DRAINAGE AND IRRIGATION ENGINEERING

DRAINAGE AND IRRIGATION ENGINEERING
INTRODUCTION

COURSE GOALS This course has two specific goals:
(i) To introduce students to basic concepts of soil, water, plants, their interactions, as well as irrigation and drainage systems design, planning and management. (ii) To develop analytical skills relevant to the areas mentioned in (i) above, particularly the design of irrigation and drainage projects.

Course Outline Basic Soil-Plant-Water Relations.
Irrigation Water Requirements, Sources, quantity and quality of irrigation water: Irrigation planning, scheduling and efficiencies. Design of irrigation systems and structures. Design of drainage systems and structures. Computer applications and simulation modeling.

Course Objectives On Completion of this course, students should be able to: (i) Understand the basic soil-plant-water parameters related to irrigation (ii) Understand how to estimate the quantity of water required by crops using manual and computer methods. (iii) Be able to plan and design irrigation and drainage projects.

COURSE OBJECTIVES CONTD.
(iv) Understand the computer applications in irrigation and drainage designs. (v) Design channels and other irrigation structures required for irrigation, drainage, soil conservation, flood control and other water-management projects.

Course Assessment (i) One (1) mid-semester test, 1-hour duration counting for 10% of the total course. (ii)One (1) Project Assignment counting for 10%. One (1) end-of-semester examination, 2 hours duration counting for 80% of the total course marks.

Reading Materials (i) James, L.G. (1988). Principles of Farm Irrigation System Design. John Wiley, New York. (ii) Chin, D.A.. (2000). Water Resources Engineering, Prentice Hall, New Jersey. (iii) Journal of Irrigation and Drainage Engineering, American Society of Civil Engineers. (iv) Course comprehensive note book and other handouts and tutorial sheets.

ME31D: DRAINAGE AND IRRIGATION ENGINEERING
1.1. Irrigation is the application of water to the soil to supplement natural precipitation and provide an environment that is optimum for crop production.

TYPES OF IRRIGATION SUPPLEMENTARY IRRIGATION: IN AREAS WITH RAINFALL FOR A PART OF THE SEASON OR YEAR TOTAL IRRIGATION: IN AREAS OF NO RAINFALL

OBJECTIVES OF IRRIGATION
To Supply Water Partially or Totally for Crop Need To Cool both the Soil and the Plant To Leach Excess Salts To improve Groundwater storage To Facilitate continuous cropping To Enhance Fertilizer Application- Fertigation

To Understand Irrigation, One Needs Knowledge of:
Basic Soil Science/Physics Plants Water Plant/Soil/Water Relations Hydraulics Hydrology General Engineering Principles

1.2 SOIL CONSTITUENTS Mineral Material: Sand, clay and silt
Organic matter - very valuable Water Air

PROPORTIONS OF SOIL CONSTITUENTS

MINERAL COMPONENTS Except in the case of organic soils, most of a soil’s solid framework consists of mineral particles. They are variable in size and composition. They can vary from small rock particles to colloids.

MINERAL COMPONENT CONTD.
The mineral can be raw quartz and other primary materials – coarse fractions which have not changed from parent material) They can also be silicate clays and iron oxides formed by the breakdown and weathering of less resistant minerals as soil formation progressed. These are called secondary minerals.

MINERAL CONSTITUENTS USDA ISSS ROCKS > 2 mm SAND 0.05 to 2 mm
SILT 0.002 to 0.05 mm 0.002 to 0.02 mm CLAY < mm

SAND COMPONENT Visible to the Naked Eye and Vary in Size.
They are Gritty when rubbed between Fingers. Sand Particles do not Adhere to one another and are therefore not Sticky.

SILT AND CLAY COMPONENTS
Silt Particles are smaller than sand. The silt particles are too small to be seen without a microscope. It feels smooth but not sticky, even when wet. Clays are the smallest class of mineral particles. They adhere together to form a sticky mass when wet and form hard clods when dry.

SOIL TEXTURE Relative proportions of the various soil separates (sand, silt and clay) in a soil. Terms such as sandy loam, silty clay, and clay loam are used to identify soil texture. Soil Components are separated using Mechanical Analysis, Sieving for Sand and Rate of Settling in Pipette for Silt and Clay.

SOIL TEXTURE CONTD. From the mechanical analysis, the proportions of sand, silt and clay are obtained. The actual soil texture is determined using the Soil Textural Triangle e.g. for a Soil with 50% sand, 20% silt and 30% clay, the texture is Sandy Clay Loam. Arranged in the increasing order of heaviness, there are 12 soil textures namely: sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, silty clay loam, clay loam, sandy clay, silty clay and clay.

1.4 COLLOIDAL MATERIAL The smaller particles (< mm) of clay and similar sized organic particles) have colloidal properties and can be seen with an electronic microscope. The colloidal particles have a very large area per unit weight so there are enough surface charges to which water and ions can be attracted. These charges make them adhere together. Humus improves the water holding capacity of the soil.

1.5 WATER Quantity of water in a soil as determined by its moisture content does not give a true indication of the soil ‘wetness’. A clay soil, which on handling feels dry, can be at the same moisture content as a sandy soil, which feels wet. A plant will have less difficulty extracting water from a sandy soil than from a clay soil at the same moisture content.

SOIL WATER CONTD. There is need for a soil ‘wetness’ which reflects the ease or difficulty of extraction of water from the soil by the plant. The Concept of Soil Water Potential is therefore used in Soil/Plant/Water Relations

1.5.1. Mechanism of Soil Water Movement
The flow of water in any hydraulic system, including the soil-plant-water system, takes place from a state of higher to one of lower potential energy. The steepness of the potential gradient from one point in the system reflects the ease with which water will flow down the potential gradient between the points.

1.5.2 Components of Soil Water Potential
As in any other hydraulic system, the total potential (or total hydraulic head) in the soil-water system is made up of a number of distinguishable components. Some of these are as follows: i ) Gravitational Potential: Reflects gravitational forces on the soil water.

Components of Soil Water Potential Contd.
ii) Pressure Potential: This is positive when greater than atmospheric pressure, and negative when below atmospheric. A negative pressure potential (or tension, or suction) is also known as the matric potential. It is characteristic of soil water above a free water surface.

Components of Soil Water Potential Contd.
iii) Osmotic Potential: reflects the effect of solutes in soil water, in the presence of a semi-permeable membrane The total potential of soil water at a point is the sum of all the components of potential, which are acting. Note that the movement of water in the soil is slow, so kinetic energy is neglected.

1.5.3 Soil Water Potential and Soil Water Content:
If a water pressure less that atmospheric (usually referred to as suction) is applied to a saturated soil, some water will drain off until equilibrium is reached. At this state of equilibrium, the total potential of the soil water relative to a free water surface at the same elevation will be negative. Its value is known as the soil suction or matric suction since it is equal to the negative pressure potential of the soil water.

Soil Water Potential and Soil Water Content Contd.
As the pressure potential is reduced ( i.e. suction increased) more water is removed from the soil. The relationship between suction and actual water content is referred to as soil water characteristic. Soil Water Potential is normally measured by tensiometers (matric potential), hanging water column (sand box) and pressure chamber.

1.5.4 Methods of Measuring Soil Water Content
i) By Feel: This is by far the easiest method. Assessment by feel is good for experienced people who have sort of calibrated their hands. The type of soil is important. ii) Gravimetric Method: This is equal to:

Gravimetric Method Contd.
Weigh wet soil in a container, put in oven at 105 oC for about 48 hours; weigh again and obtain the weight of water by subtraction. A good soil should have moisture contents between 5 and 60% and for peat or organic soils, it can be greater than 100%.

Methods of Measuring Soil Water Content Contd.
(iii) Volumetric water content, Pv. This is equal to: Recall that volume = mass/density i.e.

Soil Bulk Density Bulk Density, Db is defined as the mass of a unit volume of dry soil. This includes both solids and pores. i.e. bulk density = Ms/V ; Ms is the mass of dry soil and V is the total volume of undisturbed soil. The major method of measuring bulk density in the field is to collect a known volume of undisturbed soil (V) in a soil core, and drying it in the oven to remove all the water to obtain Ms.

Methods of Measuring Soil Water Content Contd.
(iv) Neutron Probe: It consists of a probe lowered down a hole in the soil. A box (rate meter or rate scalar) is at the top. Within the probe is a radioactive source e.g. beryllium (435 years life span). Close to the source is a detector. The source emits fast neutrons, some of which are slowed down when they collide with water molecules (due to hydrogen molecules). A cloud of slow neutrons (thermal neutrons) build up near the probe and are registered by the rate meter or rate scalar which measures the number of slowed down neutrons.

NEUTRON PROBE Fig. 1.3: Diagram and Photograph of Neutron Probe in Use
Fig. 1.3: Diagram and Photograph of Neutron Probe in Use The method is quick but very expensive. It is also dangerous since it is radioactive and must be used with care.

1.5.5 Methods of Measuring Soil Water Suction
i) Electrical Resistance Unit: This consists of a porous body with two electrodes embedded into it. The porous body when buried equilibrates with the soil water and the readings are obtained through the resistance meters attached to the electrodes. Resistance units are measured and the instrument needs to be calibrated against matric suction or volumetric moisture content (Pv). Various porous bodies needed are gypsum, nylon or fibreglass. The instrument is relatively cheap but it takes a long time to equilibrate or react e.g. 48 hours. The method is insensitive in wet soils <0.5 bars. It measures from 0.5 to 15 bars and more.

ELECTRICAL RESISTANCE UNIT
Figure 1.4 Portable meter and resistance blocks used to measure soil moisture. (Courtesy Industrial Instrument, Inc.)

Methods of Measuring Soil Water Suction Contd.
ii) Tensiometer: Tensiometer operates on the principle that a partial vacuum is developed in a closed chamber when water moves out through the porous ceramic tip to the surrounding. A vacuum gauge or a water or mercury manometer can measure the tension. The gauge is usually calibrated in centibars or millibars. After the porous cup is put in the soil, the tensiometer is filled with water. Water moves out from the porous tip to the surrounding soil (as suction is more in the soil). A point is reached when the water in the tensiometer is at equilibrium with the soil water. The reading of the gauge is then taken and correlated to moisture content using a calibration curve. Mention that a zero reading on the tensiometer means that the soil is saturated.

1.5.6 Soil Water Equilibrium Points
In a soil, which is completely saturated, large pores are filled with what is called gravitational water because it can drain out under gravity. It drains out so fast that it is not available to the crops. The time of draining out varies from one day in sandy soils to four days in clay soils.

Soil Water Equilibrium Points Contd.
Field Capacity (FC): This is the amount of water a well-drained soil contains after gravitational water movement has materially ceased. It is taken as the water content after 48 hours the soil has been subjected to heavy rainfall or irrigation sufficient to cause saturation. Field capacity can also be determined by finding the moisture content when suction is 1/3 bar for clay and 1/10 bar for sand. There still remains the water held loosely between the soil particles by surface tension at field capacity. This is called capillary water and is the main source of water for plant growth. Plants continuously take this up until there is no more water available for crop growth and wilting occurs.

SOIL MOISTURE EQUILIBRIUM POINTS CONTD.
Permanent Wilting Point (PWP): This is the soil moisture content at which crops can no longer obtain enough water to satisfy evapotranspiration needs. The plant will wilt and may die later if water is not available. Water tension of soil at PWP is generally taken as 15 bars. For field estimation, a crop is planted and when it wilts, the moisture content is the PWP. This technique requires personal judgment and prone to mistakes.

SOIL MOISTURE EQUILIBRIUM POINTS CONTD.
Available Water (AW): This is the water available to crops. It is the water content at field capacity minus that at permanent wilting point. Readily Available Water (RAW): This is the level to which the available water in the soil can be used up without causing stress in the crop. For most crops, 50 to 60% available water is taken as readily available.

Typical Soil Water Equilibrium Points
Field Capacity (FC) (By Weight) Permanent Wilting Point (PWP) Available Water (AW) Readily Available Water = 0.5 AW Clay 45 30 15 7.5 Clay Loam 40 25 Fine Sand 8 7 3.5 Sand 4 2.0

Available Water in the Soil
Saturated Excess water 100% available Field Capacity Readily Available Water Available Water Little reserve available and plants stressed Wilting Point 0% Available No water available Oven dry

1.5.7 DEFINITION OF SOIL WETNESS
Soil Wetness can be described as: By Mass (Pm): This is the gravimetric system. b) By Volume (Pv): This is the volumetric system. It is given as: Pv = Pm x Dry bulk density ( Db). c) By Equivalent Depth: This is expressed in depth eg. in mm. This is normally used in irrigation engineering. d = Pm Db D where: d is the equivalent depth of water applied (mm); Pm is the moisture content by mass (fraction or decimal); D is the root zone depth (mm). In this case, Db is the specific gravity of the soil, which is dimensionless. It has the same units as bulk density when expressed in gm/cm3. The unit of d is therefore determined by the unit of the root zone depth, D

Table: Effective Rooting Depth (mm) of Some Crops
Fruits 750 Lucerne 1200 Cotton 900 Maize, small grains, wheat 600 Most Vegetables 300 Source: Hudson’s Field Engineering

1.5.8 INFILTRATION OF WATER Infiltration is the entry of water into the soil. It is a very important variable in irrigation design since it shows the rate at which water can move into the soil mass to replenish the root zone. Infiltration rate of a soil is the maximum rate at which water will enter the soil mass through the surface. Infiltration rates into soils depend on soil texture and structure, density, organic matter content, hydraulic conductivity (permeability) and porosity. Remember to mention Hydraulic conductivity

INFILTRATION CONTD. As wetting time increases, the infiltration rate decreases and usually approaches a constant value, which in the case of heavy clays may be zero. A general equation for the Infiltration rate (I) is the Kostiakov (1932) equation: I = (a Tn ) mm/hr. Where: a and n are constants and T is the elapsed wetting time

Methods of Measuring Infiltration
Irrigation is practiced mainly in three ways: By flooding the whole surface of the soil surface; By Flooding part of the surface and By Sprinkling. The method used influences the measured intake rate of water into the soil. When designing irrigation systems, the method used for measuring the soil infiltration rate should simulate, as far as possible, the mechanism of water intake during the application.

Infiltration Measurement For Flooded Irrigation
For Flooded irrigation (border strip and basin), a double infiltrometer is normally used. This consists of two concentric cylinders, the inner about 0.4 m diameter, the outer 0.5 m. Water is maintained at the same level in each cylinder, 25 mm above the soil surface, or more if the water level is likely to be higher during irrigation. The water infiltrating from the outer ring prevents lateral seepage by the water from the center cylinder. By measuring the rate at which the water is added to the center cylinder, the infiltration rate can be found.

Double Ring Infiltrometer

Infiltration Measurement For Furrow Irrigation
For flood irrigation (furrow), in addition to the usual factors affecting infiltration, the intake of water depends on the spacing and shape of the furrow. The difference between inflows and outflows of water flowing through hydraulic flumes placed at different distances of test furrows represent the total infiltration. Furrow dimensions are used to obtain the infiltration rates. See Chapter 3 for test calculations.

Infiltration Measurement For Sprinkler Irrigation
The mechanism of infiltration under sprinkler irrigation is different from the surface methods. There is no head of water above the soil surface and the effect of sprinkler drops on the soil tends to form soil pans on the surface, reducing infiltration rate. The ideal method of measuring infiltration rates for sprinkler irrigation is to use sprinklers at various rates of spraying. Water could be sprayed into infiltrometers to obtain a small head of water and the intake rate found as described earlier.

CHAPTER TWO: BASICS IN IRRIGATION ENGINEERING
2.1. IRRIGATION ENGINEERING: This involves Conception, Planning, Design, Construction, Operation and Management of an irrigation system. An irrigation engineer is one who has a long theoretical and practical training in planning, design, construction, operation and management of irrigation systems.

Considerations in Planning Irrigation Systems
i) Location: The main point to consider in locating an irrigation project is the need to investigate available resources in the area e.g. Climate, Adequate water in quality and quantity, Land with good agricultural potential and Good topography, Availability of labour (sophisticated or not), Land tenure, Marketing, Transport facilities etc.

Considerations in Planning Irrigation Systems Contd.
ii) Crops to be grown: Should be determined by available resources as well as marketability of the crops especially in terms of what people like to eat. iii) Water Supply: Consider (a)Sources of water (b) Quantity and quality of water c) Engineering works necessary to obtain water e.g. if underground, pumping is needed d) Conveyance System: can be by gravity e.g. open channels or canals or by closed conduits e.g. pipes. (e) Water measuring devices e.g. weirs, orifice, flumes, current meters

Other Considerations iv) Systems of Applying Water:
e.g. Surface (90% worldwide), Sprinkler(5%), Trickle and Sub-irrigation(5%). v) Water Demand: The water requirement for the given crop has to be determined. This is by calculating the evapotranspiration (to be treated later) vi) Project Management: Consider how to manage the irrigation system

2.2 CROP WATER AND NET IRRIGATION REQUIREMENTS
In irrigation, it is essential to know the amount of water needed by crops. This determines the quantity of water to be added by irrigation and rainfall and helps in day to day management of irrigation systems. Total water demand of crops is made up of: i) Crop water use: includes evaporation and transpiration (evapotranspiration described in section 2.3 below) ii) Leaching requirement: iii) Losses of water due to deep seepage in canals and losses due to the inefficiency of application.

EVAPOTRANSPIRATION 2.3.1 DEFINITIONS
a) Evaporation: The process by which water is changed from the liquid or solid state into the gaseous state through the transfer of heat energy. b) Transpiration: The evaporation of water absorbed by the crop which is used directly in the building of plant tissue in a specified time. It does not include soil evaporation. c) Evapotranspiration, ET: It is the sum of the amount of water transpired by plants during the growth process and that amount that is evaporated from soil and vegetation in the domain occupied by the growing crop. ET is normally expressed in mm/day.

FACTORS THAT AFFECT EVAPOTRANSPIRATION
Weather parameters, Crop Characteristics, Management and Environmental aspects are factors affecting ET (a) Weather Parameters: The principal weather conditions affecting evapotranspiration are: Radiation, Air temperature, Humidity and Wind speed.

CROP FACTORS THAT AFFECT ET
Crop Type Variety of Crop Development Stage Crop Height Crop Roughness Ground Cover Crop Rooting Depth

Management and Environmental Factors
(a) Factors such as soil salinity, Poor land fertility, Limited application of fertilizers, Absence of control of diseases and Pests and poor soil management May limit the crop development and reduce soil evapotranspiration. Other factors that affect ET are ground cover, plant density and soil water content. The effect of soil water content on ET is conditioned primarily by the magnitude of the water deficit and the type of soil. Too much water will result in waterlogging which might damage the root and limit root water uptake by inhibiting respiration.

EVAPOTRANSPIRATION CONCEPTS
(a) Reference Crop Evapotranspiration (ETo): Used by FAO. This is ET rate from a reference plant e.g. grass or alfalfa, not short of water and is denoted as ETo. The ET of other crops can be related to the Et of the reference plant. ETo is a climatic parameter as it is only affected by climatic factors. The FAO Penman-Monteith method is recommended as the sole method for determining ETo. The method has been selected because it closely approximates grass ETo at the location evaluated, is physically based, and explicitly incorporates both physiological and aerodynamic parameters.

CROP ET UNDER STANDARD CONDITIONS (ETc)
This refers to crop ET under standard conditions, i.e. ET from disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions. ETc can be derived from ETo using the equation: ETc = Kc . ETo where Kc is crop coefficient Crop Evapotranspiration under non- standard conditions as mentioned above is called ETc (adjusted). This refers to growth of crops under non-optimal conditions.

DETERMINATION OF EVAPOTRANSPIRATION
Evapotranspiration is not easy to measure. Specific devices and accurate measurements of various physical parameters or the soil water balance in lysimeters are required to determine ET. The methods are expensive, demanding and used for research purposes. They remain important for evaluating ET estimates obtained by more indirect methods.

ENERGY BUDGET METHOD This method like the water budget approach involves solving an equation which lists all the sources and sinks of thermal energy and leaves evaporation as the only unknown. It involves a great deal of instrumentation and is still under active development. It is data intensive and is really a specialist approach.

Energy Budget Method Contd.

Water Balance Method The Water Balance or Budget Method is a measurement of continuity of flow of water. This method consists of drawing up a balance sheet of all the water entering and leaving a particular catchment or drainage basin. The water balance equation can be written as: ET = I + P – RO – DP + CR + SF + SW Where: I is Irrigation, P is rainfall, RO is surface runoff, DP is deep percolation, CR is capillary rise, SF and SW are change in sub-surface flow and change in soil water content respectively

Lysimeters For Water Balance Method
Lysimeters are normally adopted in water balance studies. By isolating the crop root zone from its environment and controlling the processes that are difficult to measure, the different terms in the soil balance equation can be determined with greater accuracy. Using Lysimeters, crop grows in isolated tanks filled with either disturbed or undisturbed soil. In weighing lysimeters, water loss is directly measured by change in mass while In non-weighing ones, the ET for a given time is determined by deducting the drainage water collected at the bottom of the lysimeters, from the total water input.

Non-Weighing Lysimeter

ET Computed from Meteorological Data:
ET is commonly computed from weather data. A large number of empirical equations have been developed for assessing crop or reference crop evapotranspiration from weather data. Some of these methods include the Blaney-Criddle, Penman, Thornthwaite, Radiation, Hargreaves, Turc and many others. Most of these methods have been found to only work in specific locations. Following an Expert Consultation by Food and Agriculture Organization in May 1990, the FAO Penman-Monteith method is now recommended as the standard method for the definition and computation of the reference evapotranspiration. The FAO Penman-Monteith equation is described in the Notes.

ET Estimated from Evaporation Pans:
Evaporation from an open water surface provides an index of integrated effect of radiation, air temperature, air humidity and wind on evapotranspiration. However, differences in the water and cropped surface produce significant differences in the water loss from an open surface and the crop. The pan is used to estimate reference ETo by observing the evaporation loss from a water surface (Epan) and applying empirical coefficients (Kpan)to relate pan evaporation to Eto thus: ETo = Kp x Epan

Standard Pan: United States Class A Pan
The most common Evaporation Pan used is the United States Class A pan. This is made up of unpainted galvanized iron, 1.2 m in diameter and 25.4 cm deep. The bottom supported on a wooded frame, is raised cm above the ground surface. The water surface is maintained between 5.0 and 7.6 cm below the rim of the pan and is measured daily with a gauge. The daily evaporation is computed as the difference between observed levels corrected for any precipitation measured in an adjacent or nearby standard rain gauge. A pan coefficient of 0.7 ( ) is normally used to convert the observed value to an estimated value for lake or reservoirs. This is because the rate of evaporation in small areas is greater than that from large areas.

US Class A Evaporation Pan
Heat Transfer Mechanisms Involved In Heating Of Water In The Standard Pans (diameter D) And Their Walls (After Jagroop,2000). Evaporation Incoming Radiation q’ Absorbed By Water Air Flow q’ conv absorbed by the water Incoming Radiation Heats Pan Wall q’’ rad Conduction Through Walls of pan Convection q”conv heats up pan walls

Types of Evaporation Pans

A Comparison of Standard Open Pans
Dimensions Pan Coefficient US Class A 1.2 m Diameter; 250 mm Deep 0.7 (0.6 to 0.8) Australian Pan 900 mm Diameter; 900 mm Deep. Large Pan: 1200 mm Diameter and 850 mm Deep 0.9 ( 0.6 to 1.2) British Tank 1.83 m Square 0.9 (Very Variable)

2.4. LEACHING REQUIREMENT Most irrigation water contain dissolved salts. Evaporation removes pure water leaving a concentration of salt in soil. Salt concentration may reach a level that is detrimental to the growth of the crop and should be controlled. The only practical way of achieving this is by leaching. Leaching requirement is an extra water needed to pass through the root zone in addition to the normal requirement to ensure that salts are placed below the root zone.

LEACHING REQUIREMENT CONTD.
Ec acceptable = 4 mmhos/cm. For water quality, Ec of 0.8 Mmhos/cm is medium, quality while Ec of 4 mmhos/cm is saline.

2.5. EFFECTIVE PRECIPITATION
This is the component of rainfall that is available to crops ie. does not runoff. It can be estimated as 65% of total rainfall. It can also be estimated as the rainfall value, which has 80% probability of being exceeded (D80).

2.6 NET IRRIGATION REQUIREMENT (Nir)
This is the moisture that must be supplied by irrigation to satisfy evapotranspiration plus that needed for leaching and not supplied by off-season storage, and the effects of precipitation and groundwater storage. Nir = ET Wl Ws Re Where: Nir is the net irrigation; ET is evapotranspiration, Wl is leaching requirement; Ws is off-season soil moisture carry-over. All parameters are in mm of water.

2.7 GROSS IRRIGATION REQUIREMENT (Gir)
Gross Irrigation Requirement is equal to: Net Irrigation Requirement Divided by Irrigation Efficiency Irrigation efficiency accounts for losses in storage and distribution systems, losses in application systems as well as operation and management losses. Irrigation Efficiency depends on the Method of Applying Irrigation Water Mention that a table for computing Net irrigation is in the note. Irrigation efficiency depends on the method of irrigation

2.8 IRRIGATION TERMS Depth of Irrigation: This is the depth of the readily available moisture. This is the net depth of water normally needed to be applied to the crops during each irrigation

Example 1 The Moisture Content at Field Capacity of a Clay Loam Soil is 28% by Weight While that at Permanent Wilting Point is 14% by Weight. Root Zone Depth Is 1 m and the Bulk Density Is 1.2 g/cm Calculate the Net and Gross Depth of Irrigation Required If the Irrigation Efficiency Is 0.7. Solution: Field Capacity = 28%; Permanent wilting point = 14% i.e. Available moisture = = 14% by weight i.e. Pm Bulk density (Db) = 1.2 g/cm3 Root Zone depth (D) = 1 m = mm Equivalent depth of available water (d) = Pm . Db D = x x mm = 168 mm This is the net depth of irrigation.

Solution to Example 1 contd.
Gross Water Application is equal to: Net Irrigation/Efficiency = 84/0.7 = 120 mm Note: This is the actual water needed to be pumped for irrigation. It is equivalent to: 120 /1000 mm x 10,000 m2 = m 3 per hectare.

2.8.2 Irrigation Interval (II):
This is the time between successive irrigations. Irrigation interval is equal to: Readily Available Moisture or Net Irrigation divided by Evapotranspiration, ET The shortest irrigation interval is normally use in design. The irrigation interval varies with ET. It is equivalent to Readily Available Water divided by the Peak ET

Example 2 For the Last Example. the Peak ET is 7.5 mm/day, Determine the Shortest Irrigation Interval. Solution: From Example 1, Readily Available Moisture (RAM) = 84 mm i.e. Shortest irrigation interval = RAM/ Peak ET = 84/7.5 = 11 days.

Irrigation Period (IP)
This is the number of days allowed to complete one irrigation cycle in a given area.

Irrigation Period Contd.
10 1 2 5 3 4 7 9 6 8 Assuming water is applied in a border in a day, the total period of irrigation is then 11 days.

Irrigation Interval and Period
In irrigation scheduling, the irrigation period should be less that the irrigation interval. This is because if the period is not smaller, before the latter parts of the area are to be irrigated, the earlier irrigated areas will need fresh irrigation. At peak evapotranspiration (used in design), irrigation interval should be equal to irrigation period. i.e. Generally IP < II

2.8.4 Desired Irrigation Design Capacity (Qc)
This is the flow rate determined by the water requirement, irrigation time, irrigation period and the irrigation application efficiency. It is the flow rate of flow of the water supply source e.g. pumps from a reservoir, or a borehole required to irrigate a given area.

Desired Irrigation Design Capacity (Qc) Contd.
Where: Qc is the Desired Design Capacity; d is the Net Irrigation Depth = Readily Available Moisture; F is the number of Days to complete the Irrigation (Irrigation Period); H is the number of Hours the System is perated (hrs/day) and Ea is the Irrigation Efficiency

Example 3 A 12-hectare field is to be irrigated with a sprinkler system. The root zone depth is 0.9 m and the field capacity of the soil is 28% while the permanent wilting point is 17% by weight. The soil bulk density is 1.36 g/cm and the water application efficiency is 70%. The soil is to be irrigated when 50% of the available water has depleted. The peak evapotranspiration is 5.0 mm/day and the system is to be run for 10 hours in a day. Determine: (i) The net irrigation depth (ii) Gross irrigation ie. the depth of water to be pumped (iii) Irrigation period (iv) Area to be irrigated per day and (v) the system capacity.

Solution to Example 3 Solution: Field Capacity = 28%; Permanent Wilting Point = 17% ie. Available Moisture = = 11% , which is Pm Root zone depth = 0.9 m; Bulk density = 1.36 g/cm3 Depth of Available Moisture, = Pm . Db. D = x x = 135 mm Allowing for 50 % depletion of Available Moisture before Irrigation, Depth of Readily Available Moisture = 0.5 x 135 mm = mm

Solution of Example 3 Contd.
i) Net irrigation depth = Depth of the Readily Available Moisture = mm ii) Gross Irrigation = Net irrigation Application efficiency = 67.5/0.7 = mm iii) Irrigation interval = Net irrigation or RAM Peak ET = 67.5/5 = days = 13.5 days = 13 days (more critical) In design, irrigation interval = irrigation period ie. irrigation period is 13 days

Solution of Example 3 Contd.
iv) Total area to be irrigated = 12 hectares Area to be irrigated per day = Total area / irrigation period = ha/ 13 days = 1 ha/day v) System Capacity, Qc = A. d m3 /s F. H. Ea Area, A = 12 ha = 12 x m2 = 120,000 m2 Net irrigation depth, d = mm = m Irrigation period , F = 13 days Number of hours of operation, H = 10 hrs/day Irrigation efficiency, Ea = 0.78

Solution of Example 3 Concluded
System capacity, Qc = ,000 m2 x m days x 10 hrs/day x 0.7 = m 3/hr Recall: 1 m 3 = L and 1 hr = 3600 s ie m3 /hr = { x L}/3600 secs = = 25 L/s The pump to be purchased for sprinkler irrigation must have capacity equal to or greater than 25 L/s. Alternatively, more than one pump can be purchased.

2.9. IRRIGATION EFFICIENCIES
These irrigation efficiencies are brought about by the desire not to waste irrigation water, no matter how cheap or abundant it is. The objective of irrigation efficiency concept is to determine whether improvements can be made in both the irrigation system and the management of the operation programmes, which will lead to an efficient irrigation water use.

2.9.1 Application Efficiency
Ea is inadequate in describing the overall quantity of water since it does not indicate the actual uniformity of irrigation, the amount of deep percolation or the magnitude of under-irrigation. See diagrams in text.

Example 4 Delivery of 10 m3/s to a 32 ha farm is continued for 4 hours. The tail water is 0.27 m3/s. Soil probing after irrigation indicates that 30 cm of water has been stored in the root zone. Compute the Application Efficiency. Solution: Total volume of water applied = 10 m3/s x 4 hrs x 3600s/hr = 144,000 m3 Total tail water = x 4 x = m3 Total water in root zone = 30 cm = 0.3 m x 32 ha x 10,000 m2/ha = 96,000 m3

Solution to Example 4 Contd.
= 96,000/144,000 = 66.7%.

2.9.2 Water Conveyance Efficiency
Farm Wd Water lost by evap And seepage Ws Stream

Example 5 45 m3 of water was pumped into a farm distribution system. 38 m3 of water is delivered to a turn out (at head ditch) which is 2 km from the well. Compute the Conveyance Efficiency. Solution: = 38/45 = 84%

2.9.3. Christiansen Uniformity Coefficient (Cu)
This measures the uniformity of irrigation W here: is the summation of deviations from the mean depth infiltered m is the mean depth unfiltered and n is the number of observations.

Example 6 A Uniformity Check is taken by probing many stations down the border. The depths of penetration (cm) recorded were: 6.4, 6.5, 6.5, 6.3, 6.2, 6.0, 6.4, 6.0, 5.8, 5.7, 5.5, 4.5, Compute the Uniformity Coefficient. Solution: Total depth of water infiltered = cm Mean depth = 76.7/13 = 5.9 cm

Locations Depths (cm) Deviations from Mean 1 6.4 0.5 2 6.5 0.6 3 4 6.3
0.4 5 6.2 0.3 6 6.0 0.1 7 8 9 5.8 10 5.7 0.2 11 5.5 12 4.5 1.4 13 4.9 1.0

Example 6 Concluded = 6.2 m = 5.9 cm; n = 13 = 92% This is a good Efficiency. 80% Efficiency is acceptable.

2.9.4 Water Storage Efficiency (Es)
2.9.5 Irrigation Efficiency ET is Evapotranspiration; Wl is Leaching Requirement; Re is Effective Precipitation; is change in storage; Wi is water diverted, stored or pumped for irrigation.

2.10 IRRIGATION SCHEDULING
This means Predicting when to Irrigate and how much to Irrigate For efficient water use on the farm, the farmer needs to be able to predict when his crops need irrigation. This can be done by: Observing the plants; Keeping a Water Balance Sheet By Measuring the Soil Moisture Content or Computer Software

Observing the Plants: This is a direct way of knowing when the crops need water. The farmer observes the plants for any signs of wilting or change in leaf colour or growth rate. The method is simple but its major disadvantage is that the signs of shortage appear after the optimum allowable depletion has already been exceeded.

2.10.2. Keeping a Water Balance Sheet
This approach works on the principle that the change in water content of the soil is represented by the difference between water added by irrigation(or rainfall) and the amount lost by evapotranspiration. The records are kept for each farm and crops as shown in Table 2.4 below. The method requires no equipment and is easy to operate. It can be operated on a daily or weekly or 10 day basis.

Table 2.3: Example of a Water Balance Sheet
Irrigation Plan: Apply 30 mm of water at 30 mm deficit. Date Estimated ET (mm) Rainfall (mm) Accumulated Deficit (mm) Irrigation Period 5.1.05 4.2 - 6.1.05 3.5 7.7 7.1.05 3.8 11.5 8.1.05 4.5 16.0 9.1.05 5.2 21.2 5.1 2.0 24.3 5.5 29.8 4.9 (34.9) 30.0 4.9 9.8 etc.

2.10.3 Measuring Soil Moisture
This is the best scheduling and the most widely used. Soil moisture can be indirectly measured using devices and instruments eg. tensiometers, resistance blocks or neutron probes. Direct measurement of soil moisture can be by weighing or the gravimetric method. These methods are either too expensive or complicated. The simplest and most practical method is to estimate the moisture content by the 'feel and appearance' of the soil. Soil is collected at the root zone and checked to guess the right time to irrigate.

2.11 IRRIGATION WATER: SOURCES, QUALITY & MEASUREMENT
Sources of Irrigation Water Supply i) Rainfall or Precipitation: This is a practical and dominant factor. The supply varies with time and place e.g. while Grenada receives 2,100 mm annual rainfall, Antigua receives only 1,100 mm. Trinidad receives 1, 950 mm (Data supplied by Gumbs, 1987). To be of greatest benefit for crop production, the rainfall amount should be enough to replace water in the root zone on a regular basis.

Sources of Irrigation Water Contd.
ii) Underground water sources: This can be shallow or bore holes. iii) Surface Sources: Streams, rivers, lakes, farm ponds etc. Streams should be gauged to ensure that there is enough water for irrigation. Rivers or streams can also be dammed to raise the height of flow and make more water available for irrigation. Farm ponds can also be dug to store water from rivers or channels (e.g. field station) or to collect water from rainfall

Sources of Irrigation Water Contd.
iv) Springs and waste water e.g. industrial water and sewage: Determine quality before use. (For details of harnessing water for irrigation in the Caribbean, see Gumb's Soil & Water Conservation Methods, Chapter 7).

2.11.2 Irrigation Water Quality:
Irrigation water quality depends on i) Amount of suspended sediment eg. silt content ii) The chemical constituents of water

i) Amount of Suspended Sediment:
The effect of sediment may depend upon the nature of the sediment and the characteristics and soil conditions of the irrigated area. Silt content in irrigation may be beneficial if it improves the texture and fertility of say sandy soil. It can also be detrimental if it is derived from a sterile sub-soil, and applied to a fertile soil. Silt accumulation can cause aggradation in canals or distribution systems. In sprinkler systems, silt can cause abrasion.

ii) The Chemical Constituents of Water:
There are three main elements or compounds that can cause hazards in irrigation water. They include: Sodium, Boron and Salts.

a) Salinity Hazards: The units of salt concentration in irrigation water can be parts per million (p.p.m), milli equivalents/litre(ME/litre) or electrical conductivity. On the basis of salinity, irrigation water can be classified as C1 to C4(see chart). They refer to low, medium, high and very high salinity levels respectively. While C1 water can easily be used for irrigation without need for leaching requirement, C4 water is not useable, except in permeable soils where adequate leaching and drainage is possible and for highly tolerant crops.

b) Sodium Hazard: It is Measured in Sodium Absorption Ratio (S.A.R).
SAR is defined as the proportion of sodium relative to other cations. Parameters are measured in ME/litre.

Sodium Hazard Contd. Irrigation Water is also divided into S1 to S4 in terms of Sodium (SAR) Content. S1 Water can be used readily S2 and S3 can be used with adequate Leaching and Drainage and addition of Organic and Chemical amendment. S4 Water has very high Sodium Content and is unsuitable for irrigation except where calcium, gypsum or other chemical amendments are possible. (See water quality chart).

Boron See Table 2.4 in Note Book for Permissible limit of Boron for several classes of irrigation water

2.11.3 Measurement of Irrigation Water
Water is the most valuable asset of irrigated agriculture and can be detrimental if used improperly. An accurate measurement permits an intelligent use. The methods to use for measurement should depend on the flow, environmental conditions and the degree of accuracy required.

Methods of Measuring Irrigation Water
a) Direct method: Collect water in a contained of known volume e.g. bucket. Measure the time required for water from an irrigation source e.g. siphon to fill the bucket. Flow rate = Volume/time m3/hr or L/s etc. b) Weirs: Weirs are regular notches over which water flows. They are used to regulate floods through rivers, overflow dams and open channels. Weirs can be sharp or broad crested; made from concrete timber, or metal and can be of cross-section rectangular, trapezoidal or triangular. Sharp crested rectangular or triangular sections are commonly used on the farm.

Weirs Contd. The discharge through a weir is usually expressed as:
Q = C L Hm where Q is the discharge; C is the coefficient dependent on the nature of weir crest and approach conditions; L is the length of crest; H is the head on the crest and m is an exponent depending on weir opening. Weirs should be calibrated to determine these parameters before use eg. for trapezoidal weirs(Cipoletti weir), Q = L H1.5 Q is discharge in L/s; L, H are in cm. More in Chapter 6.

Methods of Measuring Irrigation Water Contd.
c) Orifices: An orifice is an opening in the wall of a tank containing water. The orifice can be circular, rectangular, triangular or any other shape. The discharge through an orifice is given by: Q = C A 2 g h Where Q is the discharge rate; C is the coefficient of discharge ( ); A is the area of the orifice; g is the acceleration due to gravity and h is the head of water over an orifice.

d) Flumes: Hydraulic flumes are artificial open channels or sections of natural channels. Two major types of hydraulic flumes are Parshall or Trapezoidal ones. Flumes need to be calibrated after construction before use. See Chapter 6 for further information. e) For streams, use gauging. A current meter is used to measure velocity at 0.2 and 0.8 Depth or at only 0.6 depth. Measure areas of all sections using trapezoidal areas. Q = ai vi

Using Floats: A floating object is put in water and observe the time it takes the float e.g. a cork to go from one marked area to another. Assuming the float travels D meters in t secs Velocity of water at surface = ( D/t ) m/s Average velocity of flow = 0.8 (D/t) Flow rate, Q = Cross sectional area of flow x velocity. Object D

CHAPTER THREE: IRRIGATION METHODS AND DESIGNS
a) Surface Irrigation: Just flooding water. About 90% of the irrigated areas in the world are by this method. b) Sprinkler Irrigation: Applying water under pressure. About 5 % of the irrigated areas are by this method. c) Drip or Trickle Irrigation: Applying water slowly to the soil ideally at the same rate with crop consumption. d) Sub-Surface Irrigation: Flooding water underground and allowing it to come up by capillarity to crop roots.

3.2 SURFACE IRRIGATION Water is applied to the field in either the controlled or uncontrolled manner. Controlled: Water is applied from the head ditch and guided by corrugations, furrows, borders, or ridges. Uncontrolled: Wild flooding. Surface irrigation is entirely practised where water is abundant. The low initial cost of development is later offset by high labour cost of applying water. There are deep percolation, runoff and drainage problems

3.2.1 Furrow Irrigation In furrow irrigation, only a part of the land surface (the furrow) is wetted thus minimizing evaporation loss. Furrow irrigation is adapted for row crops like corn, banana, tobacco, and cabbage. It is also good for grains. Irrigation can be by corrugation using small irrigation streams. Furrow irrigation is adapted for irrigating on various slopes except on steep ones because of erosion and bank overflow.

Furrow Irrigation Contd.
There are different ways of applying water to the furrow. As shown in Fig. 3.1, siphons are used to divert water from the head ditch to the furrows. There can also be direct gravity flow whereby water is delivered from the head ditch to the furrows by cutting the ridge or levee separating the head ditch and the furrows (see diagram from Gumb's book). Gated pipes can also be used. Large portable pipe(up to 450 mm) with gate openings spaced to deliver water to the furrows are used. Water is pumped from the water source in closed conduits. The openings of the gated pipe can be regulated to control the discharge rate into the furrows.

Furrow Irrigation by Cutting the Ridge

Furrow Irrigation with Siphons

Fig. 3.1: A Furrow System

3.2.1.1 Design Parameters of Furrow Irrigation
The Major Design Considerations in Surface Irrigation Include: Storing the Readily Available Moisture in the Root Zone, if Possible; Obtaining As Uniform Water Application As Possible; Minimizing Soil Erosion by Applying Non-erosive Streams; Minimizing Runoff at the End of the Furrow by Using a Re-use System or a Cut -Back Stream; Minimizing Labour Requirements by Having Good Land Preparation, Good Design and Experienced Labour and Facilitating Use of Machinery for Land Preparation, Cultivation, Furrowing, Harvesting Etc.

Furrow Irrigation Contd.
The Specific Design Parameters of Furrow Irrigation Are Aimed at Achieving the Above Objectives and Include: a) Shape and Spacing of Furrows: Heights of ridges vary between 15 cm and 40 cm and the distance between the ridges should be based on the optimum crop spacing modified, if necessary to obtain adequate lateral wetting, and to accommodate the track of mechanical equipment. The range of spacing commonly used is from 0.3 to 1.8 m with 1.0 m as the average.

Design Parameters of Furrow Irrigation Contd.
b) Selection of the Advance or Initial Furrow Stream: In permeable soils, the maximum non-erosive flow within the furrow capacity can be used so as to enable wetting of the end of the furrow to begin as soon as possible. The maximum non-erosive flow (Qm) is given by: Qm = c/S where c is a constant = 0.6 when Qm is in l/s and S is slope in %. Example 1: For a soil slope of 0.1 %, the Qm is 0.6/0.1 = 6 l/s.

The actual stream size should be determined by field tests. It is desirable that this initial stream size reaches the end of the furrow in T/4 time where T is the total time required to apply the required irrigation depth. c) Cut-back Stream: This is the stream size to which the initial stream is reduced sometime after it has reached the lower end of the field. This is to reduce soil erosion. One or two cutbacks can be carried out and removing some siphons or reducing the size at the head of the furrow achieves this.

d) Field Slope: To reduce costs of land grading, longitudinal and cross slopes should be adapted to the natural topography. Small cross slopes can be tolerated. To reduce erosion problems during rainfall, furrows (which channel the runoff) should have a limited slope (see Table 3.1).

Table 3.1 : Maximum Slopes for Various Soil Types
Soil Type Maximum slopes* Sand Sandy loam Fine sandy loam Clay Loam Source: Withers & Vipond (1974) *A minimum slope of about 0.05 % is required to ensure surface drainage.

e) Furrow Length: Very long lengths lead to a lot of deep percolation involving over-irrigation at the upper end of the furrow and under-irrigation at the lower end. Typical values are given in Table 3.2, but actual furrow lengths should be got from field tests.

e) Field Widths: Widths are flexible but should not be of a size to enclose variable soil types. The widths should depend on land grading permissible.

3.2.1.2 Evaluation of a Furrow Irrigation System
The objective is to determine fairly accurately how the system is used and to suggest possible amendments or changes. Equipment: Engineers Level and Staff, 30 m Tape, Marker Stakes, Siphons of Various Sizes, Two Small Measuring Flumes, Watch with Second Hand and Spade.

Evaluation of a Furrow Irrigation System Contd.
Procedure a) Select several (say 3 or more) uniform test furrows which should be typical of those in the area. b) Measure the average furrow spacing and note the shape, condition etc. c) Set the marker stakes at 30 m intervals down the furrows. d) Take levels at each stake and determine the average slope. e) Set the flumes say 30 m apart at the head of the middle furrow. f) Pass constant flow streams down the furrows, using wide range of flows. The largest flow should just cause erosion and overtopping, the smallest might just reach the end of the furrow. The median stream should have a discharge of about Q = 3/4 S (l/s) where S is the % slope.

Evaluation of a Furrow Irrigation System Contd.
g) Record the time when flow starts and passes each marker in each flow(advance data). h) Record the flow at each flume periodically until the flows become practically constant. This may take several hours on fine textured soils(Infiltration data). i) Check for evidence of erosion or overtopping. j) Move the flumes and measure the streams at the heads only of the other furrows. Results: To be presented in a format shown: Watch Opportunity time(mins) Station A Station B Losses Time A B C Depth Flow Depth Flow Diff Infil. (mm) ( L/s) (mm) (L/s) (L/s) (mm/h) Solve problem in note.

3.2.2. Border Irrigation System
In a border irrigation, controlled surface flooding is practised whereby the field is divided up into strips by parallel ridges or dykes and each strip is irrigated separately by introducing water upstream and it progressively covers the entire strip. Border irrigation is suited for crops that can withstand flooding for a short time e.g. wheat. It can be used for all crops provided that the system is designated to provide the needed water control for irrigation of crops. It is suited to soils between extremely high and very low infiltration rates.

Border Irrigation System

Border Irrigation

Border Irrigation Contd.
In border irrigation, water is applied slowly. The root zone is applied water gradually down the field. At a time, the application flow is cut-off to reduce water loses. Ideally, there is no runoff and deep percolation. The problem is that the time to cut off the inflow is difficult to determine.

3.2.2.2 Design Parameters of Border Irrigation System
a) Strip width: Cross slopes must be eliminated by leveling. Since there are no furrows to restrict lateral movement, any cross slope will make water move down one side leading to poor application efficiency and possibly erosion. The stream size available should also be considered in choosing a strip width. The size should be enough to allow complete lateral spreading throughout the length of the strip. The width of the strip for a given water supply is a function of the length (Table 3.5). The strip width should be at least bigger than the size of vehicle tract for construction where applicable.

Design Parameters of Border Irrigation System Contd.
b) Strip Slope: Longitudinal slopes should be almost same as for the furrow irrigation. c) Construction of Levees: Levees should be big enough to withstand erosion, and of sufficient height to contain the irrigation stream. d) Selection of the Advance Stream: The maximum advance stream used should be non-erosive and therefore depends on the protection afforded by the crop cover. Clay soils are less susceptible to erosion but suffer surface panning at high water velocities. Table 3.4 gives the maximum flows recommendable for bare soils. e) The Length of the Strip: Typical lengths and widths for various flows are given in Table The ideal lengths can be obtained by field tests.

3.2.2.3 Evaluation of a Border Strip
The aim is to vary various parameters with the aim of obtaining a good irrigation profile. Steps a) Measure the infiltration rate of soils and get the cumulative infiltration curve. Measurement can be by double ring infiltrometer. Depth of Water, D (mm) D = KTn Time, T (mins) Fig 3.5: Cumulative Infiltration Curve

Evaluation of Border Strip Contd.
b) Mark some points on the border strip and check the advance of water. Also check recession. For steep slopes, recession of water can be seen unlike in gentle slopes where it may be difficult to see. In border irrigation, recession is very important because unlike furrows, there is no place water can seep into after water is turned off.

Time Distance Diagram of the Border System

Evaluation of the Border System Contd.
About two-thirds down the border, the flow is turned off and recession starts. The difference between the advance and recession curves gives the opportunity time or total time when water is in contact with the soil. For various distances, obtain the opportunity times from the advance/recession curves and from the cumulative infiltration curve, obtain the depths of water. With the depth and distance data, plot the irrigation profile depth shown below.

Depth- Distance Diagram of the Border System

Evaluation of the Border System Contd.
The depth of irrigation obtained is compared with the SMD (ideal irrigation depth). There is deep percolation and runoff at the end of the field. The variables can then be changed to give different shapes of graphs to see the one to reduce runoff and deep percolation. In this particular case above, the inflow can be stopped sooner. The recession curve then changes. The profile now obtained creates deficiency at the ends of the borders (see graph: dotted lies above). A good profile of irrigation can be obtained by varying the flow, which leads to a change in the recession curve, and by choosing a reasonable contact time each time using the infiltration curve.

3.2.3 Basin Irrigation System
Description: In basin irrigation, water is flooded in wider areas. It is ideal for irrigating rice. The area is normally flat. In basin irrigation, a very high stream size is introduced into the basin so that rapid movement of water is obtained. Water does not infiltrate a lot initially. At the end, a bond is put and water can pond the field. The opportunity time difference between the upward and the downward ends are reduced.

Basin Irrigation Diagram
rrigation time.

Size of Basins The size of basin is related to stream size and soil type(See Table 3.6 below). Table 3.6: Suggested basin areas for different soil types and rates of water flow Flow rate Soil Type Sand Sandy loam Clay loam Clay l/s m3 /hr Hectares Note: The size of basin for clays is 10 times that of sand as the infiltration rate for clay is low leading to higher irrigation time. The size of basin also increases as the flow rate increases. The table is only a guide and practical values from an area should be relied upon. There is the need for field evaluation.

3.2.3.3 Evaluation of Basin System
a) Calculate the soil moisture deficiency and irrigation depth. b) Get the cumulative infiltration using either single or double ring infiltrometer. I = c Tn Infiltered Depth (mm) Time (mins)

Evaluation of a Basin System Contd.
c) Get the advance curves using sticks to monitor rate of water movement. Plot a time versus distance graph (advance curve). Also plot recession curve or assume it to be straight It is ensured that water reaches the end of the basin at T/4 time and stays T time before it disappears. At any point on the advance and recession curves, get the contact or opportunity time and relate it to the depth-time graph above to know the amount of water that has infiltrated at any distance.

Time-Distance Graph of the Basin System

Depth-Distance Graphs of the Basin Irrigation System

Evaluation of Basin Irrigation Concluded.
Check the deficiency and decide whether improvements are necessary or not. The T/4 time can be increased or flow rate changed. The recession curve may not be a straight line but a curve due to some low points in the basin.

3.3 SPRINKLER IRRIGATION Introduction: The sprinkler system is ideal in areas where water is scarce. A Sprinkler system conveys water through pipes and applies it with a minimum amount of losses. Water is applied in form of sprays sometimes simulating natural rainfall. The difference is that this rainfall can be controlled in duration and intensity. If well planned, designed and operated, it can be used in sloping land to reduce erosion where other systems are not possible.

Components of a Sprinkler Irrigation System

3.3.2 Types of Conventional Sprinkler Systems
a) Fully portable system: The laterals, mains, sub-mains and the pumping plant are all portable. The system is designed to be moved from one field to another or other pumping sites that are in the same field. b) Semi-portable system: Water source and pumping plant are fixed in locations. Other components can be moved. The system cannot be moved from field to field or from farm to farm except when more than one fixed pumping plant is used.

Types of Conventional Sprinkler Systems Contd.
c) Fully permanent system: Permanent laterals, mains, sub-mains as well as fixed pumping plant. Sometimes laterals and mainlines may be buried. The sprinkler may be permanently located or moved along the lateral. It can be used on permanent irrigation fields and for relatively high value crops e.g. Orchards and vineyards. Labour savings throughout the life of the system may later offset high installation cost.

3.3.3 Mobile Sprinkler Types
a) Raingun: A mobile machine with a big sprinkler. The speed of the machine determines the application rate. The sprinkler has a powerful jet system. b) Lateral Move: A mobile long boom with many sprinklers attached to them. As the machine moves, it collects water from a canal into the sprinklers connected to the long boom.

Raingun Irrigation System

Linear Move

Centre Pivot c) Centre Pivot: The source of water is stationary e.g. a bore hole. The boom with many sprinklers rotates about the water source.

Centre Pivot

Pivot of a Centre Pivot System

3.3.4 Design of Sprinkler Irrigation System
Objectives and Procedures Provide Sufficient Flow Capacity to meet the Irrigation Demand Ensure that the Least Irrigated Plant receives adequate Water Ensure Uniform Distribution of Water.

Design Steps Determine Irrigation Water Requirements and Irrigation Schedule Determine Type and Spacing of Sprinklers Prepare Layout of Mainline, Submains and Laterals Design Pipework and select Valves and Fittings Determine Pumping Requirements.

Choice of Sprinkler System
Consider: Application rate or precipitation rate Uniformity of Application: Use UC Drop Size Distribution and Cost

Sprinkler Application Rate
Must be Less than Intake Rates Soil Texture Max. Appln. Rates (mm/hr.) Coarse Sand 20 to 40 Fine Sand 12 to 25 Sandy Loam 12 Silt Loam 10 Clay Loam/Clay 5 to 8

Effects of Wind In case of Wind:
Reduce the spacing between Sprinklers: See table 6 in Text. Allign Sprinkler Laterals across prevailing wind directions Build Extra Capacity Select Rotary Sprinklers with a low trajectory angle.

System Layout Layout is determined by the Physical Features of the Site e.g. Field Shape and Size, Obstacles, and topography and the type of Equipment chosen. Where there are several possibilities of preparing the layout, a cost criteria can be applied to the alternatives. Laterals should be as long as site dimensions, pressure and pipe diameter restrictions will allow. Laterals of 75 mm to 100 mm diameter can easily be moved. Etc. - See text for other considerations

Pipework Design This involves the Selection of Pipe Sizes to ensure that adequate water can be distributed as uniformly as possible throughout the system Pressure variations in the system are kept as low as possible as any changes in pressure may affect the discharge at the sprinklers

Design of Laterals Laterals supply water to the Sprinklers
Pipe Sizes are chosen to minimize the pressure variations along the Lateral, due to Friction and Elevation Changes. Select a Pipe Size which limits the total pressure change to 20% of the design operating pressure of the Sprinkler. This limits overall variations in Sprinkler Discharge to 10%.

Lateral Discharge The Discharge (QL) in a Lateral is defined as the flow at the head of the lateral where water is taken from the mainline or submain. Thus: QL = N. qL Where N is the number of sprinklers on the lateral and qL is the Sprinkler discharge (m3/h)

Selecting Lateral Pipe Sizes
Friction Loss in a Lateral is less than that in a Pipeline where all the flow passes through the entire pipe Length because flow changes at every sprinkler along the Line. First Compute the Friction Loss in the Pipe assuming no Sprinklers using a Friction Formula or Charts and then: Apply a Factor, F based on the number of Sprinklers on the Lateral (See Text for F Values)

Selecting Lateral Pipe Sizes Contd.
Lateral Pipe Size can be determined as follows: Calculate 20% of Sprinkler Operating Pressure (Pa) Divide Value by F for the number of Sprinklers to obtain Allowable Pressure Loss (Pf) Use Normal Pipeline Head Loss Charts of Friction Formulae with Calculated Pf and QL to determine Pipe Diameter, D.

Changes in Ground Elevation
Allowance must be made for Pressure changes along the Lateral when it is uphill, downhill or over undulating land. If Pe1 is the Pressure Difference Due to Elevation changes:

Pressure at Head of Lateral
The Pressure requirements (PL)where the Lateral joins the Mainline or Submain are determined as follows: PL = Pa Pf + Pr For laterals laid on Flat land PL = Pa (Pf Pe) + Pr For Laterals on gradient. The factor 0.75 is to provide for average operating pressure (Pa) at the centre of the Lateral rather than at the distal end. Pr is the height of the riser.

Diagram of Pressure at Head of Lateral

Selecting Pipe Sizes of Submains and MainLines
As a general rule, for pumped systems, the Maximum Pressure Loss in both Mainlines and Submains should not exceed 30% of the total pumping head required. This is reasonable starting point for the preliminary design. Allowance should be made for pressure changes in the mainline and submain when they are uphill, downhill or undulating.

Pumping Requirements Maximum Discharge (Qp) = qs N Where:
qs is the Sprinkler Discharge and N is the total number of Sprinklers operating at one time during irrigation cycle. The Maximum Pressure to operate the system (Total Dynamic Head, Pp) is given as shown in Example.

3.4 DRIP OR TRICKLE IRRIGATION
Introduction: In this irrigation system: i) Water is applied directly to the crop ie. entire field is not wetted. ii) Water is conserved (iii) Weeds are controlled because only the places getting water can grow weeds. (iv) There is a low pressure system. (v) There is a slow rate of water application somewhat matching the consumptive use. Application rate can be as low as l/hr. (vi) There is reduced evaporation, only potential transpiration is considered. vii) There is no need for a drainage system.

Components of a Drip Irrigation System
Control Head Unit Wetting Pattern Mainline Or Manifold Emitter Lateral

Drip Irrigation System
The Major Components of a Drip Irrigation System include: a) Head unit which contains filters to remove debris that may block emitters; fertilizer tank; water meter; and pressure regulator. b) Mainline, Laterals, and Emitters which can be easily blocked.

3.4.2 Water Use for Trickle Irrigation System
The design of drip system is similar to that of the sprinkler system except that the spacing of emitters is much less than that of sprinklers and that water must be filtered and treated to prevent blockage of emitters. Another major difference is that not all areas are irrigated. In design, the water use rate or the area irrigated may be decreased to account for this reduced area.

Water Use for Trickle Irrigation System Contd.
Karmeli and Keller (1975) suggested the following water use rate for trickle irrigation design ETt = ET x P/85 Where: ETt is average evapotranspiration rate for crops under trickle irrigation; P is the percentage of the total area shaded by crops; ET is the conventional evapotranspiration rate for the crop. E.g. If a mature orchard shades 70% of the area and the conventional ET is 7 mm/day, the trickle irrigation design rate is: 7/1 x 70/85 = 5.8 mm/day OR use potential transpiration, Tp = 0.7 Epan where Epan is the evaporation from the United States Class A pan.

Emitters Consist of fixed type and variable size types. The fixed size emitters do not have a mechanism to compensate for the friction induced pressure drop along the lateral while the variable size types have it. Emitter discharge may be described by: q = K h x Where: q is the emitter discharge; K is constant for each emitter ; h is pressure head at which the emitter operates and x is the exponent characterized by the flow regime.

Emitters Contd. The exponent, x can be determined by measuring the slope of the log-log plot of head Vs discharge. With x known, K can be determined using the above equation. Discharges are normally determined from the manufacturer's charts (see Fig. 3.7 in Note).

3.4.4 Water Distribution from Emitters
Emitter discharge variability is greater than that of sprinkler nozzles because of smaller openings(lower flow) and lower design pressures. Eu = (0.8 Cv/ n 0.5 ) Where Eu is emitter uniformity; Cv is manufacturer's coefficient of variation(s/x ); n is the number of emitters per plant. Application efficiency for trickle irrigation is defined as: Eea = Eu x Ea x 100 Where Eea is the trickle irrigation efficiency; Ea is the application efficiency as defined earlier.

Trickle System Design The diameter of the lateral should be selected so that the difference in discharge between emitters operating simultaneously will not exceed 10 %. This allowable variation is same as for sprinkler irrigation laterals already discussed. To stay within this 10 % variation in flow, the head difference between emitters should not exceed 10 to 15 % of the average operating head for long-path or 20 % for turbulent flow emitters.

Trickle System Design Contd.
The maximum difference in pressure is the head loss between the control point at the inlet and the pressure at the emitter farthest from the inlet. The inlet is usually at the manifold where the pressure is regulated. The manifold is a line to which the trickle laterals are connected.

Trickle System Design Contd.
For minimum cost, on a level area 55 % of the allowable head loss should be allocated to the lateral and 45 % to the manifold. The Friction Loss for Mains and Sub-mains can be computed from Darcy-Weisbach equation for smooth pipes in trickle systems when combined with the Blasius equation for friction factor. The equation is: Hf = K L Q 1.75 D – 4.75 Where: Hf is the friction loss in m; K is constant = x for S.I. units for water at 20 ° C; L is the pipe length in m; Q is the total pipe flow in l/s; and D is the internal diameter of pipe in mm.

Trickle System Design Contd
As with sprinkler design, F should be used to compute head loss for laterals and manifolds with multiple outlets, by multiplying a suitable F factor (See Table 8 of Sprinkler Design section) by head loss. F values shown below can also be used.

Table 3.7: Correction Factor, F for Friction Losses in Aluminium Pipes with Multiple Outlets.
Number of Outlets F* 30 or more *Values adapted from Jensen and Frantini (1957

Example Design a Trickle Irrigation System for a fully matured orchard with the layout below. Assume that the field is level, maximum time for irrigation is 12 hours per day, allowable pressure variation in the emitters is 15%, the maximum suction lift at the well is 20 m, the ET rate is 7 mm/day and the matured orchard shades 70% of the area; trickle irrigation efficiency is 80%. Sections 1 and 2 are to be irrigated at the same time and alternated with sections 3 and 4. Each tree is to be supplied by 4 emitters.

LAYOUT OF THE TRICKLE IRRIGATION SYSTEM

Solution (1) ETt = ET x P/85 Where: Ett is the average ET for crops under trickle irrigation (mm/day) ET is nomal ET rate for the crop = 7 mm/day P is the percentage of total ares shaded by the crop = 70% ETt = 7 mm/day x 70/85 = 5.8 mm/day.

Solution Contd. (2) Discharge for each tree with a spacing of 4 m x 7 m = 4 m x 7 m x 5.8 x 10-3 m/day = m3/day = m3/hr (24 hr. day) For 12 hours of opearation per day, discharge required = x 24/12 = m3/hr = L/s With an appliance efficiency of 80%, the required discharge per tree is: /0.8 = L/s The discharge per emitter, with 4 emitters per tree is then: = /4 = L/s = L/s

Required Discharge (L/s)
Discharge of Each Line Line No. of Trees No. of Emitters Required Discharge (L/s) Half Lateral 12 48 0.0576 Half Manifold 168 672 0.8060 Submain, A to Section 1 336 1344 1.6130 Main, A to Pump 2688 3.2260

Solution Contd. (4) From Fig (Soil and Water Conservation), select the medium long-path emitter with K = and x = 0.63 Substituting in equation q = K hx, with an average discharge of L/s, Log q = log K + x log h h = 87 kPa or 8.9 m ( or use Chart to obtain h). This is the Average operating head, Ha.

Solution Contd. (5) Total allowable pressure loss of 15 % of Ha in both the Lateral and Manifold = 8.9 x 0.15 =1.3 m of which 0.55 x 1.3 = 0.7 m is allowed for Lateral and 0.45 x 1.3 = 0.6 is for the Manifold. (6) Compute the Friction Loss in each of the Lines from Equation: Hf = K L Q D –4.75 by selecting a diameter to keep the loss within the allowable limits of 0.7 m and 0.6 m, already determined.

Selection of Diameters
Line Q (L/s) Pipe Diameter (mm) L (m) F Hf’ (m) Half Lateral 0.0576 12.70 46 0.36 0.51 Half Manifold 0.8060 31.75 45.5 0.68 Sub-Main, A to Section 1 1.6130 44.45 243 1 6.59 Main, A to Pump 3.2260 50.80 60 2.90

Pressure Head at Manifold Inlet
Like Sprinklers, the pressure head at inlet to the manifold: = Average Operating Head = 8.9 m + 75% of Lateral and Manifold head Loss = 0.75 ( ) + Riser Height = Zero for Trickle since no risers exist. + Elevation difference = Zero , since the field is Level = m

Solution Concluded Total Head for Pump = Manifold Pressure = 9.79 m
+ Pressure loss at Sub-main = 6.59 m + Pressure loss at Main = m + Suction Lift = 20 m + Net Positive Suction head for pump = 4 m (assumed) = m i.e. The Pump must deliver 3.23 L/s at a head of about 43 m.

3.5 SUB-SURFACE IRRIGATION
Applied in places where natural soil and topographic condition favour water application to the soil under the surface, a practice called sub-surface irrigation. These conditions include: a) Impervious layer at 15 cm depth or more b) Pervious soil underlying the restricting layer. c) Uniform topographic condition d) Moderate slopes.

SUB-SURFACE IRRIGATION Contd.
The operation of the system involves a huge reservoir of water and level is controlled by inflow and outflow. The inflow is water application and rainfall while the outflow is evapotranspiration and deep percolation. It does not disturb normal farm operations. Excess water can be removed by pumping.

3.6 CHOICE OF IRRIGATION METHODS:
The following criteria should be considered: (a) Water supply available (b) Topography of area to be irrigated c) Climate of the area (d) Soils of the area (e) Crops to be grown f) Economics (g) Local traditions and skills (For details see extract from Hudson's Field Engineering).

3.7 INFORMATION TO BE COLLECTED ON A VISIT TO A PROPOSED IRRIGATION SITE.
a) Soil Properties: Texture and structure, moisture equilibrium points, water holding capacity, agricultural potential, land classification, kinds of crops that the soil can support. b) Water Source: Water source availability eg. surface water, boreholes etc., hydrologic data of the area, water quantity, water quality, eg. sodium adsorption ratio, salt content, boron etc.; possible engineering works necessary to obtain water. c) Weather data: Temperature, relative humidity, sunshine hours and rainfall.

INFORMATION TO BE COLLECTED
d) Topography e.g. slope: This helps to determine the layout of the irrigation system and method of irrigation water application suited for the area. e) History of People and Irrigation in the area: Check past exposure of people to irrigation and land tenure and level of possible re-settlement or otherwise. f) Information about crops grown in the area: Check preference by people, market potential, adaptability to area, water demand, growth schedules and planting periods.

CHAPTER FOUR: DRAINAGE & DESIGN OF DRAINAGE SYSTEMS

4.1 INTRODUCTION Drainage means the removal of excess water from a given place. Two types of drainage can be identified: i) Land Drainage: This is large scale drainage where the objective is to drain surplus water from a large area by such means as excavating large open drains, erecting dykes and levees and pumping. Such schemes are necessary in low lying areas and are mainly Civil Engineering work.

ii) Field Drainage: This is the drainage that concerns us in agriculture. It is the removal of excess water from the root zone of crops.

Water in Soil After Heavy Rain

The main aims of Field drainage include:
i) To bring soil moisture down from saturation to field capacity. At field capacity, air is available to the soil and most soils are mesophites ie. like to grow at moisture less than saturation. ii) Drainage helps improve hydraulic conductivity: Soil structure can collapse under very wet conditions and so also engineering structures. iii) In some areas with salt disposition, especially in arid regions, drainage is used to leach excess salt.

The main aims of Field drainage Contd.
iv) In irrigated areas, drainage is needed due to poor application efficiency which means that a lot of water is applied. v) Drainage can shorten the number of occasions when cultivation is held up waiting for soil to dry out.

Two types of drainage exist: Surface and Sub-surface drainage.
4.2 DESIGN OF SURFACE DRAINAGE SYSTEMS: Surface drainage involves the removal of excess water from the surface of the soil. This is done by removing low spots where water accumulates by land forming or by excavating ditches or a combination of the two.

Surface Drainage

Surface Drainage Contd.
Land forming is mechanically changing the land surface to drain surface water. This is done by smoothing, grading, bedding or leveling. Land smoothing is the shaping of the land to a smooth surface in order to eliminate minor differences in elevation and this is accomplished by filling shallow depressions. There is no change in land contour. Smoothing is done using land levelers or planes

Surface Drainage Concluded
Land grading is shaping the land for drainage done by cutting, filling and smoothening to planned continuous surface grade e.g. using bulldozers or scrapers.

4.2.1 Design of Drainage Channels or Ditches
Estimation of Peak Flows: This can be done using the Rational formula, Cook's method, Curve Number method, Soil Conservation Service method etc. Drainage coefficients (to be treated later) are at times used in the tropics used in the tropics especially in flat areas and where peak storm runoff would require excessively large channels and culverts. This may not apply locally because of high slopes.

a) The Rational Formula:
It states that: qp = (CIA)/360 where qp is the peak flow (m3 /s); C is dimensionless runoff coefficient; I is the rainfall intensity for a given return period. Return period is the average number of years within which a given rainfall event will be expected to occur at least once. A is the area of catchment (ha).

Using the Rational Method
i) Obtain area of catchment by surveying or from maps or aerial photographs. ii) Estimate intensity using the curve in Hudson's Field Engineering, page 42. iii) The runoff coefficient C is a measure of the rain which becomes runoff. On a corrugated iron roof, almost all the rain would runoff so C = 1, while in a well drained soil, nine-tenths of the rain may soak in and so C = The table (see handout) from Hudson's Field Engineering can be used to obtain C value. Where the catchment has several different kinds of characteristics, the different values should be combined in proportion to the area of each.

Runoff Coefficient, C

b) Cook's Method: Three factors are considered: Vegetation,
Soil permeability and Slope. These are the catchment characteristics. For each catchment, these are assessed and compared with Table 3.4 of Hudson's Field Engineering

Table 3.4: Hudson’s Field Eng’g (CC)

Example A catchment may be heavy grass (10) on shallow soils with impeded drainage(30) and moderate slope(10). Catchment characteristics (CC) is then the sum of the three ie. 50. The area of the catchment is then measured, and using the Area, A and the CC, the maximum runoff can be read from Table 3.5 (Field Engineering, pp. 45).

Table 3.5: Hudson’s Field Eng’g (Runoff Values)

Cook’s Method Contd. This gives the runoff for a 10 yr return period. For other return periods, other than 10 years, the conversion factor is: Return Period (yrs): Conversion factor: Another factor to be considered is the shape of the catchment. Table 3.5 gives the runoff for a catchment, which is roughly square or round. For other catchment shapes, the following conversion factors should be used: Square or round catchment (1) Long & narrow (0.8) Broad & short (1.25)

Surface Drainage Channels
The drainage channels are normally designed using the Manning formula (see Chapter 6). The required capacity of a drainage channel is calculated from the summation of the inflowing streams (See Note)

Surface Drainage Channels Contd.
The bed level of an open drain collecting flow from field pipe drains should be such as to allow free fall from the pipe drain outlets under maximum flow conditions, with an allowance for siltation and weed growth mm is a reasonable general figure.

Surface Ditch Arrangements
The ditch arrangement can be random, parallel or cross- slope. Random ditch system: Used where only scattered wet lands require drainage. Parallel ditch system: Used in flat topography. Ditches are parallel and perpendicular to the slope. Laterals, which run in the direction of the flow, collect water from ditches.

Surface Ditch Arrangements

4.3 DESIGN OF SUB-SURFACE DRAINAGE SYSTEMS
Sub-surface drainage is the removal of excess groundwater below the soil surface. It aims at increasing the rate at which water will drain from the soil, and so lowering the water table, thus increasing the depth of drier soil above the water table. Sub-surface drainage can be done by open ditches or buried drains.

Sub-Surface Drainage Using Ditches

Sub-Surface Drainage Using Ditches
Ditches have lower initial cost than buried drains; There is ease of inspection and ditches are applicable in some organic soils where drains are unsuitable. Ditches, however, reduce the land available for cropping and require more maintenance that drains due to weed growth and erosion.

Sub-Surface Drains Using Buried Drains

Sub-Surface Drainage Using Buried Drains
Buried drains refer to any type of buried conduits having open joints or perforations, which collect and convey drainage water. They can be fabricated from clay, concrete, corrugated plastic tubes or any other suitable material. The drains can be arranged in a parallel, herringbone, double main or random fashion.

Buried Drains

Arrangements of Sub-Surface Drains

Sub-Surface Drainage Designs
The Major Considerations in Sub-surface Drainage Design Include: Drainage Coefficient; Drain Depth and Spacing; Drain Diameters and Gradient; Drainage Filters.

Drainage Coefficient This is the rate of water removal used in drainage design to obtain the desired protection of crops from excess surface or sub-surface water and can be expressed in mm/day , m/day etc. Drainage is different in Rain-Fed Areas and Irrigated Areas

Drainage Coefficient in Rain-Fed Areas
This is chosen from experience depending on rainfall. The following are guidelines. A. Table 4.1 : Drainage Coefficient for Rain-Fed Areas* Mean annual rainfall Drainage coefficient (mm/day) (mm/yr) Ministry of Agric Hudson < *From Ministry of Agric., U.K (1967) & Hudson (1975)

Other Methods For Obtaining Drainage Coefficient in Rain-Fed Areas
Note: Hudson suggests that for MAR > mm, drainage coefficient is MAR/1000 mm/day and where MAR < 1000 mm, drainage coefficient is 10 mm/day. B. From rainfall records, determine peak rainfall with a certain probability depending on the value of crops or grounds to be protected e.g. 5 day rainfall for 1: 2 return period. C. Divide the rainfall of the heaviest rainfall month by the days of the month e.g. in St. Augustine, Trinidad, the heaviest rainfall month is August with 249 mm. i.e. Drainage discharge = 249/31 = mm/day. Use this method as a last resort.

Drainage Coefficient in Irrigated Areas
In irrigated areas, water enters the groundwater from: Deep percolation, Leaching requirement, Seepage or Conveyance losses from watercourses and canals and Rainfall for some parts of the world.

Example In the design of an irrigation system, the following properties exist: Soil field capacity is 28% by weight, permanent wilting point is 17% by weight; Bulk density = 1.36 g/cm3 ; root zone depth is 1 m; peak ET is 5 mm/day; irrigation efficiency is 60%, water conveyance efficiency is 80%, 50 % of water lost in canals contribute to seepage; rainfall for January is 69 mm and evapotranspiration is 100 mm; salinity of irrigation water is 0.80 mm hos/cm while that acceptable is 4 mmhos/cm. Compute the drainage coefficient.

Solution: Readily available moisture (RAM) = ½ (FC - PWP) = 1/2( ) = 5.5%. In depth, RAM = x 1.36 x 1000 mm= mm = Net irrigation Shortest irrigation interval = RAM/peak ET = 74.8/5 = 15 days With irrigation efficiency of 60 %, Gross irrigation requirement = 74.8/0.6 = mm. This is per irrigation. (a) Water losses = Gross - Net irrigation = = mm Assuming 70% is deep percolation while 30% is wasted on the soil surface (Standard assumption), deep percolation = x = mm

Solution Contd. (b)Seepage
Conveyance efficiency, Ec = Water delivered to farm Water released at dam = 0.8 Water delivered to farm = Gross irrigation =124.7 mm i.e. Water released = /0.8 = mm Excess water or water lost in canal = = mm Since half of the water is seepage (given), the rest will be evaporation during conveyance Seepage = 1/2 x mm = mm

Solution Contd. (c) Leaching Reqd. = Ecirrig (ET - Rain ) = 0.8 ( ) Ecaccep = mm This is for one month; for 15 days, we have 3.88 mm (d) Rainfall = 69 mm; for 15 days, this is 34.5 mm Note: In surface irrigation systems, deep percolation is much higher than leaching requirement so only the former is used in computation. It is assumed that excess water going down the soil as a result of deep percolation can be used for leaching. In sprinkler system, leaching requirement may be greater than deep percolation and can be used instead.

Solution Concluded Neglecting Leaching Requirement, Total water input into drains is equal to: = mm This is per 15 days, since irrigation interval is 15 days Drainage coefficient = /15 = 5.67 = 6 mm/day.

Drain Depth and Spacing
L is drain spacing; h is mid drain water table height (m) above drain level; Do is depth of aquifer from drain level to impermeable layer(m); q is the water input rate(m/day) = specific discharge or drainage coefficient; K is hydraulic conductivity(m/day); H is the depth to water table.

Design Water table depth (H):
This is the minimum depth below the surface at which the water table should be controlled and is determined by farming needs especially crop tolerance to water. Typically, it varies from 0.5 to 1.5 m.

Design Depth of Drain The deeper a drain is put, the larger the spacing and the more economical the design becomes. Drain depth, however, is constrained by soil and machinery limitations. Table : Typical Drain Depths(D) Soil Type Drain Depth (m) Sand Sandy loam Silt loam Clay loam Peat

Drain Spacing (L) This is normally determined using the Hooghoudt equation. It states that Hooghoudt equation states that for ditches reaching the impermeable layer: L2 = 8 K Do h K h q q (See definitions of terms above) For tube drains which do not reach the impermeable layer, the equation can be modified as: L = 8 K d h K h2 q q Where d is called the Houghoudt equivalent d. The equation for tube drains can be solved using trial and error method or the graphical method.

Example For the drainage design of an irrigated area, drain pipes with a radius of 0.1 m are used. They are placed at a depth of 1.8 m below the soil surface. A relatively impermeable soil layer was found at a depth of 6.8 m below the surface. From auger hole tests, the hydraulic conductivity above this layer was estimated as 0.8 m/day. The average irrigation losses, which recharge the groundwater, are 40 mm per 20 days so the average discharge of the drain system amounts to 2 mm/day. Estimate the drain spacing, if the depth of the water table is 1.2 m.

Solution

Solution Contd. Trial One
Assume L = 75 m, from Houghout d table provided, with L = 75 m, and Do = 5 m, d = 3.49 m. From equation (1), L2 = (1920 x 3.49) = ; L = 85.3 m Comment: The L chosen is small since 75 < m Try L = 100 m, from table, d = 3.78 From (1), L 2 = (1920 x 3.78) = ; L = 88.51m Comment: Since < 100, try a smaller L; L should be between 75 and 100 m.

Analytical Solution Concluded
Try L = 90 m, d = /25( ) = 3.66 m L2 = (1920 x 3.66) = m ; L = 87 m Comment: Since 87 < 90, try a smaller L; L should be between 75 and 90. Try L = 87 m, d = /25( ) =3.63 m L2 = (1920 x 3.63) = ; L = m Comment: The difference between the assumed and calculated L is <1, so : Drain Spacing = 87 m.

Graphical Solution Calculate 4 K h2 and 8 K h q q
4 K h2 = 4 x 0.8 x = 576; q 8 K h = 8 x 0.8 x = 1920 q Locate the two points on graph given and join. For a value of Do = 5 m; produce downwards to meet the line. Read off the spacing on the diagram L = 87 m

Drain Diameters and Gradients
There are two approaches to design: (a) Transport approach: Assumes that pipes are flowing full from top to end of field. Assumes uniform flow. Widely used in United States, Canada and Germany. Used to design collector drains. (b) Drainage approach: Assumes that water enters the pipe all down the length as it is perforated. This is more realistic. Widely used in United Kingdom, Holland and Denmark. This is used to design lateral drainage pipes.

Parameters Required to use Solution Graphs
(a) Types of pipes: Pipes can be smooth or rough. Clay tiles and smooth plastic pipes are smooth while corrugated plastic pipes are rough. (b) Drainable area: The area drained by one lateral and is equal to the maximum length of a lateral multiplied by drain spacing. The whole area drained by the laterals discharging into a collector represents the drainable area of the collector.

Parameters Required to use Solution Graphs Contd.
c) Specific discharge: Earlier defined. Same as drainage coefficient. d) Silt safety factors: Used to account for the silting of pipes with time by making the pipes bigger. 60, 75 and 100 % pipe capacity factors are indicated. This means allowing 40, 25 and 0% respectively for silting. e) Average hydraulic gradient(%): normally the soil slope.

Example: The drainage design of a field is drain spacing = 30 m, length of drain lines = 200 m, slope = 0.10%, specific discharge = 10 mm/day. Estimate drain diameter. Assume 60% silt factor and clay tiles. Solution: Area to be drained by one lateral = 30 m x 200 m = m2 = 0.6 ha Slope = average hydraulic gradient = 0.10% ; q = 10 mm/day Using chart for smooth drains, nearest diameter = 70 mm inside diameter.

Drainage Filters Filters for tile drains are permeable materials eg. gravel placed around the drains for the purpose of improving the flow conditions in the area immediately surrounding the drains as well as for improving bedding conditions. Filters provide a high hydraulic conductivity around the drains which stabilizes the soil around and prevent small particles from entering the lateral drains since they are perforated.

Soils that Need Filters
a) Uniform soils will cause problems while non-uniform ones since they are widely distributed stabilize themselves. b) Clays have high cohesion so cannot be easily moved so require no filters. c) Big particles like gravel can hardly be moved due to their weight. * Fine soils are then the soils that will actually need filters especially if they are uniform.

Drainage Filters Continued
a) Filters are needed to be gravel with same uniformity with the soil to be protected. b) D15 Filter < 5 D85 Soil ; D15 Filter < 20 D15 Soil ; D50 Filter < 25 D50 Soil. These are the filtration criteria. To give adequate hydraulic conductivity, D85 Filter > 5 D15 Soil. These criteria are difficult to achieve and should serve as guidelines.

Laying Plastic Pipes: A Trench is excavated, the pipe is laid in the trench, permeable fill is added, and then the trench is filled. This is for smooth-walled rigid plastic pipes or tile drains. A Flexible Corrugated Pipe can be laid by machines, which lay the pipes without excavating an open trench (trench less machines).

PRINCIPLES OF OPEN CHANNEL FLOW
CHAPTER FIVE PRINCIPLES OF OPEN CHANNEL FLOW

5.1 FUNDAMENTAL EQUATIONS OF FLOW
Continuity Equation Inflow = Outflow Area; A1 and A2 and Velocity; V1 and V2 Area x Velocity (A . V) = Discharge, Q ie. A1 V1 = A2 V2 = Q

5.1.2 Energy Equation Energy is Capacity to do work
Work done = Force x Distance moved Forms of Energy Kinetic Energy velocity Pressure Energy - pressure Potential Energy - Height or elevation

Kinetic Energy (KE): Energy possessed by Moving objects.
Solid Mechanics : KE = 1/2 m V2 But Mass = W/g where W is the weight In hydraulics: KE = 1/2 . W/g . V2 = W V2 /2g KE per unit weight = ( W V2 /2g) / W = V2 /2g

Pressure Energy Fluid flow under pressure has ability to do work and so possesses energy by virtue of its pressure. Pressure force = P. a where P is pressure w = specific volume If W is the weight of water flowing, then Volume = W/w Distance moved by flow = W/w.a (Recall Volume/area = distance Work done = Force x distance = P.a x W/wa = WP/w Pressure Energy per unit weight = P/w

Potential Energy Total Energy available is the sum of the three:
Energy Related to Position Wt. of Fluid W at a height Z Then Potential Energy = W Z Potential Energy per unit wt. = Z Total Energy available is the sum of the three: E = P/w + V2 /2g Z : The Bernoulli Equation.

Bernoulli Theorem Total Energy of Each Particle of a Body of Fluid is the Same Provided That No Energy Enters or Leaves the System at Any Point. Division of Available Energy Between Pressure, Kinetic and Position May Change but Total Energy Remains Constant. Bernoulli Equation Is Generally Used to Determine Pressures and Velocities at Different Positions in a System. Z1 + V12/2g + P1 /w = Z2 + V22 /2g + P2 /w

5.2 UNIFORM FLOW IN OPEN CHANNELS
5.2.1 Definitions a) Open Channel: Duct through which Liquid Flows with a Free Surface - River, Canal b) Steady and Non- Steady Flow: In Steady Flows, all the characteristics of flow are constant with time. In unsteady flows, there are variations with time.

Uniform and Non-Uniform Flow
In Uniform Flow, All Characteristics of Flow Are Same Along the Whole Length of Flow. Ie. Velocity, V1 = V2 ; Flow Areas, A1 = A2 In Uniform Channel Flow, Water Surface is Parallel to Channel Bed. In Non-uniform Flow, Characteristics of Flow Vary along the Whole Length.

More Open Channel Terms
d) Normal Flow: Occurs when the Total Energy line is parallel to the bed of the Channel. f)Uniform Steady Flow: All characteristics of flow remain constant and do not vary with time.

Parameters of Open Channels
a) Wetted Perimeter, P : The Length of contact between Liquid and sides and base of Channel P = b + 2 d ; d = normal depth b)Hydraulic Mean Depth or Hydraulic Radius (R): If cross sectional area is A, then R = A/P, e.g. for rectangular channel, A = b d, P = b d Area, A d b Wetted Perimeter

Empirical Flow Equations for Estimating Normal Flow Velocities
a) Chezy Formula (1775): Can be derived from basic principles. It states that: ; Where: V is velocity; R is hydraulic radius and S is slope of the channel. C is Chezy coefficient and is a function of hydraulic radius and channel roughness.

Manning Formula (1889) Empirical Formula based on analysis of various discharge data. The formula is the most widely used. 'n' is called the Manning's Roughness Coefficient found in textbooks. It is a function of vegetation growth, channel irregularities, obstructions and shape and size of channel.

Best Hydraulic Section or Economic Channel Section
For a given Q, there are many channel shapes. There is the need to find the best proportions of B and D which will make discharge a maximum for a given area, A. Using Chezy's formula: Flow rate, Q = A = For a rectangular Channel: P = b +2d A = b d and therefore: b = A/d i.e. P = A/d + 2 d

Best Hydraulic Section Contd.
For a given Area, A, Q will be maximum when P is minimum (from equation 1) Differentiate P with respect to d dp/dd = - A/d For minimum P i.e. Pmin , - A/d = 0 A = 2 d , Since A = b d ie. b d = 2 d2 ie. b = 2 d i.e. for maximum discharge, b = 2 d OR

For a Trapezoidal Section
Zd Zd Area of cross section(A) = b d + Z d2 Width , b = A/d - Z d (1) Perimeter = b + 2 d ( 1 + Z 2 )1/2 From (1), Perimeter = A/d - Z d d(1 + Z2 )1/2 1 d Z b

For maximum flow, P has to be a minimum
i.e dp/dd = - A/d Z (1 + Z2 )1/2 For Pmin, - A/d2 - Z (1 + Z2)1/2 = 0 A/d2 = 2 (1 + Z2 ) Z A = 2 d2 ( Z2 )1/ Z d2 But Area = b d + Z d2 ie. bd + Z d2 = 2 d2 (1 + Z2 ) - Z d2 For maximum discharge, b = 2 d (1 + Z2)1/ Z d or: Try: Show that for the best hydraulic section:

NON-UNIFORM FLOW IN OPEN CHANNELS
5.3.1 Definition: By non-uniform flow, we mean that the velocity varies at each section of the channel. Velocities at Sections 1 to 4 vary (Next Slide) Non-uniform flow can be caused by i)Differences in depth of channel and ii) Differences in width of channel. iii) Differences in the nature of bed iv) Differences in slope of channel and v) Obstruction in the direction of flow.

Variations of Flow Velocities
1 3 4 2

Non-uniform Flow In Open Channels Contd.
In the non-uniform flow, the Energy Line is not parallel to the bed of the channel. The study of non-uniform flow is primarily concerned with the analysis of Surface profiles and Energy Gradients.

Energy Line Analysis

Energy Line Analysis Contd
For the Energy Line, total head is equal to the depth above datum plus energy due to velocity plus the depth of the channel. Pressure energy is not included because we are working with atmospheric pressure. ie. H = Z + d + V 2 / 2 g

Specific Energy, Es When we neglect Z, the energy obtained is called specific energy (Es). Specific energy (Es) = d + V2 /2g Non-uniform flow analysis usually involves the energy measured from the bed only, the bed forming the datum, and this is called specific energy. In Uniform flow, the specific energy is constant and the energy grade line is parallel to the bed. In non-uniform flow, although the energy grade line always slopes downwards in the direction of flow, the specific energy may increase or decrease according to the particular channel's flow conditions.

Variation of Specific Energy( Es) with depth (d)

Variation of Specific Energy( Es) with depth (d) Contd.
Es = d + V 2 /2g, since q = v d ie. v = q/d, Es = d + q 2 /2 g d2 For a given q, we can plot the variation of Es (specific energy) with flow depth, and use the graph to solve the cubic equation above. For a given value of Es, there are two values of d, indicating two different flow regimes. Flow at A is slow and deep (sub-critical) while flow at B is fast and shallow (super-critical)

Variation of Spcific Energy, Es with depth, d
Increasing Flow per unit Width, q A C. B Specific Energy, Es Minimum Es

Critical Depth Critical Depth we observe from the graph is the depth at which the hydraulic specific energy possessed by a given quantity of flowing water is minimum. CRITICAL FLOW occurs at CRITICAL DEPTH and CRITICAL VELOCITY. At Critical point C, the value of Es is minimum for a given flow rate q.

b) Some Properties of Critical Flow
Es (specific energy) = d + q 2 /2gd (1) At critical flow, E has a minimum value - obtain minimum value by differentiation: dEs/dd = q2 /gd = 0 ie. q 2 /g dc3 = , d = dc - critical depth. dc = q 2 /g (q 2 = g d3 ) This means that critical depth, d is a function of flow per unit width, q only. From above: q 2 = g d c3 but q = Vc dc ie. Vc2 dc = g dc3 and V c2 = g dc and Vc2 /2g = 1/2 dc kinetic energy ie. dc = Vc2 /g This means that when the value of velocity head is double the depth of flow, the depth is critical. The specific energy equation (1), now becomes: E = dc + 1/2 dc = 3/2 dc dc = 2/3 Ec ie. dc = q2 /g = Vc2 /g = 2/3 Ec

Some Properties of Critical Flow Contd.
It is also interesting to see how discharge q varies with depth, d for a given amount of specific energy, E d dc q Max Discharge

Variation of Es with d Concluded
Es = d + q 2 /2g d2 ie. q2 = 2g d 2 (Es - d) = 2g d2 Es - 2g d3 For maximum q, dq/dd = 0 ie. 2 dq/dd = 4 Es g d g d2 dq/dd = (4 Es g d g d2)/2 = 0 ie. 2 Es g d - 3 g d 2 = ie. dc = 2 Es/3 This means that maximum flow occurs when d = 2/3 Es. This equation is similar to the one above for critical flow. Therefore, if the specific energy available is Ec, then maximum flow occurs at 2/3 Ec ie. at the critical depth. Summarising: When discharge is constant, critical depth is the depth at minimum specific energy and when the specific energy is constant, critical depth is the depth at maximum discharge.

Sub-Critical and Super-Critical Flows
At the increase of slope of a bed of flow, the level of flow drops and velocity of flow increases. Where a condition exists such that the depth of flow is below critical depth, the flow is referred to as super critical. Super-critical velocity refers to the velocity above critical velocity. Similarly, sub-critical velocity refers to velocity below critical velocity. These flow regimes can be represented by the two limbs of the depth-specific energy curve.

Sub-Critical and Super-Critical Flows Contd.
Es

Froude Number This is a Dimensionless Ratio Characterizing Open Channel Flow. Froude Number, F = V/ gd = Stream velocity/wave velocity When F = 1, Flow is critical (d = dc and V = Vc) F < 1, Flow is sub-critical (d > dc and V < Vc) F > 1, Flow is super-critical(d< dc and V > Vc)

Hydraulic Jump If a super-critical flow suddenly changes to a sub-critical flow, a hydraulic jump is said to have occurred. The change from super-critical to sub-critical flow may occur as a result of an obstruction placed in the passage of the flow or the slope of the bed provided is not adequate to maintain super-critical flow eg. water falling from a spillway.

Diagram of Hydraulic Jump
dsub dc Water Falling From a Spillway

Depth and Energy Loss of Hydraulic Jump
As energy is lost in the hydraulic jump, we cannot use the Bernoulli equation to analyse. Momentum equation is suited to this case - no mention of energy. The aim is to find expression for d2

Depth and Energy Loss of Hydraulic Jump Contd.
V2 V1

Depth and Energy Loss of Hydraulic Jump Concluded
It can be shown that: Also: the loss of energy (m) during a hydraulic jump can be derived as: The depths are in m and: Power loss (kW) = x Flow rate , m3/s x Energy loss (m)

Hydraulic Jump Concluded
A hydraulic jump is use ful when we require: i) Dissipation of energy e.g. at the foot of a spillway ii) When mixing of fluids is required e.g. in chemical and processing plants. iii) Reduction of velocity e.g. at the base of a dam where large velocities will result in scouring. It is, however, undesirable and should not be allowed to occur where energy dissipation and turbulence are intolerable.

DESIGN OF CHANNELS AND IRRIGATION STRUCTURES
CHAPTER SIX: DESIGN OF CHANNELS AND IRRIGATION STRUCTURES

6.1 DESIGN OF CHANNELS FOR STEADY UNIFORM FLOW
Channels are very important in Engineering projects especially in Irrigation and, Drainage. Channels used for irrigation are normally called canals Channels used for drainage are normally called drains.

6.1.1 ESTIMATION OF CANAL DESIGN FLOWS (Q)
For Irrigation Canals, Design Flows are estimated Using the Peak Gross Irrigation Requirement For Example, in a Location with the Peak Gross Irrigation Requirement of 7.69 mm/day. Peak flow (Q) = /1000 m x x 1/ x 1/24 x = L/s/ha For a canal serving an area of 1000 ha, canal design flow is then 890 L/s or 0.89 m 3 /s. Typically, for humid areas, magnitude of discharges are in the range of 0.5 to 1.0 L/s/ha.

6.1.2 Dimensions of Channels and Definitions

Definitions a) Freeboard: Vertical distance between the highest water level anticipated in the design and the top of the retaining banks. It is a safety factor to prevent the overtopping of structures. b) Side Slope (Z): The ratio of the horizontal to vertical distance of the sides of the channel. Z = e/d = e’/D

Table 6.1: Maximum Canal Side Slopes (Z)
Sand, Soft Clay 3: 1 (Horizontal: Vertical) Sandy Clay, Silt Loam, Sandy Loam 2:1 Fine Clay, Clay Loam 1.5:1 Heavy Clay 1:1 Stiff Clay with Concrete Lining 0.5 to 1:1 Lined Canals

6.1.3 Estimation of Velocity in Channels
The most prominent Equation used in the design is the Manning formula described in Values of Manning's n can be found in standard texts (See Hudson's Field Engineering). Design of Channels Design of open channels can be sub-divided into 2: a) For Non-Erodible Channels (lined) b) Erodible Channels carrying clean water

Design of Non-Erodible Channels
When a channel conveying clear water is to be lined, or the earth used for its construction is non-erodible in the normal range of canal velocities, Manning's equation is used. We are not interested about maximum velocity in design. Manning's equation is: ` Q and S are basic requirements of canal determined from crop water needs. The slope of the channels follows the natural channel. Manning's n can also be got from Tables or estimated using the Strickler equation: n = d1/6 , d is the particle size diameter (m)

Design of Non-Erodible Channels Contd.
LHS of equation (1) can be calculated in terms of A R2/ termed section factor. For a trapezoidal section: A = b d + Z d ; P = b d (1 + Z)1/2 The value of Z is decided (see Table 6.1) and the value of b is chosen based on the material for the construction of the channel. The only unknown d is obtained by trial and error to contain the design flow. Check flow velocity and add freeboard.

Example 6.1 Design a Non-Erodible Channel to convey 10 m3/s flow, the slope is and the mean particle diameter of the soil is 5 mm. The side slope is 2 : 1. Solution: Q = 1/n AR 2/3 S 1/2 ….. (1) With particle diameter, d being 5 mm, Using Strickler Equation, n = d 1/6 = x /6 =

Solution of Example Contd.
Z = Choose a value of 1.5 m for 'b‘ For a trapezoidal channel, A = b d + Z d = 1.5 d + 2 d2 P = b d (Z )1/2 = d 51/2 = d Try different values of d to contain the design flow of 10 m3/s

Soln of Example 6.1 Contd. d(m) A(m2 ) P(m) R(m) R2/3 Q(m3/s) Comment
Small flow Too big slightly big slightly small O.K. The design parameters are then d = m and b = 1.5 m Check Velocity : Velocity = Q/A = 10/ = m/s Note: For earth channels, it is advisable that Velocity should be above 0.8 m/s to inhibit weed growth but this may be impracticable for small channels. Assuming freeboard of 0.2 d ie m, Final design parameters are: D = 2.5 m and b = m

Final Design Diagram T = 11.5 m D = 2.5 m Z = 2:1 b = 1.5 m
T = b + 2 Z d = x2 x 2.5 = 11.5 m

Design of Erodible Channels Carrying Clean Water
The problem here is to find the velocity at which scour is initiated and to keep safely below it. Different procedures and thresholds are involved including maximum permissible velocity and tractive force criteria. Maximum Permissible Velocities: The maximum permissible velocities for different earth materials can be found in text books e.g. Hudson's Field Engineering, Table 8.2.

Procedure For Design i) Determine the maximum permissible velocity from tables. ii) With the permissible velocity equal to Q/A, determine A. iii) With permissible velocity = 1/n S1/2 R2/3 Slope, s and n are normally given. iv) R = A/P , so determine P as A/R v) Then A = b d + Z d and P = b+ 2 d (Z2 + 1)1/2 , Solve and obtain values of b and d

Example 6.2: From previous example, design the channel using the maximum permissible velocity method. Solution: Given: Q = 10 m3 /s , Slope = , n = 0.016 , Z = 2 : 1 i) From permissible velocity table, velocity = m/s A = Q/V = 10/0.75 = m ` iv) P = A/R = /0.97 = m v) A = b d + Z d2 = b d d2 P = b d (Z )1/2 = b d 51/ = b d ie. b d + 2 d2 = m (1) b d = m (2) From previo

Solution of Equation 6.2 Contd.
From (2), b = d (3) Substitute (3) into (1), ( d)d d2 = 13.33 13.74 d d d = 13.74 d d2 = ie d d = 0 Recall the quadratic equation formula: d = 1.26 m is more practicable From (3), b = (4.5 x 1.26) = m Adding 20% freeboard, Final Dimensions are depth = 1.5 m and width = 8.07 m See Final Diagram in note book

6.1.5 Classification of Canals Based on Capacity:
Canals can be classified as: (a) Main Canal: It is the principal channel of a canal system taking off from the headworks or a reservoir or tail of a feeder. It is a large capacity channel and usually there is no direct irrigation from it. Small capacity ditch distributaries running parallel to the canal are taken off from the main canal to irrigate adjoining areas. Main canals deliver supply to branch canal and main distributaries.

Canals Contd. (b) Branch or Secondary Canal: Branch canals take their supply from the main canal and convey to the distributaries. Very little direct irrigation is done from the branch canals. Sub-branch is a canal, which takes off from the branch canal but has capacity higher than a distributary.

Canals Contd. (c) Major Distributary: It is a distributing channel, which may take off from a main canal, branch canal or sub-branch and has discharge capacity less than that of a branch canal. It supplies water to another distributary. Distributaries and minors take off from it. Irrigation is done through outlets fixed along it.

Canals Contd. (d) Distributary:
It is a channel receiving supply from branch canal or major distributary and has discharge less than that of major distributary. Minors take off from it, besides irrigation is done from it through outlets.

IRRIGATION STRUCTURES
Structures are widely used in Irrigation, water conservation, flood alleviation, river works where water level and discharge regulation are required. These are hydraulic structures that are used to regulate, measure, and/or transport water in open channels. These structures are called control structures when there is a fixed relationship between the water surface elevation upstream or downstream of the structure and the flow rate through the structure. Hydraulic structures can be grouped into three categories:

IRRIGATION STRUCTURES

Hydraulic Structures Contd.
(i) Flow measuring structures, such as weirs (ii) Regulation structures such as gates and (iii) Discharge structures such as culverts.

Weirs Weirs: Weirs are elevated structures in open channels that are used to measure flow and/or control outflow elevations from basins and channels. There are two types of weirs in common use: Sharp-crested weirs and the broad-crested weirs. The sharp-crested weirs are commonly used in irrigation practice

Sharp-Crested Weirs Sharp-crested or thin plate, weirs consist of a plastic or metal plate that is set vertically across the width of the channel. The main types of sharp-crested weirs are rectangular, V-notches and the Cipolletti or the Trapezoidal weirs. The amount of discharge flowing through the opening is non-linearly related to the width of the opening and the depth of the water level in the approach section above the height of the weir crest.

Sharp Crested Weirs Contd.
Weirs can be classified as being contracted or suppressed depending on whether or not the nappe is constrained by the edges of the channel. If the nappe is open to the atmosphere at the edges, it is said to be contracted because the flow contracts as it passes through the flow section and the width of the nappe is slightly less than the width of the weir crest (see figure). If the sides of the channel are also the sides of the weir opening, the streamlines of flow are parallel to the walls of the channel and there is no contraction of flow.

Figure 6.2: Rectangular Weirs
(b) Unsuppressed Weir (Contracted) (a) Suppressed Weir

In this case, the weir is said to be suppressed. Some type of air vent must be installed in a suppressed weir so air at atmospheric pressure is free to circulate beneath the nappe. (See Figure 6.2 for suppressed and unsuppressed weirs).

The discharge, Q (m3/s) over a rectangular suppressed weir can be derived as: Where: Cd is the discharge coefficient, b is the width of the weir crest, m (see Figure 6.2 above) and H is the head of water (m) above weir crest. According to Rouse (1946) and Blevins( 1984), ………………..(2) Where: Hw is the height of the crest of the weir above the bottom of the channel.

Weirs Contd This equation is valid when H/Hw <5, and is approximated up to H/Hw = If H/Hw < 0.4, Cd can be approximated as and equation (1) reduces to: Q = 1.83 b H ………. (3) This equation is normally used to compute flow over a rectangular suppressed weir over the usual operating range. It is recommended that the upstream head, H be measured between 4H and 5H upstream of the weir. For the unsuppressed (contracted) weir, the air beneath the nappe is in contact with the atmosphere and venting is not necessary. The effect of side contractions is to reduce the effective width of the nappe by 0.1 H and that flow rate over the weir, Q is estimated as: Q = 1.83 (b – 0.2 H) H1.5 ………………… (4) This equation is acceptable as long as b is longer than 3 H

Cipoletti Weir A type of contracted weir which is related to the rectangular sharp-crested weir is the Cipoletti weir (see Figure 6.3 below) which has a trapezoidal cross-section with side slopes 1:4 (H:V). The advantage of a Cipolletti weir is that corrections for end contractions are not necessary.

Cipolletti Weir Contd. The discharge formula can be written as:
Q = b H1.5 …………….. (5) Where: b is the bottom width of the Cipolletti weir. The minimum head on standard rectangular and Cipolletti weirs is 6 mm and at heads less than 6 mm, the nappe does not spring free of the crest. Figure 6.3: A Trapezoidal of Cipolletti Weir

Example 6.3 A weir is be installed to measure flows in the range of 0.5 to 1.0 m3/s. If the maximum depth of water that can be accommodated at the weir is 1 m and the width of the channel is 4 m, determine the height of a suppressed weir that should be used to measure the flow rate.

Solution to Example 6.3 The flow over the weir is shown in the Figure 6.4 below. The height of water is Hw and the flow rate is Q. The height of water over the crest of the weir, H is given by: H = 1 – Hw Assuming that H/Hw , 0.4, then Q is related to H by equation (3), where: Q = 1.83 b H 1.5 Figure 6.4: Weir Flow

Solution to Example 6.3 Concluded
Taking b = 0.4 m, Q = 1m3/s (the maximum flow rate will give the maximum head, H), then: The height of the weir, Hw is therefore given by: Hw = 1 – = m And H/Hw = / = 0.36 The initial assumption that H/Hw < 0.4 is therefore validated, and the height of the weir should be m.

V-Notch Weir A V-notch weir is a sharp-crested weir that has a V-shaped opening instead of a rectangular-shaped opening. These weirs, also called triangular weirs, are typically used instead of rectangular weirs under low-flow conditions ( mainly < 0.28 m3/s), where rectangular weirs tend to be less accurate. It can be derived that the flow rate, Q over the weir is given by:

V-Notch Weirs Contd.

Parshall Flume Although weirs are the simplest structures for measuring the discharge in open channels, the high head losses caused by weirs and the tendency for suspended particles to accumulate behind weirs may be important limitations. The Parshall flume provides an alternative to the weir for measuring flow rates in open channels where high head losses and sediment accumulation are of concern. Such cases include flow measurement in irrigation channels. The Parshall flume (see Figures 6.7 and 6.8 below) consists of a converging section that causes critical flow conditions, followed by a steep throat section that provides for a transition to supercritical flow.

Parshall Flume

Parshall Flumes

Parshall Flume Contd. The unique relationship between the depth of flow and the flow rate under critical flow conditions is the basic principle on which the Parshall flume operates. The transition from supercritical flow to subcritical flow at the exit of the flume usually occurs via a hydraulic jump, but under high tail water conditions the jump is sometimes submerged.

Parshall Flume Contd Within the flume structure, water depths are measured at two locations, one in the converging section, Ha and the other at the throat section, Hb. The flow depth in the throat section is measured relative to the bottom of the converging section as illustrated in the figure below. If the hydraulic jump at the exit of the Parshall flume is not submerged, then the discharge through the flume is related to the measured flow depth in the converging section, Ha by the empirical discharge relations given in Table 6.2, where Q is the discharge in ft3/s, W is the width of the throat in ft, and Ha is measured in ft.

Parshall Flume Contd Submergence of the hydraulic jump is determined by the ratio of the flow depth in the throat, Hb, to the flow depth in the converging section, Ha, and critical values for the Hb/Ha are given in Table 6.3. Whenever, the ratio exceeds the critical values in the table, the hydraulic jump is submerged and the discharge is reduced from the values given by the equations in Table 6.2. Corrections to the theoretical flow rates as a function of Ha and the percentage of submergence, Hb/Ha are given in the Figures 6.8 and 6.9 below for throat widths of 1 ft and 10 ft.

Parshall Flumes Contd.

Parshall Flumes Contd. Flow corrections for the 1 ft flume are applied to larger flumes by multiplying the correction for the 1 ft flume by a factor corresponding to the flume size given in Table 6.4. Similarly, flow corrections for flume sizes greater than 10 ft. are applied to larger flumes by multiplying the correction for the 10 ft flume by a factor corresponding to the flume size given in Table 6.5. Parshall flumes do not reliably measure flow rates when the submergence ratio, Hb/Ha exceeds 0.95.

Parshall Flume Correction

Tables For Parshall Flume Correction

Example 6.4 Example 6.4: Flow is being measured by a Parshall flume that has a throat width of 2 ft. Determine the flow rate through the flume when the water depth in the converging section is 2.00 ft and the depth in the throat section is 1.70ft.

Solution to Example 6.4 From the given data: W = 2 ft, Ha = 2 ft, and Hb = 1.7 ft. According to Table 6.2, Q is given by: In this case: Hb/Ha = 1.7/2 = 0.85 Therefore, according to Table 6.3, the flow is submerged. Figure 6.8 gives the flow rate correction for a 1 ft flume as 2ft3/s, and Table 6.4 gives the correction factor for a 2 ft flume as The flow rate correction, dQ for a 2 ft flume is therefore given by: DQ = 2 x 1.8 = 3.6 ft3/s And the flow rate through the Parshall flume is Q – dQ, where Q – dQ = – 3.6 = ft3/s

Gates Gates are used to regulate the flow in open channels.
They are designed for either over-flow or underflow operation, with overflow operation appropriate for channels in which there is a significant amount of floating debris. The common types of gates are vertical and radial (Tainter) gates, which are illustrated below. Vertical gates are supported by vertical guides with roller wheels, and large hydrostatic forces usually induce significant frictional resistance to raise and lower the gate

Diagrams of Gates

Gates Contd. Flow, Q through a gate could be established to be: `
Cc = Cc = coefficient of contraction, = y2/yg = for most vertical gates. ` For For Tainter gates, Cc is generally greater than 0.61 and is commonly expressed as a function of the angle (degrees) shown in the diagram above.

Gates Concluded It can be expressed as:
This equation applies as long as the angle is least than All the equations apply where there is free flow through the gates. See texts for situations where the flows through the gates are submerged.

Drop Structures: Drop structures, typically constructed out of concrete, can accommodate a sudden change in elevation of the channel bottom while maintaining control of the flow. Drop structures are used in channels, which must be laid along relatively steep gradients to allow for dissipation of energy without causing scour in the channel itself. In such applications, the drop structure allows the main channel to be laid on subcritical slope while the excess potential energy of the flow due to the steep topography is absorbed in the drop structure. See Figure 6.12 of a drop structure below

Diagram of Drop Structure

Example 6.5 An irrigation channel with a design discharge of m3/s is to be laid along a terrain having an average slope of m/m. To maintain subcritical flow in the channel section, the bottom of the channel must be limited to m/m. The extra fall is to be absorbed by drop structures such as the one shown above in the diagram having a width of m. Compute the number of structures required in a km length of line if the drop height (dZ) is equal to m.

Solution of Example 6.5 Solution: The total drop to be absorbed by structures, ZT = (St - So) L Where St is the terrain slope, L is total distance, and So is the slope of the channel. ZT = ( m/m m/m ) km = The number of drop structures required, N = ZT/dZ = /1.829 = 36 Structures.

DRAINAGE AND IRRIGATION ENGINEERING

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