1. Water Management Systems
Advanced Vegetable Crops Management
HS 590 E
This presentation describes a plot scale subsurface drip irrigation system intended to determine water distribution and lateral movement in a clay loam, and to establish feasible drip line spacing and depth for corn. Results obtained for the 2000 growing season and conclusions are presented.

2. Overview
Soil-Water Relationships
Water Demand
Water Supply

3. Soil Water Holding Capacity
Amount or volume of water existing in soil pore space
Factors
Soil Texture (particle size)
Particle shape/orientation

4. Water Holding Capacity2 1
Which has greater porosity?

5. Water Holding Capacity2 1
Porosity = 47.6%
Porosity = 47.6%
Porosity is more than just particle size. It also is related to pore size, particle orientation, and aggregation of particles.
It is related to bulk density by (1-(db/dp))*100 where db is bulk density and dp is particle density, usually taken to be 2.65 g/cc

6. General Relationships
As can be seen, and using the equation found in the notes of the previous slide, porosity is inversely proportional to bulk density.

7. Capillarity Sand Clay Weight Surface Tension of Water Water
A smaller tube creates a greater rise in a water column as surface tension forces (cohesion and adhesion) play a greater role relative to gravitational forces. Water does not rise as high in a tube of greater diameter since the mass of water (which gravity acts upon) at a given height is greater.
Water

8. Water Holding Capacity
Finer textured soils hold more water then coarser textured soils (field capacity)
Due to lower bulk density (greater total void space) and smaller pore size (holds more water against gravity, or free drainage), finer textured soils hold more water than coarser textured soils. This does not necessarily mean that the finest textured soils have more plant available water.

9. Water Holding Capacity
Gravitational Water - drains by gravity (saturated conditions)
Field Capacity (1-3 day drainage)
Wilting Point
Plant Available Water (PAW)
Field Capacity - Wilting Point
Gravitational water is the water than drains form a soil freely and is sometimes called “free drainage”.
Field capacity is normally considered to be the water holding capacity of a soil after it has drained for 1-3 days. The resulting tension or matric potential in the soil is normally about -0.1 to bars (10 to 33 centibars of tension). For sandy soils, field capacity may be attained in less than one day of drainage and may occur at a tension of less than 10 centibars.
The permanent wilting point is related to a soil moisture tension or “dryness” at which the plant cannot recover. The tension associated with permanent wilting point is -15 bars (1500 centibars of tension). There is still water in the soil at the permanent wilting point, however it in unavailable to the plant
Plant available water sometimes called available water holding capacity, is the amount of water between field capacity and the permanent wilting point. This amount is normally expressed as inches of water per inch or soil, or inches of water per foot of soil, or by percent water by volume

10. Soil Reservoir (“bank account”)
When managing water for crop production, the soil can be be thought of as a reservoir in which water is stored. This “reservoir” is empty at the permanent wilting point (PWP) and and full at field capacity. There is a point above the PWP at which the plant will become stressed. This varies by plant. To avoid stress, and therefore lower yield or quality, irrigation is scheduled to occur at a point above the PWP. For many crops, this is a level when about 50% of the plant available water (PAW) is gone. The level at which irrigation is planned is called the management allowed depletion or MAD.
Example: PAW = 2 inches/foot ; effective root zone is 1 foot MAD=50%
Irrigate at 2 in/ft x 0.5 x 1ft = 1 inch of depletion. Schedule a net irrigation of 1 inch to refill the root zone.
Full
Field Capacity
PAW
PAW = Root depth x WHC/ft
Irrigate at PAW x Management Allowed Depletion, MAD
MAD
Stress
Wilting Point
Empty

11. Plant Available Water
In general plant available water increases when decreasing soil particle size, however a medium textured soil, such as a loam can hold more water than a clay. Although clay will hold more water at field capacity, it will also have more water at the permanent wilting point, and the net water available to the plant will be greater in the loam.

12. Measuring Soil Moisture
“Feel” Method
Gravimetric
Tensiometer
Electrical Resistance
Other Devices
Checkbook
There are several “direct” and “indirect” ways of measuring soil moisture. We will talk about these two categories later.

13. Feel Method Subjective Relate to physical characteristics
Use push probe
Advantages of this method are that it is cheap and adaptable, i.e, that you can sample from any site
The disadvantage is that it is subjective, which can especially create problems when more than one person is estimating soil moisture for the same site.

14. Gravimetric Method
Field sample weighed moist-oven dried and re-weighed
Good for calibrating other methods to particular soil
This is a very accurate way to determine soil moisture, however it is impractical for use in irrigation scheduling due to the time required to obtain an answer. It is, however, a standard measure by which other methods are calibrated.
Soil moisture by weight (Pw)= ((weight wet soil-weight dry soil)/weight dry soil)*100
Soil moisture by volume (Pv) = Pw x bulk density of soil
Soil moisture expressed as inches per foot of soil = Pv x 12

15. Tensiometers Measures soil-water suction (vacuum) Easy to use
Suitable for medium to coarse soils
0-80 centibars (kilopascals) of tension
Tensiometers are sometimes referred to as dummy roots. There advantages are they are inexpensive, and can be permanently installed at various depths. Their disadvantage is that they operate over a limited range of soil tension

16. Electrical Resistance Blocks
Gypsum blocks
up to -15 bars (wilting point)
Watermarks
0-200 centibars (2.0 bars)
Resistance blocks can operate to a greater tension than tensiometers. However, they require a meter to read them.

17. Soil Moisture Measurement
Neutron Probes
Time Domain Reflectometry (TDR)
Capacitance Probes
Other methods, which are more expensive but can be highly accurate are available.

18. Calculating Soil Moisture
Gravimetric, Neutron, TDR, etc. measure total soil moisture
Rule of thumb is that plant available water (FC - WP) is one half this amount at field capacity
Most often expressed as inches of water per foot of soil (in/ft) = %vol * 12in/ft
e.g. 33%/100 x 12 = 4 inches
Remember that total soil water is not the same measure as plant available water. Total soil moisture does not account for that portion of water unavailable to the plant

19. Moisture Release Curve
Silty Clay
Clay
Sandy Loam
Clay Loam
Moisture release curves are developed to related water content to tension. The reflect the water yielded by the soil as an increasing tension is applied.
Sand
Coarse Sand

20. Moisture Release Curve
Silty Clay FC
Clay
( ) x 12 in/ft = 1.32 in/ft x 0.5 = 0.66
Sandy Loam
Clay Loam
The above graph illustrates how PAW can be obtained from a moisture release curve. Projecting the curve at 15 bars of tension (PWP) to the y-axis one can obtain the percent moisture by volume at PWP. Likewise projecting from the curve at field capacity ~ .10 bars for the sand and ~0.3 bars for the clay loam, percent moisture by volume is obtained at field capacity. The difference can be converted to inches per foot then multiplied by the management allowable depletion (MAD) to obtain the soil moisture deficit at which irrigation should occur.
Using the graph above, we can find the soil tension at which to irrigate.
For the sandy soil, at a MAD of 50%, when Pv = 0.05 or 5%, the soil tension is at about 0.5 bar or 50 cbars. In reality a sandy soil may be irrigated at less of a tension
The clay loam is at 16.5% water at MAD which correlates to a tension of about 1.5 bars. Note that a tensiometer should not be used to schedule irrigations in this type of soil. WP FC
( ) x 12 in/ft = 0.72 in/ft x 0.5 = 0.36
Sand
Coarse Sand WP FC
PWP

21. Implications Coarse Soils and/or shallow rooted crops
Heavy Soils/ and or deep rooted crops
Frequent, shallow irrigations
Infrequent, deep irrigations

22. Infiltration
In general, infiltration rates decrease with decreasing particle size

23. The graph above illustrates the change of infiltration rate over time
The graph above illustrates the change of infiltration rate over time. Infiltration rates decrease over time as the soil becomes saturated. The level at which the curve flattens out is referred to as the “basic infiltration rate”. Knowledge of infiltration rates are important when designing irrigation systems, especially surface irrigation and sprinkler irrigation.

24. Water Demand

25. Evapotranspiration
Sum of evaporation from the soil plus transpiration, the loss of water from plants in the form of water vapor
Most water used by a plant is used to cool the plant. This is called transpiration. A very small percentage of the water used by a plant goes into maintaining cell rigidity and transporting nutrients.

26. Evaptranspiration (ET)
Temperature 
Cloudiness 
Humidity 
Wind 
ET 
ET 
Evapotranspiration is controlled by environmental demands. The only time a plant controls evapotranspiration is when the plant becomes stressed enough that the guard cells around the stomata start to close.

27. Evapotranspiration “Direct” Methods Indirect or Estimation Methods
Measure soil water (water balance)
Lysimeters
Indirect or Estimation Methods
Direct methods involve the actual measurement of soil water, such as the gravimetric method or using a lysimeter. A lysimeter normally determines water content by weighing the block of soil it contains and tracking the change in weight.
Indirect methods, as the name implies, do not measure soil water directly, but rather measure another property of the soil, such as tension (tensiometer), electrical resistance (soil blocks), hydrogen atoms (neutron probes), or soil capacitance or dialectric constants (Capacitance probes and TDR respectively).
ET then can be determined by a water balance equation, accounting for the change in soil moisture and any other contributors to water balance such as rainfall, irrigation, or drainage.

28. Reference Evapotranspiration, ET0
Evapotranspiration of a well-watered clipped grass
Standard method becoming Penman-Monteith Equation
One one to determine ET is to estimate it using equations and weather data.

29. Reference Evapotranspiration, ET0
ET0 = kp Epan
Kc can be obtained from tables or can be calibrated to local conditions with the PM or with lysimeter data or checkbook with measured soil moisture.

30. Crop Coefficient, Kc
Adjusts reference evapotranspiration to particular crop
Reflects stage of crop
Can be less or more than ET0
Must be related to ET0 method used
It is important to use a crop coefficient that was developed for the equation that you use.

31. Tomato Small vegetables
Crop curves vary by crop and reflect plant stage. The adjust a particular crop’s water use from the reference crop, e.g. a well watered cool season grass, or alfalfa.

32. Crop Evapotranspiration
Crop Evapotranspiration is the reference evapotranspiration adjusted by the crop coefficient….or
ETc = kc x ET0

33. Evapotranspiration varies not only by crop, but when the crop is planted. This is due to the differing environmental demands at different times of the year, and also by differing rates of plant development.

34. Crop Irrigation Requirement
Water Required by Crop less that provided by rain
CIR = ETC- ppteff
The water required from irrigation over the season should take into account the amount supplied from rainfall.

35. Effective Precipitation
The amount of rainfall that can be used by the crop
Not all rainfall is “effective”. Some may be intercepted by foliage then evaporated, and some may runoff the field or percolate beneath the root zone.

36. Effective Precipitation
subtract interception by plant canopy
subtract intense rainfall that runs off
subtract rainfall above that which can be stored in root zone

37. Note that the above graph shows a lower CIR than ET
Note that the above graph shows a lower CIR than ET. When summing the CIR over the season this would be appropriate in planning a water supply. However, this plot assumes an even distribution of monthly rainfall, which in actuality will not occur.
When designing an irrigation system in most cases, rainfall should not be considered as you need to design for the peak demand, e., g. a one week period during which you may very likely not receive rain.

38. CIR in System Design
CIR more important for considering total water supply (whole season) than for designing system (peak use rate)

39. Seasonal Irrigation Requirement
Definition:
Example:
Annual Crop Irrigation Demand (depth) times irrigated area (area) = volume
Tomato CIR = 10 inches; 10 acres irrigated;
Seasonal irrigation water requirement= 10 inches x 10 acres = 100 acre-inches x 27,152 gallons/ac-in = 2.7 M gallons = 8.3 acre-feet
Seasonal irrigation requirement is used in planning water supplies. The supply can either be obtained from storage such as a pond or an aquifer, or by “live” flows such as a stream.

40. Irrigation System Design
Typically design for 80% exceedence (low 20% rainfall)
For drought sensitive crop, look at probability for 1 week interval during peak period
Rule of thumb…design for no rainfall

41. Irrigation System Requirements small vegetables early
Peak Use Rate inches per day- no rainfall
0.25 inches per day x 27,150 gallons/acre-inch /1440 minute per day
=4.7 gpm/acre
4.71 gpm/acre /.75 (efficiency = 75%) = 6.3 gpm/acre
6.3/acre x 24 hours/12 hours operating = 12.6 gpm
When sizing a pumping plant, the peak use rate (crop demand) along with operating time and efficiency is used in calculating the pumping plant capacity. This required pumping plant rate is often expressed in gallons per minute per acre.

42. Water Supply
Quantity
Quality
Location
Accessibility

43. Water Supply Volume Rate (V/t)
Two important factors when considering water supplies for irrigation are volume or storage and the flow rate at which is can be withdrawn. For instance, an aquifer may have plenty of water but if the transmissivity is low, it may not be able to irrigate a field of any size.

44. Qt = Ad Vol/t *t = Vol Supply Application
Supply must be adequate for the demand. The gross depth or irrigation on a given area is related to a supply flow rate and time by the above equation. A conversion factor is applied when using specific units.
Qt = Ad
Vol/t *t = Vol

45. Qt = Ad Vol = Vol Water Field Supply Sprinkler Pumping Plant Main
Lateral
Main
Submain
Pumping
Plant
Sprinkler
Qt = Ad
Vol = Vol
Water
Supply
Field

46. Qt = Ad Vol = Vol Zone - to match system application rate Water Field
Lateral
Main
Submain
Pumping
Plant
Sprinkler
Zone - to match system application rate
When sizing a pumping plant, the peak use rate (crop demand) along with operating time and efficiency is used in calculating the pumping plant capacity. This required pumping plant rate is often expressed in gallons per minute per acre.
A given irrigation system most likely will require a flow rate that is greater than this amount. The field is therefore zoned to match the pumping rate to the system requirement.
Sizing a pump to irrigate the the entire field at one time will result in an overdesigned system with associated higher costs.
Qt = Ad
Vol = Vol
Water
Supply
Field

47. Frost Protection Greater System requirements
All field needs to be covered
0.1 in/hr (45 gpm/acre) to 0.15 in/hr (65 gpm/acre)
Greater pressure requirements, psi
smaller droplets/ better coverage
Oftentimes greater (up to 120 gpm/acre) depending upon nozzle selection and spacing

48. Efficiency System Uniformity Management Environment
Efficiency (or inefficiency) needs to be considered when determining an irrigation water supply. Understanding the sources of inefficiencies can help you manage the irrigation system or design the irrigation system for greater efficiency.
Inefficiency can be “partitioned” into systems, management and environmental factors.
System efficiencies relate to distribution uniformity, management efficiencies relate to how the system is operated, and the efficiency due to the environment relates to drift and evaporation.

49. Efficiency
Application efficiency = Volume (Depth) of Water Applied/Volume (Depth) of Water in Root zone
Distribution Uniformity (DU)=Average depth received in low-quarter of field/average depth received
Definitions may vary. Note that you can have an application efficiency of %100 and with a deficit irrigation.

50. Causes of non-uniformity
System
Variation in Pressure
Variation in manufacture
Spacing (for sprinkler systems)
Environment
Wind, evaporation, interception by canopy
Operation/Management/Maintenance
Deep percolation, runoff

51. Irrigation System Efficiencies1
1 From Allen, Irrigation Engineering Principles

52. Irrigation System Application Rates
It is important to match the system application rates to the soil infiltration rates such that runoff does not occur. The exception to this would be surface irrigation such as furrow where there usually will be runoff (tailwater).

53. Irrigation System Costs