb it. This water occurs as a film around soil particles and in the pores between them and is the main source of plant moisture. As this water is withdrawn, the larger pores drain first. The finer the pores, the more resistant they are to removal of water. As water is withdrawn, the film becomes thinner and harder to detach from the soil particles. This capillary water can move in all directions in response to suction and can move upwards through soil for up to two metres, the particles and pores of the soil acting like a wick.

70 Source: Department of Agriculture Bulletin 462, 1960
WHC - TEKSTUR Water holding capacity (mm/cm depth of soil) of main texture groups.  Figures are averages and vary with structure and organic matter differences. Texture Field Capacity Wilting point Available water Coarse sand 0.6 0.2 0.4 Fine sand 1.0 Loamy sand 1.4 0.8 Sandy loam 2.0 1.2 Light sandy clay loam 2.3 1.3 Loam 2.7 1.5 Sandy clay loam 2.8 Clay loam 3.2 1.8 Clay 4.0 2.5 Self-mulching clay 4.5 Source: Department of Agriculture Bulletin 462, 1960

71 Potential Air Tanah Deskripsi:
Measure of the energy status of the soil water Important because it reflects how hard plants must work to extract water Units of measure are normally bars or atmospheres Soil water potentials are negative pressures (tension or suction) Water flows from a higher (less negative) potential to a lower (more negative) potential

72 Potential Air Tanah Komponennya: t = total soil water potential
g = gravitational potential (force of gravity pulling on the water) m = matric potential (force placed on the water by the soil matrix – soil water “tension”) o = osmotic potential (due to the difference in salt concentration across a semi-permeable membrane, such as a plant root) Matric potential, m, normally has the greatest effect on release of water from soil to plants

73 Soil Water Release Curve
Curve of matric potential (tension) vs. water content Less water  more tension At a given tension, finer-textured soils retain more water (larger number of small pores)

74 Matric Potential and Soil Texture
The tension or suction created by small capillary tubes (small soil pores) is greater that that created by large tubes (large soil pores). At any given matric potential coarse soils hold less water than fine-textured soils. Height of capillary rise inversely related to tube diameter

Soil water content where gravity drainage becomes negligible Soil is not saturated but still a very wet condition Traditionally defined as the water content corresponding to a soil water potential of -1/10 to -1/3 bar Permanent Wilting Point (WP or wp) Soil water content beyond which plants cannot recover from water stress (dead) Still some water in the soil but not enough to be of use to plants Traditionally defined as the water content corresponding to -15 bars of SWP

76 KAPASITAS LAPANG "The amount of water held in soil after excess water has drained away and the rate of downward movement has materially decreased, which usually takes place within days after a rain or irrigation in pervious soils of uniform structure and texture". Diunduh dari: -….. 15/11/2012

77 Factors affecting field capacity
KAPASITAS LAPANG Factors affecting field capacity Texture. The finer the texture of the soil particles, the higher is the apparent field capacity and the slower it is attained. Type of Clay. Soils high in montmorillonite have higher field capacity values Organic Matter. Increases field capacity (as high as 100% in organic soils) Depth of initial wetting. In general (but not always), The wetter the lower soil profile at the beginning of redistribution, and the greater the depth of wetting, the slower the rate of redistribution, and the greater the value of field capacity Impeding layers. Inhibit redistribution and increase field capacity Evapotranspiration. Modifies redistribution and affects field capacity. Diunduh dari: -….. 15/11/2012

78 KAPASITAS LAPANG Field capacity minus wilting point is the amount of water available to plants. Diunduh dari: ….. 15/11/2012

79 KAPASITAS LAPANG Field capacity is the volumetric water content at a soil water suction of 0.33 bars or remaining after a prolonged period of gravity drainage without additional water supply. Wilting point is the volumetric water content at a suction of 15 bars or the lowest volumetric water content that can be achieved by plant transpiration. These moisture retention parameters are used to define moisture storage and relative unsaturated hydraulic conductivity. The field capacity must be greater than the wilting point and less than the porosity. Total porosity must be greater than the field capacity but less than 1 (one). The general relation among moisture parameters and soil texture class is shown below. Diunduh dari: ….. 15/11/2012

80 KAPASITAS LAPANG Conservation practices resulting in available water capacity favorable to soil function include: Conservation Crop Rotation Cover Crop Prescribed Grazing Crop Residue and Tillage Management Salinity and Sodic Soil Management Diunduh dari: /11/2012

81 KAPASITAS LAPANG Hygroscopic water content (θH) and bulk density (ρb) as a function of organic matter content (OM). The effect of OM content on ρb and θH is significant. . Both properties were linearly correlated to OM content (R2 = 0.91 for ρb and 0.97 for θH). Even though the different treatments imposed some changes on the OM content, for a given soil these changes resulted in a limited range of both θH and ρb values. The increasing trend of θH as a function of OM content can be explained by the significantly higher vapor adsorption capacity of OM substances (Chen and Schnitzer, 1976). Diunduh dari: ….. 15/11/2012

82 Effect of amount of soil cover on rainwater runoff and infiltration.
KAPASITAS LAPANG The proportion of rainwater that infiltrates into the soil depends on the amount of soil cover provided. The figure shows that on bare soils (cover = 0 tonnes/ha) runoff and thus soil erosion is greater than when the soil is protected with mulch. Crop residues left on the soil surface lead to improved soil aggregation and porosity, and an increase in the number of macropores, and thus to greater infiltration rates. Effect of amount of soil cover on rainwater runoff and infiltration. Diunduh dari: /11/2012

. Soil is a porous structure. Ideally about 50% of the volume is pore space, with half of it consisting of “macropores” of visible to invisible size (0.05 mm to several millimetres in diameter) and half being smaller “micro-” or “capillary” pores. During an extended, heavy rainfall, all pores may become filled with water. The water then drains from the larger pores by downward gravitational flow, leaving them air-filled; this is important for supplying oxygen to roots and soil life. The larger the pores, the more rapidly they drain, providing that such drainage is not impeded (e.g. by a hardpan). After those larger pores have drained—about 1 day after the rain stops in lighter soils, and 3 days in heavier soils—all of the micropores remain filled with water and the soil is said to be at field capacity. The micropores do not drain by gravitational flow; rather they hold water by a type of electrostatic attraction between the water and the soil surfaces. The smaller the pores or the closer you get to the surface of the particles, the more tightly the water is held. Diunduh dari: ….. 15/11/2012

84 Available water capacity (AWC)
Water that is held in macropores drains too quickly to be of much use to plants; water held in micropores below the permanent wilting point cannot be used at all. Thus the water available to plants is that held between the field capacity and the permanent wilting point. This quantity of water is called the available water capacity (AWC) of the soil. Of this amount, one half is usually readily available, meaning that until AWC has dropped to one half, there are no serious water limitations. THE AWC values for soils of different textures. In general, loams hold the most available water, and also have the best balance of macro- and micropores. Consequently, loams have both good water storage as well as good drainage and aeration. Sandy soils, with a predominance of larger pores, are well drained, but they do not hold much water and they dry out quickly. At the other end of the textural scale, clay with many very small pores holds a lot of water but much of it is not available. Clay also lacks larger pores so drainage and aeration are poor. Diunduh dari: ….. 15/11/2012

85 Diunduh dari: ….. 15/11/2012
KEDALAMAN PERAKARAN Roots can pull water from a distance of only 1–2 cm. So, the depth of soil from which crops can extract water is, in general, the depth to which roots are growing. This means that shallow rooting crops are, in general, more prone to water limitations than are deep rooting crops. On the other hand, deep rooting crops can take advantage of water deeper in the soil only if their growth downwards is unrestricted. Removing barriers (such as plowpans) facilitates both water movement and root growth into deeper horizons. Double-digging, with deep incorporation of compost, is a good way to do it in gardens. Planting deep rooting cover crops with large tap roots, such as oilradish, or using rotations with alfalfa or sweet clover are effective biological ways to open up the subsoil. The direction of root growth is always towards the more moist regions. For that reason, frequent watering discourages deep growth of roots. Watering deep and less frequently encourages deep growth. Diunduh dari: ….. 15/11/2012

Increasing available water capacity The total water holding capacity of a soil can be improved by: Adding more soil, Opening up the subsoil as discussed above, Changing the texture, or Increasing the organic matter content. Texture should be a consideration when soil is being imported to make a garden. Manufactured topsoil used in new developments is often much too sandy for optimal water retention in a garden. Diunduh dari: ….. 15/11/2012

Organic matter is about 5 fold lighter than mineral soil, and a small amount of organic matter by weight has a big impact on pore space. “Within all textural groups, as organic matter increased from 1 to 3%, the available water capacity approximately doubled. When organic matter content increased to 4%, it then accounted for more than 60% of total AWC“. Organic matter increases the bulk of the soil so you actually have more soil depth as well as greater AWC per unit depth. It also improves infiltration, drainage and aeration, not to mention the benefits of improved soil nutrition and soil life. Diunduh dari: ….. 15/11/2012

88 Diunduh dari: ….. 15/11/2012
MEMANEN AIR HUJAN We want to capture as much of the precipitation as possible. The maximum rate at which water can be absorbed is referred to as the infiltration rate. When rainfall intensity exceeds the infiltration rate, water runs off, or forms ponds or puddles on the soil surface. The infiltration rate can vary from a few millimetres to more than ten centimetres per hour. The rate increases with the soil macro-porosity, which is naturally high in pure sands. In finer soils, the infiltration rate is very dependent on the formation of soil crumbs through binding of particles by humus, microbial gums and fungal hyphae, and on the channels formed by soil fauna and roots. Hence, it can be increased by adding compost and by regularly feeding the soil biota with plant residues and manures. Worm holes are especially important as they carry water quickly into deeper layers. Pulverizing soil through intensive cultivation or compacting soil with heavy machinery can quickly and drastically reduce macroporosity and infiltration. Diunduh dari: ….. 15/11/2012

89 . Ketersediaan air (Water availability)
Ketersediaan air adalah berapa besar cadangan air yang tersedia untuk keperluan irigasi. Ketersediaan air ini biasanya terdapat pada air permukaan seperti sungai, danau, dan rawa-rawa, serta sumber air di bawah permukaan tanah. Pada prinsipnya perhitungan ketersediaan air ini bersumber dari banyaknya curah hujan, atau dengan perkataan lain hujan yang jatuh pada daerah tangkapan hujan (catchment area/ watershed) sebagian akan hilang menjadi evapotranspirasi, sebagian lagi menjadi limpasan langsung (direct run off), sebagian yang lain akan masuk sebagai infiltrasi. Infiltrasi ini akan menjenuhkan tanah atas (top soil), kemudian menjadi perkolasi ke ground water yang akan keluar menjadi base flow Di samping data meteorologi, dibutuhkan pula data cahaya permukaan (exposed surface), dan data kelembaban tanah (soil moisture). Untuk rumus run off adalah Run off = base flow + direct run off. Diunduh dari: ….. 12/11/2012

90 Available water content in mm water depth per m soil depth (mm/m)
Air Tersedia Definisi Water held in the soil between field capacity and permanent wilting point “Available” for plant use Available Water Capacity (AWC) AWC = fc - wp Units: depth of available water per unit depth of soil, “unitless” (in/in, or mm/mm) Measured using field or laboratory methods (described in text) Kandungan air tersedia sangat tergantung pada tekstur dan struktur tanah. Kisaran nilai-nilai pada beragam tipe tanah sbb: Soil Available water content in mm water depth per m soil depth (mm/m) Sand 25 to 100 Loam 100 to 175 Clay 175 to 250

91 Sifat hidraulik tanah & Tekstur tanah

92 Fraction available water depleted (fd)
(fc - v) = soil water deficit (SWD) v = current soil volumetric water content Fraction available water remaining (fr) (v - wp) = soil water balance (SWB)

93 Total Available Water (TAW)
TAW = (AWC) (Rd) TAW = total available water capacity within the plant root zone, (inches) AWC = available water capacity of the soil, (inches of H2O/inch of soil) Rd = depth of the plant root zone, (inches) If different soil layers have different AWC’s, need to sum up the layer-by-layer TAW’s TAW = (AWC1) (L1) + (AWC2) (L2) (AWCN) (LN) - L = thickness of soil layer, (inches) - 1, 2, N: subscripts represent each successive soil layer [Error on page 26 of text: change SWD  TAW ]

94 Ketersediaan Air Tanah
Ketersediaan air dalam tanah dipengaruhi: Banyaknya curah hujan atau air irigasi, Kemampuan tanah menahan air, Besarnya evapotranspirasi (penguapan langsung melalui tanah dan melalui vegetasi), Tingginya muka air tanah, Kadar bahan organik tanah, Senyawa kimiawi atau kandungan garam-garam, dan Kedalaman solum tanah atau lapisan tanah. Jumlah air yang tersedia bagi tanaman adalah jumlah air yang disimpan dalam tanah pada kondisi kapasitas lapang dikurangi dengan jumlah air yang masih tertinggal dalam kondisi titik layu permanen. Kandungan air tersedia = Kandungan air pada kapasitas lapang – kandungan air pada titik layu permanen Diunduh dari: ….. 12/11/2012

95 Gravity vs. Capillarity
Horizontal movement due to capillarity Vertical movement due largely to gravity

96 Infiltrasi Air Masuknya air ke dalam tanah
Faktor-faktor yang berpengaruh : Soil texture Initial soil water content Surface sealing (structure, etc.) Soil cracking Tillage practices Method of application (e.g., Basin vs. Furrow) Water temperature

97 Infiltrasi Kumulatif vs. Waktu Berbagai Tekstur tanah

98 Infiltration Rate vs. Time For Different Soil Textures

99 Laju Infiltrasi dan Tekstur Tanah

100 Soil Infiltration Rate vs. Constant Irrigation Application Rate

101 Soil Infiltration Rate vs. Variable Irrigation Application Rate

As water infiltrates into the soil, the length of the transmission zone increases, and the infiltrating water wets the soil’s wetting zone, which subsequently moves down in the soil profile. Diunduh dari: -….. 15/11/2012

For small times, vertical infiltration behaves as if horizontal infiltration because soil water pressure potential gradients dominate over the gravitational gradient. Example: Infiltration from an irrigation furrow into an initially dry soil. Diunduh dari: -….. 15/11/2012

Effect of Soil Moistture on Infiltration Diunduh dari: -….. 15/11/2012

105 INFILTRASI & SIFAT-SIFAT TANAH Effect of Soil Texture on Infiltration
Diunduh dari: -….. 15/11/2012

Effects of Surface crusts on infiltration: Soil surface crusts Can develop by: · Soil surface compaction · Slaking of soil agggregates at soil surface by rainfall / irrigation Diunduh dari: -….. 15/11/2012

Soil layering: · Effect of soil layering depends on soil texture variations between layers · Generally, any soil layers that are present will decrease water infiltration If clay layer: Clay impedes infiltration because of lower saturated hydraulic conductivity If sand layer: Sandy layer will reduce infiltration rate temporarily and retards moving of the wetting front due to its lower unsaturated hydraulic conductivity Diunduh dari: -….. 15/11/2012

108 Kedalaman Penetrasi Can be viewed as sequentially filling the soil profile in layers Deep percolation: water penetrating deeper than the bottom of the root zone Leaching: transport of chemicals from the root zone due to deep percolation Deep Percolation Deep percolation is when water moves down through the soil profile below the root zone and cannot be utilized by plants. Diunduh dari: /11/2012

109 Simpanan Air dalam Profil Tanah


111 . SIMPANAN LENGAS TANAH = Soil Moisture Storage (ST).
Simpanan lengas tanah adalah jumlah air yang ditahan dalam tanah selama waktu tertentu. Jumlah air dalam tanah tergantung pada sifat tanah seperti tekstur tanah dan kandungan bahan organik tanah. Jumlah maksimum air yang dapat ditahan dalam tanah disebut KAPASITAS LAPANG. Tanah-tanah berbutir halus mempunyai kapasitas lapang lebih besar dibandingkan dengan tanah-tanah berbutir kasar (tanah berpasir). Dengan demikian lebih banyak air yang tersedia untuk evapotranspirasi aktual dari tanah-tanah yang teksturnya halus daripada tanah yang teksturnya kasar. Batas maksimum simpanan lengas rtanah adalah KAPASITAS LAPANG, batas minimumnya adalah nol (0) ketika tanah telah mengering.

112 SIMPANAN LENGAS TANAH Pemanfaatan lengas tanah merupakan faktor penting yang membatasi produksi tanaman. Pengetahuan mengenai simpanan air tersedia dalam tanah sangat penitng dalam pengelolaan pertanian lahan kering. Kajian tentang simpanan air tanah, komponen-komponen dari siklus air, sangat diperlukan dalam perhitungan neraca air lahan. Neraca air lahan menyajikan informasi tentang masukan air (hujan dan irigasi), kehilangan air (evapotranspirasi, run-off dan drainage), serta perubahan simpanan lengas tanah yang terjadi selama periode waktu tertentu. Pengelolaan simpanan lengas tanah secara efisien dapat dicapai dengan jalan memanipulasi neraca air lahan. Hal ini melibatkan pemantauan dan pengendalian berbagai proses aliran lengas tanah, termasuk infiltrasi, redistribusi, drainage, evaporasi dan penyerapan air oleh tanaman. Bahan organik tanah mempunyai peran penting dalam mengendalikan semua pproses-proses fisika ini.

Air hujan yang dipanen disimpan dalam tanah di daerah lahan budidaya tanaman. Kapasitas tanah untuk menyimpan air dan membuat air tersebut tersedia abagi tanaman disebut KAPASITAS SIMPANAN AIR TERSEDIA. Kapasitas ini tergantung pada (i) jumlah dan ukuran pori tanah (tekstur) dan (ii) kedalaman tanah. Kapasitas simpanan air tersedia dinyatakan dalam mm kedalaman air (air simpanan) per meter kedalaman tanah, mm/m. Kapasitas Simpanan Air Tersedia: Tipe Tanah Air Tersedia (mm/m) Pasir = sand 55 Lempung Berpasir = sandy loam 120 Lempung Liat = clay loam 150 Liat = clay 135 Suatu tanah lempung dengan WHC air tersedia yang cukup baik 120 mm per meter kedalaman tanah, akan kehilangan nilainya kalau SOLUM TANAH hilang dan tanah menjadi dangkal. Misalnya, 40 cm tanah pada suatu batuan induk hanya menyediakan 48 mm air tersedia bagi tanaman.

Kapasitas simpanan air tersedia dan kedalaman tanah mempunyai implikasi penting bagi disain sistem pemanenan air tersedia. Pada tanah yang solumnya dalam, misalnya, 2 m dengan kapasitas simpanan air yang besar (150 mm/m) , maka kapasitas simpanan airnya sebesar 300 mm air dan tidak akan terjadi genangan air runoff pada lahan pengolahan hingga kedalaman lebih dari 300 mm (30 cm). Kalau jumlah air melebihi 30 cm kedalaman, maka akan hilang melalui drainage-dalam dan juga akan dapat menimbulkan bahanya penggenangan. Kapasitas air tersedia dan kedalaman solum tanah juga mempengaruhi jenis tanaman yang akan ditanam. Suatu tanah yang solumnya dalam dengan kapasitas simpanan air tersedia yang besar hanya dapat digunakan secara efektif oleh tanaman yang mempunyai perakaran yang dalam. Misalnya, bawang-Onions, mempunyai perakaran sedalam cm, sehingga ia tidak dapat memanfaatkan sepenuhnya semua lengas tanah yang tersimpan dalam solum. Tabel 7 menyajikan kedalaman perakaran beberapa jenis tanaman. Kedalaman perakaran efektif beberapa tanaman (Doorenbos et al., 1979). Jenis Tanaman Kedalaman akar efektif (m) Kacang buncis = Bean Jagung = Maize Bawang merah = Onion Padi = Rice Sorghum Bunga matahari = Sunflower

115 KEDALAMAN TANAH This gives an indication of the soil volume which can be utilised by the plant and which is conducive to moisture retention. Effective soil depth is the depth where adequate moisture, nutrients and air occur. Effective soil depth can be lowered by rocky layers, a high clay content, waterlogged layers, limestone layers, acid subsoil and compacted layers. Various crops have different requirements concerning effective soil depth.

116 A hypothetical soil profile.
Diunduh dari: /11/2012

117 Generalized Soil Profile
Diunduh dari: /11/2012

118 The top layer of soil is called the SURFACE LITTER LAYER
The top layer of soil is called the SURFACE LITTER LAYER. This is where all the “litter” of any ecosystem lies. It includes leaves, branches, animal scats and bodies, mushrooms and other rotting matter. Diunduh dari: /11/2012

119 PROFIL TANAH ALFISOL Alfisols develop in humid and subhumid climates, have average annual precipitation of mm. They are frequently under forest vegetation. Characteristic features: Clay accumulation in a Bt horizon, Thick E horizon, Available water much of the growing season, Slightly to moderately acid. Diunduh dari: /11/2012

. Inceptisols, especially in humid regions, have weak to moderated horizon development. Horizon development have been retarded because of cold climated, waterloged soils, or lack of time for stronger development. Characteristic feature: Texture has to be finer than loamy very fine sand. Diunduh dari: /11/2012

121 PROFIL TANAH ENTISOL Entisols have no profile development except a shallow marginal A horizon. Many recent river floodplains, volcanic ash deposits, unconsolidated deposits with horizons eroded away, and sands are Entisols. Diunduh dari: /11/2012

122 PROFIL TANAH MOLISOL Mollisols are frequently under grassland, but with some broadleaf forest-covered soils. Characteristic features: Deep, dark A horizons, they may have B horizons and lime accumulation. Diunduh dari: /11/2012

123 PROFIL TANAH ULTISOL Ultisols are ectensively weathered soils of tropical and subtropical climates. Characteristic features: Thick A horizon, clay accumulation in a Bt, strongly acid. Diunduh dari: /11/2012

124 Characteristic features:
PROFIL TANAH VERTISOL Vertisols exist most in temperate to tropical climated with distinct wet and dry seasons. They have a high content of clays that swell when wetted and show cracks when dry. Characteristic features: Deep self-mixed A horizon , Top soil falls into cracks seasonally, Gradually mixing the soil to the depth of the cracking. Diunduh dari: /11/2012

125 PROFIL TANAH OKSISOL Oxisols are excessively weathered, whereas few original minerals are left unweathered. They develop only in tropical and subtropical climates. Characteristic features: Often Oxisols are over 3 m deep, Have low fertility, Have dominantely iron and aluminium clays, Acid. Diunduh dari: /11/2012


127 .. Water balance, transpiration and canopy conductance in two beech stands
Agricultural and Forest Meteorology (4) Granier, A., Biron, P., Lemoine, D. Measurements of sap flow, vapour fluxes, throughfall and soil water content were conducted for 19 months in a young stand dominated by beech (Fagus sylvatica) growing at low elevation, in the Hesse forest, Germany. Study of the radial variation of sap flow within tree stems showed a general pattern of sap flux density in relation to the depth below cambium. Among-tree variation of sap flow was also assessed, in order to determine the contribution of the different crown classes to the total stand transpiration. Stand sap flow and vapour flux, measured by the eddy covariance technique, were well correlated, for half hourly as well for daily values, the ratios of the fluxes for both averaging periods being 0.77. A strong canopy coupling to the atmosphere was found, with the omega factor ranging between 0.05 and 0.20 relative to the wind speed. Canopy conductance variation was related to a range of environmental variables: global radiation, vapour pressure deficit, air temperature and soil water deficit. In addition to the effect of radiation and of vapour pressure deficit often found in various other tree species, here beech exhibited a strong reduction in canopy conductance when air temperature decreased below 17oC.

128 Agricultural and Forest Meteorology. 1998. 89 (3/4). 169-184
Maize canopies under two soil water regimes. I. Diurnal patterns of energy balance, carbon dioxide flux, and canopy conductance Agricultural and Forest Meteorology (3/4) Steduto, P., Hsiao, T. C. . Diurnal patterns of fluxes of CO2, latent heat and other components of energy were determined for adjacent fields of maize located at the University of California, Davis, one being well watered and one growing only on water stored in the soil, under clear and variable cloud conditions. In addition, it was demonstrated that the canopy responded to the movement of clouds without measurable lag, with Acf, lambda E and sensible heat (H) fluctuating in unison as the sun disappeared and reappeared. Acf, reaching peak values of about micro mol m-2 s-1, was higher in the morning than in the afternoon for the same level of Qp (photosynthetic photon flux density), and dropped to zero late in the afternoon when Qp was still substantial as a result of increases in respiration as the temperature rose from the morning to the afternoon. Senescence of the canopy, whether normal as maturity was approached or accelerated by water stress, was associated with a general reduction in Acf for a given green leaf area index. Late in the life cycle, Acf was light saturated at 1.3 to 1.6 mmol m-2 s-1. At that time, abnormally high radiation due to focusing by clouds caused an increase in lambda E and H, but not in Acf. Canopy conductances were calculated using the Penman-Monteith big leaf model, after aerodynamic conductances were derived from wind data. Diurnal patterns indicated that canopy conductance was strongly influenced by radiation and was independent from, and usually lower than, aerodynamic conductance. Canopy conductance for water vapour reached noon-time values /up to/ 40 mm s-1 under the most favourable water conditions. Water stress and senescence reduced canopy conductance. Aerodynamic conductance for water vapour ranged between 15 and 90 mm s-1 for wind velocity between 1 and 5 m s-1

129 .. Water and energy balance of a forested Appalachian watershed
Agricultural and Forest Meteorology (1/2) Tajchman, S. J., Fu, H., Kochenderfer, J. N. This study focuses on the water and energy balance of a central Appalachian watershed with an area of 39.2 ha, covered with upland oaks (Quercus spp.) and cove hardwoods (Acer saccharum, Prunus serotina, Liriodendron tulipifera, Fagus grandifolia), 80 years old, located near Parsons, West Virginia, USA. For 40 hydrological years (May-April, ) the average yearly sum of precipitation (P) was cm, stream flow (R) was 63.8 cm, and evapotranspiration (Et = P - R) was 81.7 cm. Neglecting the energy used in photosynthesis, the yearly sum of net radiation of the watershed (Rn) equals latent heat of evapotranspiration (Et) plus sensible heat exchange with the atmosphere (H). Rn was computed for 432 terrain segments covering the whole watershed area. Its average value was 2.2 GJ m-2 year-1. From the balance LEt = 2.02 GJ m-2 year-1, where L is the latent heat of evaporation of water. The sensible heat exchange (H = Rn - Et) was 0.18 GJ m-2 year-1 or 8% of Rn

130 .. Difficulties of estimating evapotranspiration from the water balance equation
Agricultural and Forest Meteorology (3/4) Villagra, M. M., Bacchi O. O. S., Tuon R. L., Reichardt, K. The effect of soil spatial variability on the estimation of evapotranspiration, using the water balance equation, was evaluated using data from 25 experimental plots, distributed along a transect of 125 m, on a dark red Brazilian latosol. The variability of soil water storage, total hydraulic gradients, soil hydraulic conductivity and soil water flux densities, and their influence on the calculation of evapotranspiration, are discussed. The variability of these parameters confers a coefficient of variation of the order of 40% to evapotranspiration estimates, indicating that aerodynamic and empiric approaches are a better choice for evapotranspiration estimation of extensive field areas, in which spatial variability of soil hydraulic characteristics is relevant.

131 What is Evapotranspiration?
Evapotranspiration is the loss of water to the atmosphere through two processes:  evaporation from soil and plant surfaces, and transpiration from plant tissues.  Evapotranspiration is an indicator of how much water crops, lawns, gardens and trees need for healthy growth and productivity. By measuring evapotranspiration, only the amount of water that is lost will be put back into the soil, therefore reducing water waste. Diunduh dari: /11/2012

132 Agricultural Water Management. 1998. 36 (2). 99-109
Grouping water storage properties of Indian soils for soil water balance model applications Agricultural Water Management (2) Rao, N. H. Soil water balance models are useful tools of agricultural and water resources planning and management. Data of water storage limits of soil profiles, field capacity (FC) and permanent wilting point (PWP), are essential for running soil water balance models. A procedure for estimating these parameters for Indian soils is presented. The values of FC and PWP for each textural class are estimated independently for alluvial, black and red soils. The estimated parameters are tested by running a soil water balance model for a case study area. The use of the model in saving irrigation water is also demonstrated. The estimated parameters of Indian soils are also compared with corresponding values of US soils. The comparisons are favourable for FC for all soil texture groups, and for PWP for coarse to medium and medium texture groups. Differences in PWP between US and Indian soils increase at finer soil textures as a result of differences in clay fraction, mineralogy, and organic matter content.

133 Water balance and water use efficiency of different land uses in western Himalayan valley region
Agricultural Water Management (3) Narain, P., Singh, R. K., Sindhwal ,N. S., Joshie, P. . Various land uses, including sole plantations of Leucaena and Eucalyptus, maize-wheat, Chrysopogon grass or turmeric and their tree crop mixtures were compared for nine years in two sequences for runoff, water use and water use efficiency on nine large erosion plots on a 4% slope. Availability of water during summers and climatic evaporative (EP) demand during winters appeared to be the governing factors for seasonal water use. Approximately 70% of annual water consumption occurs during the four months (July to October) of the rainy season. During this season water use was approx equal to 3-4 times EP for trees and grass and 2.5 times for maize. The water use equals EP regardless of the land use during the winter season, while it was reduced to approx. one-third to half of EP in the summer season. Annual water use was closely linked with runoff reduction efficiency of the land use. Sole plantations of Leucaena and Eucalyptus showed negligible runoff losses and their water use approximated annual rainfall. Agroforestry land uses also reduced runoff and increased water use and water use efficiency. Seasonal crops exploited 1.5 m depth of profile more exhaustively than trees, whereas trees used soil water down to 3.0 m depth. Therefore, in tree-crop mixtures more efficient soil water use was observed as compared to monocropping systems. Results of this study indicate that water conserved under sole tree plantations and due to tree intervention in agroforestry land uses through runoff reduction, is utilised to meet increased evapotranspiration demand, and hence groundwater recharge in appreciable quantities is unlikely.

FAG = P/U where P is crop production (total dry matter or the marketable product, as the case may be) and U is the volume of water applied. As only a fraction of the applied water is actually absorbed and utilized by the crop, it is necessary to consider the various components of the denominator U: U = R + D + Ep + Es + Tw + Tc where R is the volume of water lost by runoff from the field, D the volume drained below the root zone (deep percolation), Ep the volume lost by evaporation during the conveyance and application to the field,2 Es the volume evaporated from the soil surface (mainly between the rows of crop plants), Tw the volume transpired by weeds, and Tc the volume transpired by the crop. All these volumes pertain to the same unit area. Accordingly: FAG = P / (R + D + EP + ES + TW + TC ) Diunduh dari: ….. 15/11/2012

135 The water balance of a field
Under flood irrigation as commonly practised in river diversion schemes, excessive water application often results in considerable runoff, evaporation from open water surfaces and transpiration by weeds. In the experience of the author, these losses commonly amount to 20 percent or even 30 percent of the water applied. In addition, the loss of water due to percolation below the root zone may be of the order of 30 percent or even 40 percent of the water applied. Consequently, the fraction actually taken up by the crop is often below 50 percent and may even be as low as 30 percent. Diunduh dari: ….. 15/11/2012

136 Water balance estimates over Greece
Agricultural Water Management (1) Kerkides, P., Michalopoulou, H., Papaioannou, G., Pollatou, R. . Water balance for 31 locations in Greece was calculated on the basis of long-term average monthly precipitation, evapotranspiration and combined soil and vegetation characteristics, using the method of Thornthwaite and Mather. Monthly evapotranspiration estimates were calculated from 27 years ( ) of routine meteorological data using the original Penman method. Soil and vegetation characteristics specific for the locations under study were combined in the water capacity of the root zone (WCRZ). Similar water balance calculations were carried out using fixed values of WCRZ for all stations, to evaluate the effects of soil and vegetation through the WCRZ in the final estimates of soil moisture deficits. Water balance calculations were performed using average monthly evapotranspiration estimates calculated according to the empirical Thornthwaite method. Results were compared to show differences that could be attributed to the method of estimating evapotranspiration. Results obtained with a value of WCRZ fixed at 300 mm and potential evapotranspiration estimated by the Thornthwaite method for the period were compared with existing similar results over a longer period in the past ( ), in order to detect diachronic changes in the water balance components over the same regions in Greece.

137 Agricultural Water Management. 1994. 26 (3). 155-168
. A soil water balance model for no-tillage and conventional till systems Agricultural Water Management (3) Shanholtz, V. O., Younos, T. M. A soil water balance model to simulate hydrological aspects of no-tillage and conventional tillage systems when selected soils are subjected to a range of climatic conditions was developed. Model components include procedures for determining plant water interception, plant growth, root development, surface runoff and infiltration, plant available soil water, evaporation, and evapotranspiration. A database for two growing seasons of maize (Zea mays) production was used to develop mathematical expressions for various model components and to calibrate and verify the model. Procedures for parameter determination and adjustment are presented. Results include a comparison of simulated soil water status to field measurements for two tillage systems. Good agreement between simulation results and field measurements of soil water was obtained at 0-30 cm soil depth for two different climatic conditions.

138 Components of the water balance for tree species under evaluation for agroforestry to control salinity in the wheatbelt of Western Australia Agroforestry Systems (3) Eastham, J., Scott, P. R., Steckis, R. . The soil water balance technique was used to study evaporation (total water loss by the processes of transpiration, evaporation from the soil surface and evaporation of water intercepted by plant canopies) in agroforestry trials using 2 fodder tree species (Acacia blakeyi and Chamaecytisus proliferus), 10 Eucalyptus species (4 of single stemmed form, and 6 of mallee form), and annual pasture over a 3-yr period in the Western Australian wheatbelt. Evaporation from both fodder trees and from 7 of the Eucalyptus spp. (E. calycogona, E. camaldulensis, E. horistes, E. kochii subsp. kochii and plenissima [E. plenissima?], E. loxophleba and E. spathulata) was greater than from pasture for one or more of the study years. The maximum difference in evaporation between trees and pasture was 82, 84 and 70 mm in the first, second and third study years, respectively. Higher evaporation from trees was associated with greater depletions in soil water than occurred beneath pasture. Upward movement of water from wet soil beneath the root zone was found under trees, with a maximum flux of 30 mm observed over a one year period beneath E. camaldulensis. The water use efficiency of fodder trees was significantly higher than for most Eucalyptus species, due to greater yields from fodder trees. Biomass production was a good indicator of the water use of eucalypts over the first 2 yr of growth, but the relation between productivity and water use differed for species with tree and mallee forms. In the third year of study, obvious differences in the relation between water use and yield were observed for some species of eucalypts with high evaporation.

139 Australian Journal of Agricultural Research. 9. 50 (2). 283-289
Obtaining soil hydraulic properties for water balance and leaching models from survey data. 1. Water retention Australian Journal of Agricultural Research (2) Smettem, K. R. J., Oliver, Y. M., Heng. L. K., Bristow, K. L., Ford, E. J.. A physico-empirical 2-parameter power law model of the draining water retention curve (WRC) based solely on clay content was described and further developed using 6 datasets obtained from Australian and New Zealand soils. The slope of the WRC, or pore-size distribution index, was well described by the model but the bubbling pressure, or inflection point was poorly described. Without a good estimation of the bubbling pressure it was not possible to scale the physico-empirical model to the WRC. To achieve the scaling, a single measured point on the WRC in the unsaturated range was required. The resulting estimated water contents may be satisfactory for application within broad-scale leaching risk models and for generalized extrapolation of results from detailed experimental sites but caution is still required for quantitative applications of nitrate leaching models at a particular site. It is concluded that soil surveys could usefully include a single WRC measurement in the field at each sampling location to improve their utility for water and chemical transport modelling.

140 Typical soil - water retention curve
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141 Australian Journal of Agricultural Research. 1999. 50 (7).
Obtaining soil hydraulic properties for water balance and leaching models from survey data. 2. Hydraulic conductivity Australian Journal of Agricultural Research (7). Smettem, K. R. J., Bristow, K. L. . A simplified physico-empirical model to estimate the 'matrix', or textural saturated hydraulic conductivity, Km, using estimates of the bubbling pressure derived entirely from clay content data that are readily available in soil surveys. Model estimates were compared with in situ measurements on surface soils obtained using a disc permeameter with a negative pressure head at the supply surface of 40 mm. Results were satisfactory for broad-scale water balance and leaching risk models that require specification of a matching point for the unsaturated hydraulic conductivity function and for modelling applications requiring generalized application of results from experimental sites.

Hydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation. Saturated hydraulic conductivity, Ksat, describes water movement through saturated media. Soil hydraulic conductivity is a function of the water potential of the soil. Conductivity measures the ease with which water moves through the soil. As water content (and hence the water potential) decreases, the hydraulic conductivity decreases drastically. The decrease in conductivity as the soil dries is due primarily to the movement of air into the soil to replace the water. As air moves in, the pathways for water flow between soil particles become smaller and more tortuous, and flow becomes more difficult. Diunduh dari: /11/2012

Soil hydraulic conductivity as a function of the water potential of the soil. Conductivity measures the ease with which water moves through the soil. The overall shape of this curve is representative of many soils, but the shape for a particular soil may be influenced by the size distribution of its particles and by its organic matter content. The field capacity is the amount of water the soil is able to retain against gravitational forces. The permanent wilting point is the soil water potential value at which plants cannot regain turgor pressure even at night, in the absence of transpiration. Diunduh dari: /11/2012

144 Forest Ecology and Management. 1998. 105 (1/3). 121-128
. Water balance of Pinus halepensis Mill. afforestation in an arid region Forest Ecology and Management (1/3) Schiller, G., Cohen, Y. Transpiration (T), needle water stress (MPa) and the water balance of an Aleppo pine (Pinus halepensis) plantation, growing in an arid region at the edge of the Israeli Negev desert, were studied over 1 yr ( ). The heat pulse technique for the measurement of the heat flow velocity was used for the estimation of the sap flow velocity (i.e. transpiration), in the stems of 16 trees (27% of the trees in a plot of 1000 m2). A pressure chamber was used to determine the needle water potential (MPa). Climatic parameters were measured in the forest for the computation of the potential transpiration (Tp) by means of the Penman-Monteith equation. During the rainy period, the transpiration (T) rate was maintained at a level between 1 and 2 mm day-1, which then dropped after the last rain; the decrease of T in small trees (diameter at breast height (DBH) <12 cm) was much faster than that in larger trees (DBH >12 cm). In May, T dropped to a nondetectable rate (about 0.02 mm day-1) and remained at this level until the next significant rain in December. A maximum T/Tp ratio of 0.3, which is nearly half of the ratio found in a previous study by the authors in an Aleppo pine plantation under Mediterranean climatic conditions (Schiller and Cohen, 1995), was recorded in February. This difference in T/Tp ratio between the 2 sites is attributed to their difference in basal area. Needle water potential at sunrise decreased from -0.8 MPa in the rainy period to more than -3.0 MPa during the dry period. The integrated T throughout the measurement period was used to estimate the total water uptake by the stand, which was mm (93% of the effective rainfall, or 80.2% of the total annual rainfall).

145 Irrigation Science. 1994. 15 (1). 17-23
.. Hydraulic properties and water balance of a clay soil cropped with cotton Irrigation Science (1) Aydin, M. A field study was carried out in Cukurova Region, Turkey, to investigate the soil water balance and water uptake by roots in relation to hydraulic properties of a clay (Vertic Luvisol) soil. Bare soils and cotton plots were equipped with tensiometers, gypsum blocks and neutron probe access tubes. Hydraulic conductivity, evaporation, drainage and water withdrawal by roots were determined from water flow equations using field data. Evaporation from bare soil was generally high from May to July, varying between 1.0 and 4.5 mm/day. However, when the soil water potential at 10 cm depth decreased to between and MPa, soil evaporation decreased to 0.4 mm/day. Drainage below 150 cm was highest (3.7 mm/day) at the start of wetting cycles, but quickly decreased. The highest values of capillary flux toward the surface layer and drainage rate from cropped soil were 2.0 and 1.8 mm/day resp. Rates of root water uptake from the soil profile, excluding the 0-10 cm layer, were high when compared with drainage and upward fluxes, with values of mm/day. Good agreement between root length densities and water uptake was found: up to 80% of roots grew in the top 50 cm of soil, where 78% of water was extracted. Evapotranspiration declined as a cubic function of available water in the top 120 cm of soil

146 .. Soil water balance changes in engineered soil surface
Journal of Environmental Quality (2) Sackschewsky, M. R., Kemp, C. J., Link, S. O., Waugh, W. J. Recharge can be prevented by storing precipitation near the surface so that it will be returned to the atmosphere via evapotranspiration. Erosion can be reduced with gravel mulch, but thick gravel layers may increase recharge. Gravel mixed into the surface soil may provide erosion protection without increasing recharge. To compare the effects of erosion control on infiltration, two lysimeter experiments were conducted to examine the effects of sand and gravel mulches and gravel admixtures, using two precipitation regimes and with or without vegetation. Sand and gravel mulch increased soil-column water storage and decreased evapotranspiration compared with a plain soil surface. Gravel admixtures did not significantly affect the soil water balance compared with plain soil surfaces. Vegetation increased evapotranspiration and decreased soil moisture storage compared with nonvegetated treatments. Irrigation greatly increased evapotranspiration but had little effect on soil water storage. Drainage was detected from sand and gravel-mulch lysimeters, but not from lysimeters with a plain-soil or gravel-admixture surface. Results are significant for isolation barrier designs in arid sites: (i) a nonvegetated gravel-mulch surface eventually will result in recharge, even under low precipitation (160 mm/yr); and (ii) a soil column with a plain-soil or gravel-admixture surface is capable of recycling all water back to the atmosphere, even under high-precipitation (450 mm/yr)

147 Journal of Hydrology (Amsterdam) 1995. 173 (1/4). 41-50
.. A conceptual model of catchment water balance: Application to runoff and baseflow modeling Journal of Hydrology (Amsterdam) (1/4) Ponce, V. M.; Shetty, A. V. A conceptual model of catchment water balance was used to simulate changes in runoff and baseflow with annual precipitation. The model is based on the sequential separation of annual precipitation into surface runoff and wetting, and wetting into baseflow and vaporization. Runoff is the sum of surface runoff and baseflow. Runoff gain is defined as the derivative of runoff coefficient with respect to precipitation. Baseflow gain is defined as the derivative of baseflow coefficient with respect to precipitation. Catchment data show that runoff and baseflow gains are always positive. Runoff gain reaches a peak value at a threshold precipitation Prt; baseflow gain reaches a peak value at a threshold precipitation Put. Analysis of the runoff and baseflow functions helps show the nature of the competition between runoff and vaporization, and baseflow and vaporization.

148 Soil & Tillage Research. 1998. 48 (1/2). 1-19
.. Modelling effects of soil structure on the water balance of soil-crop systems: a review Soil & Tillage Research (1/2). 1-19 Connolly, R. D. Poor soil structure, i.e. aggregation and porosity, is widely acknowledged as a major limitation to infiltration, redistribution and storage of water in a soil profile, leading to more runoff and erosion, reduced available water for plants and reduced crop production. Models of soil-crop systems are useful tools for evaluating interactions between soil physical condition, climate, management and crop growth. An outline of the principal components of soil structure which influence the water balance, hence crop growth, is given. Models that represent these components are reviewed and classified according to their representation of soil structure and the water balance. Few of the models reviewed explicitly represented soil structure and none attempted to simulate long-term changes in soil physical properties. About half the models used a USDA Curve Number approach to represent runoff and a 'tipping bucket' approach to represent soil water redistribution, and were classed as 'functional'. Functional models were mostly used for predicting long-term trends in erosion, soil fertility and crop growth. The remainder were termed 'mechanistic' as they used more physically based representations of the water balance. Mechanistic models were typically used for detailed simulations of soil water dynamics and solute movement and were more complex than functional models. No one model or modelling approach suites all applications; the problem the model is being used to resolve define the features and complexity needed. Future development of models of the soil-crop system would benefit from better methods of parameterising and applying models, extension of point-source models to simulate spatial variability and to handle varying catchment scales, and a more complete representation of soil physical properties

149 STRUKTUR TANAH The components of soil structure. Soil minerals with organic matter form soil structure units, called "peds". Micropores inside the peds and macropores between the peds carry air and water and facilitate root penetration. Soil aggregation or soil structure refers to how the sand, silt and clay come together to form larger granules. Good aggregation is apparent in a crumbly soil with water-stable granules that do not disintegrate easily. Well-aggregated soil has greater water entry at the surface, better aeration, and more waterholding capacity than poorly aggregated soil. A stable system of soil pores allows easy exchange of air and water. Diunduh dari: ….. 15/11/2012

150 .. Tillage and furrow diking effects on water balance and yields of sorghum and cotton
Soil Science Society of America Journal (4) Baumhardt, R. L., Wendt, C. W., Keeling, J. W. The amount of rain conserved and the yields of forage sorghum and cotton as affected by furrow dikes and tillage were compared for a 3-yr period at a site in Texas, USA. A clay loam soil was alternately cropped to cotton and sorghum. Forage sorghum was grown in (i) disc or (ii) chisel-disc tilled 16 by 23.8 m field plots with and without diking. Cotton was grown in rotation following sorghum after (i) conventional mouldboard-disc or (ii) no-tillage, with furrow dikes in one-half of the tillage treatment plots. Crop yield, rainfall amount, soil water content, and runoff of natural rainfall and of simulated rainfall, applied at 80 mm/h for 1 h, were measured. Compared with conventionally tilled undiked plots, cumulative nonponded infiltration of simulated rainfall was significantly greater with no-tillage treatments and greater (not significant) in furrow-diked treatments. Runoff of natural rainfall from plots with furrow dikes averaged approx equal to 22 mm less than from undiked plots, it was as much as 57 mm less; however, runoff from diked fields was observed. Under the conditions of this 3-yr study, diking did not significantly increase crop water use and yield. It was concluded that furrow dikes installed during the growing season did not increase water conservation and crop yields under the conditions of this 3-yr study due to seasonal dike consolidation that reduced the detention capacity and to the limited runoff from level fields. No-tillage was more effective than chisel tillage for increasing water conservation and crop yields for the conditions of this study

151 .. Transpiration from coppiced poplar and willow measured using sap-flow methods
Agricultural and Forest Meteorology (4) Hall, R. L., Allen S. J., Rosier P. T. W., Hopkins, R. Transpiration rates from clones of poplar (Populus trichocarpa x deltoides [P. interamericana] 'Beaupre') and willow (Salix burjatica [S. aquatica]) clones, grown as short-rotation coppice (3-year-old stems on 4-year-old stools) at a site in south-west England, were measured through the summer of Area-averaged transpiration was estimated by scaling sap-flow rates measured in individual stems to a stand area basis using measurements of leaf area and stem diameter distribution. Sap flow in poplar was measured using the stem heat balance, heat pulse velocity and deuterium tracing techniques, while in willow only the stem heat balance method was used. In June and early July the mean daily transpiration from the poplar was 6 plus or minus 0.5 mm/day, stomatal conductances averaged 0.33 mol m-2/s for leaves in the upper layer of the canopy, and daily latent heat flux often exceeded the daily net radiation flux. Similarly high transpiration was estimated for the willow. Transpiration rates were higher than any reported rates from agricultural or tree crops grown in the UK and arose because of high aerodynamic and stomatal conductances. The high stomatal conductances were maintained even when atmospheric humidity deficits and soil water deficits were large. Much lower rates from both clones were recorded in August at the end of a drought period. It is suggested that extensive plantation of poplar or willow short-rotation coppice may result in reduced drainage to stream flow and aquifer recharge

152 TRANSPIRASI Oak trees will lose water through a process called transpiration as illustrated at the left. Transpiration is part of the water cycle, and it is the loss of water vapor from parts of the plant, mainly the leaves, which also triggers a flow of mineral nutrients and water from the roots back to the shoots of tree.  Diunduh dari: ….. 15/11/2012

153 . Evaporation of intercepted precipitation based on an energy balance in unlogged and logged forest areas of central Kalimantan, Indonesia Agricultural and Forest Meteorology (3) Asdak C.; Jarvis P. G.; Gardingen P. V. The effect of logging practices on rainfall interception loss was investigated in a dipterocarp humid tropical rain forest in central Kalimantan. The traditional volume balance method was used to measure throughfall, stemflow and interception loss. The evaporation rate during and after rainfall had ceased in canopy-saturated conditions was calculated by an energy balance method, which relied on the modified Penman equation using directly determined microclimatic and canopy structure variables as inputs. The results obtained showed that the evaporation from wet canopies in this research area is driven more by advected energy than by radiative energy. In the unlogged plot (581 trees/ha with diameter at breast height (dbh) >10 cm), advective energy accounted for 0.38 mm h-1 of the 0.51 mm h-1 of evaporation, whereas radiative energy accounted for only 0.13 mm h-1. A similar relationship between the major driving variables and the rate of evaporation was also found in the logged over plot (278 trees/ha with dbh >10 cm) and this implies that logging activities did not change the proportion of energy used for interception loss. The Priestley-Taylor equation was found to be a poor model for evaporation of intercepted water in tropical forests because advected energy is very important at the canopy scale

154 Influence of vegetation on infiltration capacity
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155 Agricultural and Forest Meteorology. 1997. 84 (1/2). 69-82
. Hydrological impacts of converting native forests and grasslands to pine plantations, South Island, New Zealand Agricultural and Forest Meteorology (1/2) Fahey B.; Jackson R. Changes in water yield, flood hydrology, and low flows caused by replacing indigenous forests and grasslands with commercial softwoods have been investigated in New Zealand since the mid-1970s. Planting the harvested areas caused the water yield from both catchments to return to pre-harvesting levels within 8 yr, and an estimated reduction in runoff of 340 mm within 5 yr at DC4. Mean flood peaks increased after harvesting, especially for small and medium storms on the skidder logged catchment (75-100%). The response of the storm quickflows to harvesting was similar but much more subdued. Low flows also increased after harvesting. Tree growth brought storm peak flows, quickflows, and low flows back to the levels of those in the original beech forest within 10 yr. The second study examined the impact of converting tussock grasslands to pine (Pinus radiata) plantations using data collected from two catchments in the eastern uplands of southern New Zealand. After a 3-year calibration period ( ) one catchment was planted with pines over 67% of its area and the other was left as tussock. By 1989 the difference in annual water yield from the planted catchment was 130 mm, and between 1991 and 1994 it averaged 260 mm (27% of total runoff from the control). Differences in low flows (represented by the minimum annual 7-day mean) showed a similar trend, and suggest that in dry periods, afforestation of tussock grasslands can reduce water yields by 0.18 mm/day. Higher interception losses from increased canopy evaporation was the main reason for the reduction in water yield. After yr of tree growth mean flood peaks had fallen by between 55 and 65%, and quickflows had decreased by approx equal to 50%

The structure of a tree’s canopy causes it to intercept raindrops as they fall, with the broad surfaces of the many layers of leaves and branches either catching rainwater and holding it until it evaporates, or letting it drip slowly to the ground. This provides two primary benefits.   First, it minimizes the erosive effect of the droplets on barren surfaces by decreasing their velocity as they reach the ground.  Second, it lessens the total amount of water that reaches the ground, which reduces the volume of runoff on the ground; this also makes rainwater less erosive. Diunduh dari: ….. 15/11/2012

157 Agricultural and Forest Meteorology. 1997. 85 (3/4). 135-147
. Evaporation from an eastern Siberian larch forest Agricultural and Forest Meteorology (3/4) Kelliher F M., Hollinger D Y., Schulze E D., Vygodskaya N N., Byers J N Hunt J E McSeveny T M., Milukova I., Sogatchev A., Varlargin A., Ziegler W., Arneth A., Bauer G. Total forest evaporation ( lambda E), understorey evaporation, and environmental variables were measured on 9 summer days under different weather conditions in a 130-yr-old stand of Larix gmelinii located 160 km south of Yakutsk in eastern Siberia, Russia. Tree and broadleaved understorey vegetation one-sided leaf area indices were 1.5 and 1.0, respectively. Agreement of lambda E and sensible heat flux (H), both measured by eddy covariance, and the available energy (Ra) was generally good: (H + lambda E ) = 0.83Ra + 9W m-2 with r2=0.92 for 364 half-hour periods and the mean plus or minus 95% confidence limit was 129 plus or minus 17 for (H + lambda E) and 144 plus or minus 19 for Ra. Daily E was mm, less than half of the potential evaporation rate and accounting for 31-50% of Ra, with the lowest percentage on clear days. A perusal of the sparse literature revealed that average daily E of boreal coniferous forest during the tree growing season (1.9 mm day-1 for this study) is relatively conservative, suggesting that low evaporation rates are a feature of this biome's energy balance. Using the Penman-Monteith equation, the maximum bulk-surface conductance (Gsmax) was 10 mm s-1. E and Gs were regulated by irradiance, air saturation deficit, and surface soil water content during a week-long dry period following 20 mm rainfall. From lysimeter measurements, 50% of E emanated from the understorey at a rate proportional to Ra. Based on the measurements and published climatological data, including average annual precipitation of 213 mm, water balance calculations indicated a growing season forest E of 169 mm, the occurrence of a late summer-autumn soil water deficit, and annual runoff of 44 mm by snowmelt.

158 Estimation of transpiration by single trees: comparison of sap flow measurements with a combination equation Agricultural and Forest Meteorology (2/3) Zhang, H., Simmonds L. P., Morison J.I. L., Payne D, . Sap flow estimates for whole trees (scaled from measurements on selected branches using the heat balance method) were compared with estimates of transpiration based on porometry in a study of poplar (Populus trichocarpa x Populus tacamahaca [P. balsamifera]) trees in an agroforestry system in the south of the UK. Sap flow showed good agreement with the transpiration rate estimated using the Penman-Monteith equation with measured stomatal conductance (R2 = 0.886) on six selected days during the season. The dominant environmental variable influencing transpiration was the vapour pressure deficit, as the aerodynamic term in the Penman-Monteith equation accounted for more than 70% of daily total transpiration, with the rest due to the radiation component. Stomatal conductance, estimated by inverting the Penman-Monteith equation from continuous measurements of sap flow over 55 days, was used to determine the parameters for a multiplicative stomatal conductance model. For an independent data set there was better agreement between measured sap flow and transpiration predicted from the stomatal conductance than for calculated and predicted stomatal conductance.

159 Agricultural and Forest Meteorology. 1995. 74 (3/4). 181-193
. Water regime of a pine forest under a Mediterranean climate Agricultural and Forest Meteorology (3/4) Schiller G., Cohen Y. The transpiration (T) of a 14-yr-old Aleppo pine (Pinus halepensis) plantation on Mt. Carmel, at Ramat ha'Nadiv, Israel, was studied during 1 year (autumn 1990 to the end of summer 1991) to evaluate soil water storage and tree response to water stress. The heat pulse method was used for continuous measurement (excluding rainy days) of sap flow in the stems of 8 trees at hourly intervals. Climatic parameters were measured in the forest for computation of potential transpiration (Tp) using the Penman formula. Extrapolating T of sampled trees to stand T was difficult because of a poor correlation between T and stem diameter. During the rainy season the average daily rate of T was linearly related to Tp and the ratio T/Tp was 0.62, independent of Tp rate. After the last rainfall, T/Tp decreased steadily, reaching a value of 0.05 approximately 30 days later; the leaf water potential ( psi l) at sunrise also decreased, from -0.8 to -2.9 MPa. The drop in T and psi l indicates that most of the available water in the main root zone had been extracted during less than 30 days after the last rain. Decreasing values of wood thermal diffusivity with the progress of the dry season show that trees may use internally stored water when soil water stress is intense. The integrated T throughout the measurement period was used for estimation of total water uptake by the stand and for evaluation of the amount of water which was available to the trees following the last rain.

160 Soil-moisture regime :
The changing state of soil moisture through the year, which reflects the changing balance of monthly precipitation and potential evapotranspiration above the ground surface. When the latter exceeds the former the period is one of soil-moisture deficit in the annual regime. The water regime of the soil is determined by the physical properties and arrangement of the soil particles. The pores in a soil determine its water-retention characteristics. When all the pores are full of water, the soil is said to be saturated. (Source: DUNSTE) Diunduh dari: ….. 15/11/2012

161 Soil Moisture Regimes - Descriptions
Soil moisture regimes are defined based on the watertable level and the presence or absence of available water (water that can be used by plants).  All moisture regimes, except aquic, are based on regional climate.  Aquic moisture regimes are based on the length of the period that the soil was saturated.  Soil moisture regimes are used as a soil classification criterion because they affect soil genesis (formation), affect the use and management of soils, and can be used to group soils with similar properties and morphology. The soil moisture regime classes include: Aquic (or Perudic):  Saturated with water long enough to cause oxygen depletion. Udic:  Humid or subhumid climate. Ustic:  Semiarid climate. Aridic (or Torric):  Arid climate. Xeric:  Mediterranean climate (moist, cool winters and dry, warm summers). Management considerations vary based on different moisture regimes.  Soils with an aridic (torris) moisture regime require irrigation to be used for crops. Soils with a ustic moisture regime can grow rain-fed crops, but moisture will be limited during some of the growing season. Soils with a udic moisture regime have sufficient moisture for crops.  Crops may be grown in the udic moisture regime without irrigation, but irrigation is needed for crops in most years in an ustic moisture regime.  Soils with an aquic (perudic) moisture regime need artificial drainage for most cropping practices. Diunduh dari: ….. 15/11/2012

162 Agricultural and Forest Meteorology. 1995. 76 (1). 1-17
Evaporation from bare soil in a temperate humid climate-measurement using micro-lysimeters and time domain reflectometry Agricultural and Forest Meteorology (1). 1-17 Plauborg, F.. Direct measurements of evaporation from a loamy sand in Denmark were made using micro-lysimeters. The accuracy of daily measurement was approx equal to plus or minus 0.5 mm H2O/d. The micro-lysimeter method was not valid in periods with high precipitation. The use of time domain reflectometry (TDR) for measuring soil water content was investigated using a manual interpretation of the trace. The precision of changes in soil water content calculated from daily measurements with TDR was approx equal to 1.3 mm H2O, when using probes of 50 cm length. However, improved precision may be obtained by the use of an automatic interpretation of the trace. Estimates of daily evaporation from bare soil calculated from the water balance equation and measurements of soil water content with TDR were compared with measurements with micro-lysimeters. The TDR technique was suitable for estimating bare soil evaporation when the soil water content was integrated over a 0-50 cm soil profile and drainage had ceased at the lower depths of the profile. Evaporation during a 13 d drying period in spring, just after the soil had been fully rewetted, was approx equal to 26 mm. During a 23 d drying period later in the season the evaporation from the bare soil was approx equal to 30 mm. In both periods the accumulated evaporation was rather high and equivalent to approx equal to 65 and 50% of the accumulated potential evapotranspiration in the first and second drying period, resp., even though the soil water content in the 0-50 cm profile was well below field capacity at the beginning of the second drying period.

163 Cienciala E., Eckersten H., Lindroth A., Hallgren J.E..
Simulated and measured water uptake by Picea abies under non-limiting soil water conditions Agricultural and Forest Meteorology (1/2) CD Volume:15 Cienciala E., Eckersten H., Lindroth A., Hallgren J.E.. A one-dimensional non-steady-state soil-plant-atmosphere continuum (SPAC) model was applied to a stand of Picea abies trees in southern Sweden. The simulated root water uptake was compared with measured sap flow under non-limiting soil water conditions. Sap flow was measured during the growing season using the tree-trunk heat balance method. The model included four resistances against water (soil-root, plant, stomatal and aerodynamic), one pool of easily available plant water and one compartment of intercepted water on the needle surface. The bulk stomatal resistance was estimated as the product of the combined effect of two independent variables - vapour pressure deficit and solar radiation. Good agreement between simulated and 'measured' water uptake was obtained both on short- and long-term scales, and the model explained 92-93% of the variation of measured uptake for both hourly and daily values. The pool of easily available water was found to be small (0.5 mm), i.e. of the same order as for agricultural crops.

164 Agricultural and Forest Meteorology. 1993. 64 (3/4). 210-221
Radiation balance, transpiration and photosynthesis of an isolated tree Agricultural and Forest Meteorology (3/4) Green S. R. . Radiation balance of an isolated walnut [Juglans nigra] tree was measured using an experimental Whirligig device. The total amount of all-wave radiation absorbed by the tree canopy was used to estimate transpiration rates using a Penman-Monteith model. Results compared favourably with tree water use, measured by the heat-pulse technique. Total amount of photosynthetically active radiation (PAR) absorbed by the tree canopy was combined with a photosynthetic light response curve to estimate net photosynthesis rates. Results compare favourably with published data from other tree canopies. Daily energy balance calculations showed that on average, about two-thirds of total radiant energy absorbed by the tree canopy was dissipated as latent heat in the form of transpiration. Dominant environmental variable influencing transpiration was vapour pressure deficit of air. Almost two-thirds of net latent heat flux was attributable to vapour pressure deficit component, with the remainder owing to the radiation component. Daily transpiration-assimilation ratios varied from day to day in response to changing environmental conditions, but generally decreased with increasing net photosynthesis and with increasing transpiration. This appears to be the first time that such a direct measurement of energy balance and photosynthesis of a single tree has been made.

165 . Effect of planting methods and soil moisture on cassava performance in the semi-arid Sudan savanna belt of Nigeria African Crop Science Journal (1) Okogbenin E., Ekanayake I J., Porto M. C. M. The effects of planting methods and soil moisture on cassava (Manihot esculenta) performance in the Sudan savanna region of Nigeria were assessed under field conditions on Eutric Regosols at the International Institute of Tropical Agriculture (IITA) in Minjibir, Kano State. Six planting methods in monoculture were evaluated in two crop seasons. These were horizontal planting on furrows or ridges, inclined planting on flat land or ridges, and vertical planting on flat land or ridges. Two genotypes were compared: TMS 91934, an improved IITA clone; and Dakata Uwariya, a land race. Dakata Uwariya was significantly better than TMS in plant height and root dry matter content; TMS was better in leaf formation and leaf retention. Ridge-based methods positively influenced root yield production and leaf formation, while flat or furrow methods were advantageous in terms of the number of plants at harvesting. Horizontal and inclined planting were the best methods in general.

166 Water use and soil moisture depletion by olive trees under different irrigation conditions
Agricultural Water Management (3) Michelakis N., Vouyoukalou E., Clapaki G, . 12-year-old olive cv. Kalamon trees irrigated at soil water potential ranges of to MPa ( PSI 0.2) and -1 to -1.5 MPa ( PSI 15) were compared with non-irrigated trees in Crete, Greece. Irrigation water was applied by two drip laterals per row (DR2) or a microtube-fed basin at each tree (BAS) for PSI 0.2 and one drip lateral per row (DR1) or a microtube-fed basin at each tree for PSI 15. Soil moisture depletion in the non-wetted areas of the irrigated treatments was greater than that of the wetted ones, but about the same as the non-irrigated treatment. Soil moisture depletion in the non-wetted areas of the irrigated and non-irrigated treatments occurred during May-June. Soil moisture reached wilting percentage at the end of September in the non-wetted areas of the PSI 0.2, at the end of August in the non-wetted areas of PSI 15 and at the end of July in the non-irrigated treatment. Total soil moisture reserve depletion was greater in the upper soil layers of the wetted and non-wetted areas. The amount of irrigation water used was higher at the PSI 0.2 than at the PSI 15 treatment, but it was not noticeably different between the drip and basin irrigation methods within the same soil water potential levels. Evapotranspiration of irrigated trees during May-September period was met mainly from irrigation water (75-90%), 10-20% from the reserves of soil moisture and 2.5-5% from rainfall. Crop coefficient, expressed as an evapotranspiration to class 'A' pan evaporation ratio, increased from 0.4 to 0.65 for the PSI 0.2, maintained at approx equal to 0.3 to 0.4 for the PSI 15 and decreased from 0.2 to 0.05 for the non-irrigated treatment. Crop coefficients were not markedly different between drip and basin methods within the same soil water potential levels.

167 Improving water-use efficiency
Conservation of water Reduce conveyance losses by lining channels or, preferably, by using closed conduits. Reduce direct evaporation during irrigation by avoiding midday sprinkling. Minimize foliar interception by under-canopy, rather than by overhead sprinkling. Reduce runoff and percolation losses due to overirrigation. Reduce evaporation from bare soil by mulch-ing and by keeping the inter-row strips dry. Reduce transpiration by weeds, keeping the inter-row strips dry and applying weed control measures where needed. Enhancement of crop growth Select most suitable and marketable crops for the region. Use optimal timing for planting and harvesting. Use optimal tillage (avoid excessive cultivation). Use appropriate insect, parasite and disease control. Apply manures and green manures where possible and fertilize effectively (preferably by injecting the necessary nutrients into the irrigation water). Practise soil conservation for long-term sustainability. Avoid progressive salinization by mon-itoring water-table elevation and early signs of salt accumulation, and by appropriate drainage. Irrigate at high frequency and in the exact amounts needed to prevent water deficits, taking account of weather conditions and crop growth stage. Diunduh dari: /11/2012

168 Soil moisture relations at the tree/crop interface in black locust alleys
Agroforestry Systems (2) Ssekabembe C. K., Henderlong P. R., Larson M. . A study was undertaken at the Ohio State Agricultural Farm to determine whether the presence of black locust (Robinia pseudoacacia) hedgerows would increase water shortage on crop land. Water was applied to bare soil which had carried a pure stand of maize in the previous growing season, and to previously established (2 yr old) alley cropping plots, some of which had 100 cm deep below-ground fibreglass partitions to prevent root competition for soil moisture in the alleys. Spacing within hedgerows was 1.5 m and alleys were 5.5 m wide. No crops were grown during the course of the experiment and all plants (weeds) other than the trees were removed. In alleys without below-ground partitions, the hedgerows reduced soil moisture content of the alleys by about 8% at site 1, and 32% at site 2, 8 days after water application. In the top 45 cm depth of the soil at site 1, the influence of the hedgerows in the same treatment was large within 76 cm of the hedgerows but declined farther inside the alleys. For the soil at site 2 (where there was more gravel in the lower soil layers preventing deep growth of black locust roots), the influence of the hedgerows was pronounced throughout the alleys but was also most marked within 76 cm of the hedges.

169 Agroforestry Systems. 1994. 26 (2). 89-99
Pattern of soil moisture depletion in alley cropping under semiarid conditions in Zambia Agroforestry Systems (2) Chirwa P W., Nair P K R., Nkedi Kizza P.. The pattern of soil moisture changes was studied during a cropping season in an alley cropping experiment with maize (var. MM603) and 2 hedgerow species (Leucaena leucocephala and Flemingia macrophylla) at the SADC/ICRAF Agroforestry Research Station (Chalimbana Agricultural Research Station) in a semiarid region near Lusaka, Zambia (28 deg 29'56"E. and 15 deg 21'32"S.). Factorial combinations of the 2 hedgerow species and 2 fertilizer rates (none, and 150 kg/ha urea applied as a split dose + a basal dose of diammonium phosphate at 200 kg/ha) formed the main plots of a split-plot experiment, and the maize rows formed the subplots. Each treatment unit consisted of 2 double hedgerows with 6 rows of maize between them and 3 on each side. Maize was planted at within- and between-row spacings of 25 and 75 cm. Soil moisture potential was monitored at regular intervals (daily after every major rainfall event for 4-5 days, and then less frequently) over the maize growing season (0-90 days after planting) using tensiometers installed at 15, 30 and 45 cm depths in fertilized and unfertilized alleys within the double hedgerow, and in the first, second and third rows of maize in the alleys. Soil moisture moved mostly towards the top horizon during very dry conditions. Alleys that had received a combination of fertilizer and hedgerow prunings depleted more moisture than those that had received only prunings. There were no differences in moisture utilization patterns between leucaena and flemingia hedgerows. The hedgerows depleted the same amount of moisture as the maize plants. However, during dry conditions, there was a higher soil moisture content under the hedgerows than in maize rows, indicating that there was no apparent competition for moisture between the hedgerows and the maize plants.

170 Root distribution and soil moisture depletion pattern of a drought-resistant soybean plant introduction Agronomy Journal (3) Hudak C. M., Patterson R. P. . The drought-resistant soyabean genotype PI was compared with cv. Forrest for soil moisture utilization and root distribution on a Dothan loamy sand (fine-loamy, siliceous, thermic Plinthic Paleudult) in Johnston County, North Carolina. Water treatments (well-watered and water-deficient) were established during R4 to late R6 with rain exclusion shelters and differential irrigation. Soil moisture utilization in row and interrow areas was monitored via neutron attenuation and tensiometry. A direct measurement of root distribution was obtained on a second site by a trench profile technique. It is suggested that PI 's advantage may reside in its ability to exploit upper soil horizons (above 68 cm) with a network of fibrous roots. Localized measurements of soil moisture tension indicated that PI 's rate of soil desiccation was slower than Forrest's, but it appeared to exploit a larger total soil volume. Trench profile results indicated that the lateral spread of PI 's root system was greater than that of Forrest. The usefulness of PI may lie in the opportunity it provides soyabean breeders for adding diversity to the root morphology of present soyabean cultivars.

171 . A weather-soil variable for estimating soil moisture stress and corn yield probabilities
Agronomy Journal (6) Dale R. F., Daniels J. A. The SIMBAL (simulation of soil water balance) program was used to calculate the soil moisture under maize and the ratio of actual to potential evapotranspiration (ET/PET) for each day of the growing seasons for poorly drained and well-drained soils in Tippecanoe County, Indiana. The interaction regression model of maize yield on the soil moisture stress variable sc (the sum of modelled daily ET/PET ratios) and technology trend (T = year, average annual yield increase due to improving agricultural technology) to Tippecanoe County was associated with 70% of the variance in the average county maize yields when Sc was a 90-d period (S90) from 39 d before silking to 50 d after. With no moisture stress (S90 = 90), the technology trend over the last 32 years was 0.17 t ha-1 year-1 (2.7 bu acre-1 year-1). With 1992 technology, each deficit unit of S90 reduced the yield 0.19 t ha-1 (3.1 bu acre-1). The distributions of S90 and predicted maize yield were highly negatively skewed. The probability of having an S90 less than 85 (at least some moisture stress), and a county maize yield less than 9.5 t ha-1 (152 bu acre-1) is 69%, but the probability of severe stress (S90 <75) and maize yield less than 7.5 plus or minus 0.8 t ha-1 (139 plus or minus 13 bu acre-1) is 22%. For the same weather regime, the probability of moisture stress and resulting maize yields differs greatly for individual soils. For a poorly drained soil (Typic Argiaquoll) the probability of have an S90 less than 85 is 41%, but for a well-drained soil (Typic Argiudoll) the probability is 90%.

172 . Fine-root dynamics, soil moisture and soil carbon content in a Eucalyptus globulus plantation under different irrigation and fertilisation regimes Forest Ecology and Management (1/3). 1-12 Katterer T., Fabiao A., Madeira M Ribeiro C., Steen E. The minirhizotron technique was used to study the temporal dynamics of fine roots over a 10 month period in 1991 in a Eucalyptus globulus plantation (established with 3-yr-old plants in 1986) in central Portugal. Four treatments were applied: a control without irrigation or fertilizer application (C), NPK fertilizer twice per year (F), irrigated without fertilizer (I), and irrigated with fertilizer once each week, with fertilizer in the irrigation water (IL). In I and IL a drip-tube system was used, and fertilizer rates were adjusted based on the estimated plant nutrient demand. Soil moisture content was measured during the same period at 5 cm depth intervals down to 90 cm depth. Soil carbon content was measured at planting, 30 months and 54 months after planting. Fine-root counts peaked in late autumn in all treatments and declined thereafter until March. Fine-root growth in spring and summer seemed to be dependent on water supply; i.e. with an ample water supply (within rows, close to the drip-tubes in I and IL), root counts increased almost linearly between April and November. In the non-irrigated treatments (C and F, as well as between rows in I and IL), no marked increase in root counts occurred until late August, when it increased immediately after a heavy rain. Root growth in I was shallowest during spring and summer, while in F it was shallowest during autumn and winter. In general, treatment means of root counts were highest in IL, somewhat lower in I, and considerably lower in C and F. In addition to irrigation effects, treatment differences in soil water content were enhanced by differences in soil carbon content, which in turn could be attributed to root turnover, as reflected by the temporal dynamics of root counts. The carbon flow from the trees to the soil, which was probably associated mainly with root death, was highest in IL. Thus this treatment should have enhanced soil fertility.

173 . Water absorption by roots
Intimate contact between the surface of root and the soil is essential for effective water absorption. Root hairs are filamentous outgrowths of root epidermal cells that greatly increase the surface area of the root, thus providing greater capacity for absorption of ions and water from the soil. Water enters the root most readily near the root tip. The intimate contact between the soil and the root surface is easily ruptured when the soil is disturbed. It is for this reason that newly transplanted seedlings and plants need to be protected from water loss for the first few days after transplantation. Root hairs intimate contact with soil particles and greatly amplify the surface area used for water absorption by the plant (source: Taiz L., Zeiger E., 2010) Diunduh dari: /11/2012

174 . Comparing different methods for estimating the soil moisture supply capacity of a soil series subjected to different types of management Geoderma (3/4) Bouma J., Droogers P The capacity of soils to supply water to growing plants was expressed in a static and dynamic manner by calculating 'available water' (AW) and the 'soil moisture supply capacity' (MSC), respectively. Four methods of increasing complexity were compared for the AW and two for the MSC, using measured moisture retention and hydraulic conductivity curves and data derived from class- and continuous pedotransfer functions. Calculations were made for a Dutch Typic Fluvaquent (genoform). Three phenoforms, defined by long-duration management, were distinguished: BIO (biodynamic); CONV (conventional, high-tech) and PERM (permanent grassland). Values for AW for a given treatment were significantly different when using measured data or pedotransfer functions. AW can be used to rank different soils but does not reflect the amount of water the crop can take up. Simulation models are needed to estimate MSC which cannot directly be measured. Simple empirical models, using AW for the root zone in a 'tipping-bucket' approach, did not produce realistic values because upward flow from the relatively shallow water table could not be distinguished. The more complex mechanistic WAVE model, also including hydraulic conductivity data, but still operating under the implicit and incorrect assumption of soil homogeneity and isotropy, produced soil water contents that were too high. Realistic values were only obtained when considering bypass flow, internal catchment and accessibility of peds for rooting, using a modified WAVE model incorporating hydraulic data derived from continuous pedotransfer functions. Values for AW and MSC were significantly different for the three phenoforms, illustrating the need to distinguish phenoforms, rather than only genoforms, when reporting basic physical data for soil series

175 Development of a rainwater harvesting system for increasing soil moisture in arid rangelands of Pakistan Journal of Arid Environments (4) Suleman S., Wood M K., Shah B H., Murray L. . Micro catchments 4-5 m long with 7-15% slopes increased soil moisture by 59, 63 and 80% at depths of 0-15, 15-30, and cm, resp. Soil moisture increased in late summer and in late winter when precipitation is greatest. Rill erosion increased with microcatchment length and gradient, with erosion volumes of litres from areas of 120 and 150 m2.

176 To increase RAINWATER capture and storage in soil:
PANEN AIR BHUJAN DAN SIMPAN DALAM TANAH To increase RAINWATER capture and storage in soil: Build up soil organic matter and feed soil regularly Break up hardpans, open up subsoil Deep dig gardens and incorporate compost Mulch or cover crop over winter Mulch between rows On slopes, plant along contours Practice water harvesting (e.g. by making water basins around plants, hilling between rows) Apply water below the canopy and close to plants Apply more water at fewer times rather than a little water frequently, Allow more drying down as deep rooting crops grow deeper Plant shelterbelts in windy landscapes . Diunduh dari: /11/2012

177 WATER BALANCE IN SOIL Diunduh dari: ….. 15/11/2012

178 TEKSTUR TANAH & LENGAS Diunduh dari: ….. 15/11/2012

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