Irrigation can be defined as the artificial application of water to the crop root zone to meet its consumptive use which can not be provided by rainfall.

Therefore, need for irrigation is as follows :

Where, Rainfall is not sufficient to supply crop water needs Where rainfall is not uniformly distributed to supplement the crop requirement If there is variable crop water requirement

1. Introduction

Irrigation maximizes crop productivity and thereby ensures food securityIt makes agricultural industry profitable and competitive Irrigation improves land productivity and value of landPromotes employment generation and lively hood of society Main conveyance/irrigation canals can be used for flood

protection, recreation and navigation.It promotes fish and wildlife preservationRapid development of irrigation facilities through construction

of multipurpose dams enhances hydroelectric power generation.

Irrigation promotes a whole array of agro-based industries which are drive engines of economy.

Direct/indirect benefits of Irrigation

1. Irrigation supplies moisture to the soil essential for, germination of seeds and various growth processes of crop.

2. Cools the soil and the surroundings thus making the environment more favorable for crop growth.

3. Washes out or dilutes salts present in the soil.4. Softens the clods and thus helps in tillage operations. 5. Enables application of fertilizers.6. Reduces the adverse effects of frost on crops.7. Ensures crop success against short duration drought.

Functions of Irrigation

Stabilized Yield of Crops Protection from Famine Improvement of Cash Crops Prosperity of Farmers Source of revenue Navigation Hydroelectric Power Generation. Water Supply General Communication Development of Fishery

Advantages of Irrigation

At the micro level: Irrigation leads to an increase in yield per hectare Subsequent increases in income, consumption and food security.

Irrigation enables smallholders to diversify cropping patterns To switch from low- value market subsistence production to high-value market –oriented production.

Contribution of Irrigation to Country’s Economy

Irrigation benefits through :higher production, higher yields, lower risks of crop failure, and higher and all year round farm as well as non-farm employment.

In Ethiopia during dry season, farmers grow Cereals and a variety of Vegetables including Onions, Tomatoes, and Leafy Green Vegetables like Lettuce under full irrigation.

Farmers also grow perennial crops like Mango, Banana, Chat, Sugarcane etc. which are sometimes intercropped with seasonal crops

Benefits of Irrigation in Ethiopian Scenario

Therefore, we can summarize the

To ensure enough moisture essential for plant life.To provide crop insurance against short duration

drought.To cool the soil and atmosphere to provide a

favorable environment for plant growth.To wash out or dilute harmful salts in the soilTo soften the tillage pans.

Objectives of irrigation

Full Irrigation :Under adequate water supply (i.e., full irrigation), the crop

water requirement is fully met. Crop water consumption (ETa) in this case is equal to the

maximum evapotranspiration (ETm), i.e., ETa = ETm. No water stress develops under such conditions, and crop

yield (Ya) is expected to be potential yield (Ym), i.e.Ya = Ym (if other production factors are not limiting).

1.2. Types of Irrigation

Under limited water supply (i.e., water stress or water deficit) :

- the level of soil water status within the plant root zone is less than what would be under full irrigation.

Crop water consumption in this case (ETa) falls below maximum evapotranspiration (ETm), i.e. ETa < ETm. Under such conditions, water stress will develop in the plant, which will adversely affect crop growth, and therefore the expected yield (i.e., actual yield, Ya) will be less than the potential yield, i.e. Ya < Ym.

Deficit Irrigation

Supplemental irrigation (SI) can be defined as the addition of small amounts of water to essentially rainfed crops during times when rainfall fails to provide sufficient moisture for normal plant growth, in order to improve and stabilize yields. SI in areas with limited water resources is based on the following three grounds:

Water is applied to a rainfed crop which would normally produce some yield without irrigation.

SI is only applied when precipitation fails to provide essential moisture for improved and stabilized production.

Supplemental Irrigation

The amount and timing of SI are not scheduled to provide moisture-stress-free conditions throughout the growing season, but to ensure that the minimum amount of water required for optimal (not maximum) yield is available during the critical stages of crop growth.

Supplemental irrigation is the opposite of full or conventional irrigation (FI). SI is dependent on the precipitation of a basic source of water for the crop.

Supplemental Irrigation (cont..)

Soil can be regarded as a porous medium, consisting of ;Solid, Liquid and Gaseous phases. Solid phase comprises : Mineral material, Organic Matter (OM) and living Micro Organisms which decomposes various residues

into beneficial Humus. Organic Matter content in a mineral soil is usually between

1 and 4% by dry weight.

Soil and Its Constituents

Organic soil is clay soil with more than 30% OM or a sandy soil with more than 20% OM.

If a soil contains more than 65% OM it is called a peat soil.

Mineral and organic compounds of the soil form the soil matrix or 'skeleton' of the soil.

Part of the soil not occupied by the matrix is the pore space or voids.

For most agricultural crops the ideal soil consists of approximately 50% solid material and 50% pores, half of which are filled with water.

Soil and Its Constituents (cont..)

Many of the physical and chemical properties of soil are affected by soil texture.

Soil texture is described by classifications which are determined by the particle size, distribution of sand, silt, and clay within the soil.

Particle size distribution of soil can be measured in the laboratory.

After pretreatment of the soil sample, the sand fractions are measured using mesh sieves of various-sized openings.

Soil texture

Soil Properties Soil Properties

Soil TextureDefinition: relative proportions of various

sizes of individual soil particles

USDA classifications Sand: 0.05 – 2.0 mm Silt: 0.002 - 0.05 mm Clay: <0.002 mm Textural triangle: USDA Textural Classes (previous

slide) Affects water movement and storage

Soil StructureDefinition: how soil particles are grouped or

arrangedAffects root penetration and water intake and

movement

Soil Properties Soil Properties (cont…)(cont…)

Bulk density is the ratio of the mass of solids to the total soil volume.

It can be used to estimate the degree of compaction

It is needed to calculate soil moisture content and porosity.

Bulk density of soil is influenced by soil structure due to its looseness or degree of compaction and by its swelling and shrinking characteristics.

Bulk density of Soil

Bulk Density (b)

◦b = soil bulk density, g/cm3

◦Ms = mass of dry soil, g◦Vb = volume of soil sample, cm3

Typical values: 1.1 - 1.6 g/cm3

Particle Density (p)

◦P = soil particle density, g/cm3

◦Ms = mass of dry soil, g◦Vs = volume of solids, cm3

Typical values: 2.6 - 2.7 g/cm3

b

sb

V

M=ρ

s

sp

V

M=ρ

Bulk density of Soil

Pore spaces in the soil matrix vary in amount, size, shape, and continuity.

Porosity is an important physical property of the soil, especially with regard to the retention of soil water.

Porosity is important because it is a measure of the soil’s capacity to infiltrate water.

Porosity is the volume fraction of pores and ranges from 0.3 to 0.6 for most soils.

Porosity is equal to the pore volume divided by the original soil volume.

Porosity

Porosity ()

Typical values: 30 - 60%

soil of volume

pores of volumeφ =

Porosity Porosity

Development of sustainable irrigation practices will require that we understand better the biophysical processes of root-water uptake in soil, and transpiration to plant canopies.

Solar energy is the driving force for most of the biophysical processes in the plant system and water movement from soil to the atmosphere.

A plant grows in soil and opens to atmosphere. About 99% of all the water that enters the roots leaves the plant’s leaves via the stomata without taking part in metabolism.

On a dry, warm, sunny day, a leaf can evaporate 100% of its water weight in just an hour. Water loss from the leaves must be compensated for by the uptake of water from the soil.

Water movement is due to differences in potential between soil, root, stem, leaf, and atmosphere

Under normal conditions, the water potential in soil is higher than that in root saps or fluids.

- (Negative potential)Typical moist soil might have a water potential of about 0.3

to 1.0 Bar, - root tissue about 4.0 Bar, stem about 7.0 Bar, - leaf about 10.0 to 12.0 Bar, - dry atmosphere about 400 to 600 Bar. These differences in water potential (and hence potential

gradient) are the driving force for causing water movement.

Water Potential

Chapter - 3

Water potential of the atmosphere is computed as:

Water Potential of Atmosphere

Water movement in soil–plant–atmospheric continuum

Soil water content

Mass water content (m)

m = mass water content (fraction)

Mw = mass of water evaporated, g (24 hours @ 105oC)

Ms = mass of dry soil, g

s

wm

M

M=θ

Water in SoilsWater in Soils

V = volumetric water content (fraction)Vw = volume of waterVb = volume of soil sample

V = As mAs = apparent soil specific gravity = b/w

(w = density of water = 1 g/cm3)As = b numerically when units of g/cm3 are used

Equivalent depth of water (d)d = volume of water per unit land area =

(v A L) / A = v Ld = equivalent depth of water in a soil layerL = depth (thickness) of the soil layer

b

wv

V

V=θ

Volumetric water content (v)

(g) (g)

(cm3)

(cm3)

Equivalent DepthEquivalent Depth

Volumetric water content & Equivalent DepthEquivalent Depth

1 in.1 in.

0.50 in.0.50 in.

0.15 in.0.15 in.

0.20 in.0.20 in.

0.15 in.0.15 in.

Soil Solids (Particles): 50%Soil Solids (Particles): 50%

Total Pore Total Pore Space: 50%Space: 50%

Very Large Pores: 15% Very Large Pores: 15% (Gravitational Water)(Gravitational Water)

Medium-sized Pores: 20% Medium-sized Pores: 20% (Plant Available Water)(Plant Available Water)

Very Small Pores: 15% Very Small Pores: 15% (Unavailable Water)(Unavailable Water)

Volumetric Water Content &Volumetric Water Content & Equivalent DepthEquivalent Depth : : Typical Values for Typical Values for

Agricultural SoilsAgricultural Soils

Coarse SandCoarse Sand Silty Clay Loam Silty Clay Loam

Gravitational WaterGravitational Water

Water Holding CapacityWater Holding Capacity

Available WaterAvailable Water

Unavailable WaterUnavailable Water

Dry SoilDry Soil

Water-Holding Capacity of Soil :Water-Holding Capacity of Soil :Effect of Soil TextureEffect of Soil Texture

• Soil-water content refers to the amount of water present in the soil. The soil-water content can be expressed in two ways:

• on the basis of its quantity• on the basis of its energy or potential • 2.2. Forces acting on soil-water• Several forces at the molecular level interact : • Molecular-level cohesive and adhesive forces-• Cohesion – attraction between like molecules• Adhesion – attraction between differing molecules• - hold water between soil particles and on the surfaces of

soil particles. • Cohesive forces hold capillary water and adhesive forces

hold the hygroscopic water.

Soil-water content

• Hydrogen bonding• Van der Waals - London forces

Cohesive forces

Water molecules (+ve charged) is attracted to the negatively charged surfaces of soil solids (adheision force)

Negatively charged soil solid surfaces

Figure: Adsorbed (hygroscopic) water adheres so tightly to soil particles that it can be removed only by oven-drying the soil. It is not available to plants.

Adsorbed water

Capillary water

Adhesion results from double-layer forces. Adsorbed (hygroscopic) water adheres so tightly to soil particles; it can be removed only by oven-drying the soil.

Capillary water coheres to adsorbed water and to itself. (Cohesion force)•Gravitational force – acts on soil-water in the macro pores under saturated conditions and tends to move water downward.•Osmotic forces – are very important in saline soils and move water across plant membranes.

Some points about potential concept : • The concept of energy is the most important concept

in nature. • This is because energy is a fundamental entity common

to all forms of matter.• Water retention in the soil is a consequence of energy

effects. • Capacity of doing work is called energy.• The concept of energy is, therefore, closely linked to

the concept of work.

• Depending on how work is done, two kinds of mechanical energy are distinguished:

• Potential energy – if by virtue of its position or state a body is able to do work.

• Kinetic energy- if a body is able to do work by virtue of its motion.

• Since the movement of water in the soil is very slow, the contribution of kinetic energy to flow of water in soil is negligible compared to that of potential energy.

• In the soil system, the state and movement of water is determined by its potential energy.

• The amount of work done, or potential energy stored, per unit mass in bringing any mass, m, from the reference to the point in question is called potential.

• Expression in terms of energy makes it more easy to compare availability of the moisture in soils of different textures.

• Most commonly accepted unit at present is bars of suction.

• Suction is negative pressure, the higher the numerical value - the lower the energy status of the water.

• Soils at field capacity - 0.1 - 0.3 bars of suction • When soils at wilting point - 15 bars of suction• CONCEPT Managing soil moisture is the critical

management component of irrigated agriculture.

• Water will move in a soil from one point to another if the water at the first point has higher energy status than the water at the second.

• The rapidity of movement depends on the size of the energy difference and soil characteristics.

• If water is applied to the surface by rain or irrigation is much faster than it can enter soil and be transmitted downward, the excess water accumulates on the surface.

• If the slope is great, erosion will likely result (unless surface stable or protected by plant residues).

Measure of the energy status of the soil water

Important because it reflects how hard plants must work to extract water

Units of measure are normally bars or atmospheres

Soil water potentials are negative pressures (tension or suction)

Water flows from a higher (less negative) potential to a lower (more negative) potential

• Gravity is the dominant force acting on soil water when soils are wet.

• Water tends to flow downward- from a region of more positive potential to one of less positive potential- under the influence of gravity, until the force of gravity is balanced by that of capillarity.

• Matric potential is due to the attraction of soil surfaces for water as well as to the influence of soil pores.

• Matric potential is, therefore, the result of two phenomena

• Adsorption • Capillarity• The soil solids tightly adsorb water, whereas

capillary forces are responsible for the water being held in the capillary pores.

• The osmotic potential is attributable to the presence of solutes in the soil.

• It results from dissolved solutes lowering the free energy of soil-water and is always negative.

Components : Components :

◦t = total soil water potential◦g = gravitational potential◦m = matric potential◦o = osmotic potential

◦Matric potential, m, normally has the greatest effect on release of water from soil to plants

omgt ψψψψ ++=

Curve of matric potential (tension) vs. water content

Less water more tensionAt a given tension, finer-textured soils retain

more water (larger number of small pores)

Soil Water Release Curve

Height of capillary rise inversely related to tube diameter

Tension or suction created by small capillary tubes (small soil pores) is greater that that created by large tubes (large soil pores).

At any given matric potential coarse soils hold less water than fine-textured soils..

Matric Potential and Soil Texture

Small soil poresLarge soil pores

–Soil water content where gravity drainage becomes negligible

–Soil is not saturated but still a very wet condition

–Defined as the water content corresponding to a soil water potential of -1/10 to -1/3 bar

–Soil water content beyond which plants cannot recover from water stress (dead)

– (Plants can no longer absorb water)

–Still some water in the soil but not enough to be of use to plants

–Traditionally defined as the water content corresponding to -15 bars of suction

Field Capacity (FC or fcfc)

Permanent Wilting Point (WP or wpwp)

Definition :Water held in the soil between field

capacity and permanent wilting point “Available” for plant use

Available Water Capacity (AWC)AWC = fc - wp

Units: depth of available water per unit depth of soil (in/in, or mm/mm)

Measured using field or laboratory methods

Available WaterAvailable Water

(fc - v) = Soil Water Deficit (SWD)v = current soil volumetric water content

Fraction available water remaining (fr)

(v - wp) = soil water balance (SWB)

=wpfc

vfcdf

θθ

θθ

=wpfc

wpvrf

θθ

θθ

Fraction available water depleted (fd)

TAW = (AWC) (Rd)TAW = total available water capacity

within the plant root zone(mm).AWC = available water capacity of the soil,

(mm of H2O/mm of soil)Rd = depth of the plant root zone, (mm)If different soil layers have different AWC’s,

need to sum up the layer-by-layer TAW’sTAW = (AWC1) (L1) + (AWC2) (L2) + . . .

(AWCN) (LN) - L = thickness of soil layer, (mm) - 1, 2, N: subscripts represent each successive

soil layer

Total Available Water (TAW)

Horizontal movement Horizontal movement due to capillaritydue to capillarityVertical movement Vertical movement

due largely to gravitydue largely to gravity

Gravity vs. CapillarityGravity vs. Capillarity

Influencing FactorsInfluencing FactorsSoil textureInitial soil water contentSurface sealing (structure, etc.)Soil crackingTillage practicesMethod of application (e.g., Basin vs. Furrow)Water temperature

Water Infiltration -Water Infiltration - Entry of water into the soil Entry of water into the soil

Cumulative Infiltration Depth vs. TimeFor Different Soil Textures

Infiltration Rate vs. Infiltration Rate vs.

Time Time for Different Soil Texturesfor Different Soil Textures

GravimetricMeasures mass water content (m)Take field samples weigh oven dry weighAdvantages: accurate; Multiple locationsDisadvantages: labor; Time delay

Feel and appearanceTake field samples and feel them by handAdvantages: low cost; Multiple locationsDisadvantages: experience required; Not highly

accurate

Soil Water MeasurementSoil Water Measurement

Neutron scattering (attenuation)Measures volumetric water content (v)Attenuation of high-energy neutrons by hydrogen

nucleusAdvantages:

samples a relatively large soil sphere repeatedly sample same site and several depths accurate

Disadvantages: high cost instrument radioactive licensing and safety not reliable for shallow measurements near the soil surface

Dielectric constantA soil’s dielectric constant is dependent on soil

moistureTime domain reflectometry (TDR)Frequency domain reflectometry (FDR)Primarily used for research purposes at this time

Soil Water Measurement Soil Water Measurement (cont..)(cont..)

Soil Water Measurement Soil Water Measurement (cont..)(cont..)

Neutron AttenuationNeutron Attenuation

TensiometersMeasure soil water potential (tension)Practical operating range is about 0 to 0.75 bar

of tension (this can be a limitation on medium- and fine-textured soils)

Electrical resistance blocksMeasure soil water potential (tension)Tend to work better at higher tensions (lower

water contents)Thermal dissipation blocks

Measure soil water potential (tension)Require individual calibration

Soil Water Measurement Soil Water Measurement (cont..)(cont..)

Porous Ceramic Tip

Vacuum Gauge (0-100 centibar)Vacuum Gauge (0-100 centibar)

Water ReservoirWater Reservoir

Variable Tube Length (12 in- 48 in) Based on Root Zone Depth

Tensiometer for Measuring Soil Water PotentialTensiometer for Measuring Soil Water Potential

Electrical Resistance Blocks & MetersElectrical Resistance Blocks & Meters

Chapter-4Chapter-44. Irrigation water requirement and scheduling4. Irrigation water requirement and scheduling

4.1. Introduction• The irrigation system is usually not expected to supply all of the

moisture required for maximum crop production. To do so would ignore the valuable contribution of other water sources such as rain and thereby force the irrigation system to be larger and more expensive than necessary.

• In arriving at the contribution an irrigation system will make to an irrigated area, particularly a surface irrigation system, four major factors require consideration. These are:

The concept of water balance in the region encompassing the plant environment

The body of soil supplying moisture, nutrient, and anchorage for the crop and the associated characteristics of this porous medium

The crop water requirements, including drainage for aeration and salt leaching

The efficiency and uniformity of the irrigation system

4.2. Water balance• The employment of a water balance is a useful concept for characterizing,

evaluating, or monitoring any surface irrigation system. A schematic of the water balance parameters used for characterizing a surface-irrigated field is shown in Fig. 4.1. The terms are defined as:

• Da = depth of applied irrigation water

• Ds = depth of change in soil moisture storage in the root zone where Ds is positive for increasing soil moisture storage

• Ddp = depth of deep percolation

• De = depth of evaporation from soil surface or ponded water surface

• Det = depth of evapotranspiration

• Dgw = depth of capillary rise from the groundwater table entering the root zone

• Dp = depth of precipitation

• Dpl = depth of precipitation intercepted by the plants (crop)

• Dpr = depth of precipitation that occurs as surface runoff

• Dpz = depth of precipitation that infiltrates into the soil

• Dt = depth of transpiration from plants

• Dtw = depth of tail water (surface) runoff resulting from overland flow of the irrigation water supply

• Dz = depth of infiltrated water resulting from overland flow of the irrigation water supply

• There are two additional terms that are useful to define at this point:

• Dpn = depth of net precipitation, or the depth of precipitation that is made available to the plant system

• Dd = depth of drainage requirement for maintaining a salt balance in the root zone

• Figure 4.1: The water balance parameters for a surface-irrigated field.

• The principle of continuity requires The principle of continuity requires that inflow (I) minus outflow (O) equals the change in storage (S) within the defined boundaries of a system:

• Of primary concern in surface irrigation are boundaries A, B, C, and D as shown in Fig. 4.1, for which the continuity equation can be written as

• in which

• And

I O S

a gw p et pr tw dp pl sInflows Outflows

D D D D D D D D D

a z twD D D p pz pl prD D D D

et e tD D D

4.3. Crop water requirement

• Crop water requirement is defined as the depth of water the depth of water (mm) needed to raise crop in a given period. (mm) needed to raise crop in a given period.

• It comprises of the water lost through evaporation from crop field, water transpired and metabolically used by crop plants, water lost during application which is economically unavailable and the water used for salt leaching and so on.

• Determination of crop water requirement is used for the planning, design, and operation of both rain fed and irrigated agriculture.

4.4. Evapotranspiration4.4. Evapotranspiration

• The concept of crop evapotranspiration (ETc) is intimately connected with CWR.

• It is defined as the rate of ET (mm/day) of a given crop as influenced by its growth stages, environmental conditions, and crop management.

• To determine the crop ET, the reference evapotranspiration (ET0) is used and it refers to a reference crop cultivated in reference conditions such that its rate of ET (mm/day) reflects the climatic conditions characterizing the local environment.

• The transfer from ET0 to ETc is done by adopting the crop coefficients (Kc), which represents the ratio between the rates of ET of the cultivated crop and of the reference crop, that is,

Kc =ETc/ET0.

• For irrigated crops, another main concept is the irrigation water requirement (IWR), defined as the net depth of water (mm) that is required to be applied to a crop to fully satisfy its specific CWR. The IWR is the fraction of CWR not satisfied by rainfall, soil water storage, and groundwater contribution.

• The rate and amount of ET is the core information needed to design irrigation projects, and it is also essential for managing water quality and other environmental concerns.

• Anyone involved with resource management will likely need to understand the methods available for estimating evapotranspiration.

Figure 4.2: Sub-processes in evapotranspiration

Evaporation essentially occurs on the surfaces of open water such as lakes, reservoirs, or puddles, or from vegetation and ground surfaces. Transpiration involves the removal of water from the soil by plant roots, transport of the water through the plant into the leaf, and evaporation of the water from the leaf interior into the atmosphere.

Evaporation Transpiration

Open water Soil Vegetation surfaces Plants

Transpiration Ratio and Consumptive UseTranspiration Ratio and Consumptive UseTranspiration Ratio and Consumptive UseTranspiration Ratio and Consumptive Use• Two terms frequently used relating to transpiration are

transpiration ratio and consumptive use.

Transpiration ratio is the ratio of the weight of water transpired to the weight of dry matter produced by the plant. This ratio is a measure of how efficiently crops use water.

For example, the transpiration ratios are approximately 900 for alfalfa, 640 for potatoes, 500 for wheat, 450 for red clover, 350 for corn and 250 for sorghum.

The least efficient crop, in terms of water use, would be alfalfa because it uses 900 kilograms of water for every kilogram of dry alfalfa it produces. Sorghum is the most efficient crop listed because it uses only 250 kg of water for every kg of dry matter produced.

• Consumptive use is the total amount of water needed to grow a crop (the sum of the water used in evapotranspiration plus the water stored in the plant's tissues). The term consumptive use is generally used interchangeably with evapotranspiration because the amount of water retained in plant tissue is negligible compared to the amount of evapotranspiration.

4.5 4.5 Potential Evapotranspiration Potential Evapotranspiration 4.5 4.5 Potential Evapotranspiration Potential Evapotranspiration

• Evaporation and especially evapotranspiration are complex processes because the rate of water vapor loss depends on the amount of solar radiation reaching the surface, the amount of wind, the opening of the stomates, the soil water content, the soil type and the type of plant.

• In order to simplify the situation, researchers have attempted to remove all the unknowns such as aperture of the stomates and soil water content, and focus on climatic conditions.

• The simplified calculations are termed potential evaporation and potential evapotranspiration. The definition given by Jensen et al. (1990) for potential evaporation (Ep) is the "...evaporation from a surface when all surface-atmosphere interfaces are wet so there is no restriction on the rate of evaporation from the surface.

• The magnitude of Ep depends primarily on atmospheric conditions and surface albedo but will vary with the surface geometry characteristics such as aerodynamic roughness". Surface albedo is the proportion of solar radiation which is reflected from a soil and crop and crop surface.

• Potential evapotranspiration (ETp)

was originally defined as " the amount of water transpired in unit time by a short green crop, completely shading the ground, of uniform height and never short of water."

• The crop is assumed to be short and uniform, and completely shading the ground so that no soil is exposed. The crop is never short of water, so soil water content is no longer a variable and presumably (most probably) the stomates would always be fully open. These conditions theoretically provide the maximum evapotranspiration rate based on the given climatic conditions.

• Many authors treat potential evapotranspiration and potential evaporation as synonymous but the original intent was that potential evapotranspiration involved on actively growing crop and potential evaporation did not.

4.6. Actual Evapotranspiration4.6. Actual Evapotranspiration• Potential evapotranspiration and evaporation may be easier to estimate

but do not represent reality. In general, watersheds are not entirely covered by well-watered short green crops.

• Actual evapotranspiration (ET) or actual evaporation (E) is the amount or rate of ET occurring from a place of interest and it is the value we want to estimate. In practice, actual ET is obtained by first calculating potential evapotranspiration and then multiplying by suitable crop coefficients to estimate the actual crop evapotranspiration.

• Crop coefficients are usually residual terms from a statistical analysis of field data, so it is essential that the methods for estimating potential evapotranspiration be consistent with the crop coefficients.

• some studies have used alfalfa and some have used grasses to measure potential evapotranspiration. Other methods correlate evaporation from free water surfaces to actual ET. The result is that the methods for determining actual ET are variable and confusing. Scientists have attempted to remedy this problem by introducing reference crop evapotranspiration.

4.7. Reference Crop Evapotranspiration

• Reference crop evapotranspiration (ETr) is defined as "...the rate at which water, if available, would be removed from the soil and plant surface of a specific crop, arbitrarily called a reference crop”.

• Typical reference crops are grasses and alfalfa. The crop is assumed to be well-watered with a full canopy cover (foliage completely shading the ground). The major advantage of relating ETr to a specific crop is that it is easier to select consistent crop coefficients and to calibrate reference equations in new areas.

4.8. Measuring Evaporation or Evapotranspiration4.8. Measuring Evaporation or Evapotranspiration4.8. Measuring Evaporation or Evapotranspiration4.8. Measuring Evaporation or Evapotranspiration• There are several methods available for measuring evaporation or

evapotranspiration.

• Since vapor flux is difficult to measure directly, most methods measure the change of water in the system. Figure 4.3 shows schematically options available for measuring potential evaporation, potential evapotranspiration, or actual evapotranspiration.

• An evaporation pan or ET gage can be used to measure potential evapotranspiration. Actual evapotranspiration can be measured in several ways.

• If you need a simpler, less expensive technique, measuring soil water depletion or using a water balance would be possibilities. More precise, but also more complex methods include lysimeters, either weighing or non-weighing, or using an energy balance or mass transfer technique.

Figure 4.3: Options available for measuring potential or actual evapotranspiration

Actual ET

Simpler, less expensive

More complex and expensive

Soil water depletion

Water balance

Lysimeters Energy balance

Mass transfer

Weighing Non-weighing

Percolation Constant water table

Potential ET

Pan evaporation

ET Gage

Soil Water Depletion

Lysimeters

Empirical methods

Soil conservation service modified Blaney-Criddle

FAO Modified Blaney-Criddle

Pan Evaporation

Combination Method

Penman Method

FAO Modified Penman Method

4.9. Depth of Irrigation 4.9. Depth of Irrigation • In planning and managing irrigation, the soil’s capacity to store

available water can be thought of as the soil water reservoir, which must be filled periodically by irrigation or rainfall.

It is slowly depleted by evapotranspiration. Water application in excess of the reservoir capacity is wasted unless it is required for leaching or to meet a specific management need.

Irrigation must be scheduled to prevent the soil water reservoir from becoming so low as to inhibit plant growth.

• The difference between FC and PWP is the available water (AW) and can be estimated from

• Where.AW = (FCv–PWPv)Dr/100

FCv and PWPv = the volumetric field capacity and permanent wilting point percentages, respectively,

Dr = depth of the root zone or depth a layer of soil with in the root zone (L)AW = depth of water available to plants (L)Plants can remove only a portion of the available water before growth and yield are affected. This portion is readily available water (RAW) and for most crops ranges between 40 and 65 percent of AW in the root zone. Readily available water can be estimated from

RAW = (MAD)(AW)In which MAD is the management allowed deficiency or the portion (decimal) of the available water that management determines can be removed from the root zone without adversely affecting yield and/or economic return.

• The supply requirements at the field level are determined by the depth and interval of irrigation. These data can be obtained from the soil water balance and are primarily determined by:

i. the total available soil water (Sa = Sfc – Sw where Sfc and Sw are the soil water content in mm/m soil depth at field capacity and wilting point, respectively);

ii. the fraction of available soil water permitting unrestricted evapotranspiration and/or optimal crop growth; and

iii. the rooting depth (D). The depth of irrigation application (d) including application losses is:

and frequency of irrigation expressed as irrigation interval of the individual field (i) is:

. ..a

a

p s Dd mm

E

. ap s Di days

ET

• where:

p = fraction of available soil water permitting unrestricted evapotranspiration, fraction

Sa = total avalable soil water, mm/m soil depth

D = rooting depth, m

Ea = application efficiency, fraction

• Since p, D and ET will vary over the growing season, the depth in mm and interval of irrigation in days will vary.

• For design and operation of the water distribution system, the requirements of the individual fields will need to be expressed in flow rates or stream size (q in m3/sec) and supply duration (t in seconds, hours or days). The field supply (q.t) is:

310. . . . a

a

q t p s D A mE

where:

q = stream size, m3/sec

t = supply duration, seconds

Ea = application efficiency, fraction

p = fraction of available soil water permitting unrestricted evapotranspiration, fraction

Sa = total available soil water, mm/m soil depth

D = rooting depth, m

A = acreage, ha

In determining the relative values of q and t, the soil intake rate and method of irrigation must be taken into account. For instance, t will be greater for heavy as compared to light soil and also sprinkler and furrow irrigation as compared to basin. Furthermore, the stream size (q) must be handled easily by the irrigator.

• Field irrigation requirements will vary for each crop during the growing season and the supply must follow those changes over area and time. Analysis of the system and selection of the method of supply (continuous, rotation or demand) should therefore start with an evaluation of the field variables.

• Example: A clay soil having an average field capacity (FC) and permanent wilting point (PWP) of 47.2 and 30.1%, respectively at the first 45 cm of the soil profile is planted to corn. If actual evapotranspiration (AET) is 6.2 mm/d, irrigation interval is 1 week and application efficiency is 65%, find the following:

A, Available water in the root zone (AW) if apparent specific gravity (As) is 1.2,

B, Depth of irrigation (DI) based on allowable depletion (MAD) of 50% and application efficiency (Ea) of 65%

C, Depth of irrigation based on consumptive use (AET) and irrigation interval

• Solution:A) Available water in the root zone (AW),

Aw = (FC-PWP)/100 *As*Dr = (47.2-30.1)/100 *1.2*0.45

= 0.092m = 92 mmb) Depth of irrigation (DI) based on allowable depletion (MAD)

Readily available water in the root zone (RAW)RAW = AW*MAD= 92 mm*0.5 = 46mmDI = RAW/Ea= 46mm/0.65 = 71 mm

• c) Depth of irrigation based on consumptive use (AET) and irrigation interval DI = AET*I/Ea= 6.2mm/d*7d/0.65 = 67mm

4.10. Crop Coefficient Curves 4.10. Crop Coefficient Curves

• FAO Crop Coefficient Method

• The FAO methods, all modifications of the Penman equation, and many other evapotranspiration estimating methods result in an evapotranspiration estimate for a reference surface of water or reference crop of grass or alfalfa. To determine water use for a crop other than the reference, crop coefficients must be applied. This is demonstrated by the following equation:

ETc = KcETr

where

ETc = crop evapotranspiration

Kc = crop coefficient

ETr = reference evapotranspiration

• the Kc values have to be with respect to the same crop as the ETr values used in the equation.

• The FAO method divides the crop coefficient curve into four linear line segments which approximate the curve. The first step is to divide the growth season into the following growth periods:

1. Initial period1. Initial period: : Time of planting to time of 10 percent ground cover.

2. Crop development period2. Crop development period: : From end of initial period to time of effective full cover – that is, 70 to 80 percent ground cover.

3. Mid-season period3. Mid-season period: : From end of crop development period to start of plant maturity as indicated by leaf discoloration (e.g., beans, maize), leaves falling off (e.g., cotton), or leaves curling and discolouring (e.g., tomatoes, potatoes).

4. Late season period4. Late season period: : From end of mid-season period to time of full maturity or harvest.

• After the season has been divided into the four growth periods, coefficients must be determined corresponding to the initial, mid- and late season periods to allow plotting of the total seasonal Kc function. The initial period crop coefficient, Kci, is related to the evaporation from basically a bare soil. This coefficient is a function of soil wetness and the reference evapotranspiration rate during the initial period.

Fig. Generalised crop coefficient curve

Irrigation Methods are mainly classified into :

1. Surface Irrigation or Gravity Irrigation 2. Subsurface Irrigation or Sub-irrigation 3. Sprinkler or overhead irrigation 4. Drip or Trickle irrigation

◦

1) SURFACE IRRIGATION Irrigation water flows across the field to the point

of infiltration Primarily used for field crops and orchards Water is applied to the soil surface and the water

flows by gravity either through furrows, strips or basins.

Water is applied from a channel located at the upper reach of the field.

Loss of water by conveyance and deep percolation is high and the efficiency of irrigation is only 40-50% at field level in surface method of irrigation.

Properly constructed water distribution systems to give sufficient control of water to the fields

And effective land preparation to permit uniform distribution of water over the field are very important.

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. Low initial cost of development is later offset by high

labour cost of applying water. Deep percolation, runoff and drainage problems

1. Furrow irrigation

2. Border irrigation

3. Basin irrigation

Furrow irrigation - in which the water poured on the field is directed to flow through narrow channels dug between the rows of crops, instead of distributing the water throughout the whole field evenly.

The furrows must all have equal dimensions, in order to guarantee that the water is distributed evenly.

Like flood irrigation, furrow irrigation is rather cheap in areas where water is inexpensive.

In furrow irrigation, only a part of the land surface (the furrow) is wetted thus minimizing evaporation loss.

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.

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.

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.

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.

In this, Parallel ridges are made to guide a sheet of flowing water when the water moves down the slope.

The field is divided into several long parallel strips called borders that are separated by low ridges.

Field should be even surface over which the water can flow down the slope with a nearly uniform depth.

Every strip is independently irrigated by turning a stream of water at the upper end.

Then water spreads and flows down the strip in a thin sheet.

Water moves towards the lower end without erosion covering the entire width of the border.

Sufficient moisture is provided to the soil to entire length of the border.

Border method is suitable for most of the soils, while it is best suited for soils having moderately low to high infiltration rates.

However, it is not suitable for course sandy and clay textured soils.

Border Irrigation SystemBorder 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 SystemBorder Irrigation System

Border IrrigationBorder Irrigation

Border Irrigation Contd.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.

 

Basin method of irrigation is adopted mainly in orchards.

Usually round basins are made for small trees and square basin for large trees.

These basins allow more water to be impounded as the root zones of orchard plants are usually very deep.

Each basin is flooded and water is allowed to infiltrate into the soil.

Based on type of crop and soil, nearly 5-10 cm depth of water may be needed for every irrigation.

The advantage of basin method is that unskilled labour can be used as there is no risk of erosion.

Disadvantages : there is difficulty in using modern machinery and it is also labour intensive.

Basin irrigation is suitable for many field crops. Rice grows best when its roots are submerged

in water and so basin irrigation is the best method to use for this crop.

Other crops which are suited to basin irrigation include:

Pastures, e.g. alfalfa, clover; Citrus, banana; Crops which are broadcast, such as cereals, and To some extent row crops such as tobacco.

Basin Irrigation DiagramBasin Irrigation Diagram

I

rrigation time.

Size of BasinsSize of Basins The size of basin is related to stream size and soil type(See

Table below). Table : 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................................ 30 108 0.02 0.06 0.12 0.20 60 216 0.04 0.12 0.24 0.40 90 324 0.06 0.18 0.36 0.60 120 432 0.08 0.24 0.48 0.80 150 540 0.10 0.30 0.60 1.00 180 648 0.12 0.36 0.72 1.20 210 756 0.14 0.42 0.84 1.40 240 864 0.16 0.48 0.96 1.60 300 1080 0.20 0.60 1.20 2.00 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.

Most common among surface irrigation Suitable for close growing crops like

groundnut, wheat, finger millet, pearl millet, paragrass etc.

In this method field is divided into small plots surrounded by bunds on all four sides.

Water from head channel is supplied into the field channel one after the other.

Each field channel supplies water to two rows of check basins and water is applied to one basin after other.

In this, field is laid out into long, narrow, strips, bordering with small bunds.

Most common size of strips are 30-50 m length and 3-5 m width.

Borders are laid out along the general slope.

Water from the channel is allowed into each strip at a time.

This method is suitable for close growing crops and medium to heavy textured soils.

Not suitable for sandy soils.

It should be applied only to flat lands that do not concave or slope downhill so that the water can evenly flow to all parts of the field.

Yet even so, about 50% of the water is wasted and does not get used by the crops.

Some of this wasted water accumulates at the edges of a field and is called run-off.

In order to conserve some of this water, growers can trap the run-off in ponds and reuse it during the next round of flood irrigation.

In flood irrigation, a large amount of water is brought to the field and flows on the ground among the crops.

In regions where water is abundant, flood irrigation is the cheapest method

This low tech irrigation method is commonly used by societies in developing countries.

However a large part of the wasted water can not be reused due to massive loss via evaporation and transpiration.

One of the advantages of flood irrigation is its ability to flush salts out of the soil, which is important for many saline intolerant crops.

However, the flooding causes an anaerobic environment around the crop which can increase microbial conversion of nitrogen from the soil to atmospheric nitrogen, or denitrification, thus creating low nitrogen soil.

Surge flooding is an attempt at a more efficient version of conventional flood irrigation in which water is released onto a field at scheduled times, thus reducing excess run-off.

- Irrigation to crops by applying water from beneath the soil surface either by constructing trenches or installing underground perforated pipe lines.

In this system, water is discharged into trenches.

And allowed to stand during the whole period of irrigation for lateral and upward movement of water by capillarity to wet the soil between the trenches.

Conditions that favor subsurface irrigation An impervious subsoil at a depth of 2 m or more. A very permeable subsoil of reasonably uniform texture

permitting good lateral and upward movement of water. Permeable loam or sandy loam surface soil. Uniform topographic conditions and moderate slope. Existence of high water table. Irrigation water is scarce and costly. Soils should be free of any salinity problem. It must be ensured that no water is lost by deep

percolation. Subsurface irrigation is made by constructing a series of

ditches or trenches 60 to 100 cm deep. Width of the trenches is about 30 cm and vertical. Spacing between the trenches varies between 15 to 30

m depending on soil types and lateral movement of water in soils.

Various types of crops, particularly with shallow root systems are well adapted to subsurface irrigation system.

Wheat, potato, beet, peas, fodder crops etc.

Advantages Maintenance of soil water at favorable

tension Loss of water by evaporation is held at

minimum Can be used for soils with low water holding

capacity and high infiltration rate where surface irrigation methods cannot be adopted and sprinkler irrigation is expensive.

Presence of high water table. Poor quality irrigation water cannot be used-good quality water must be available.

Chances of saline and alkali conditions being developed by upward movement of salts with water.

Soils should have a good hydraulic conductivity for upward movement of water.

Sprinkler irrigation is a method of applying irrigation water which is similar to natural rainfall.

Water is distributed through a system of pipes usually by pumping.

Water under pressure is carried and sprayed into the air above the crop through a system of:

Overhead perforated pipes, nozzle lines, or through nozzles fitted to riser pipes attached to a system of pipes laid on the ground.

Nozzles of fixed type or rotating under the pressure of water are set at suitable intervals in the distribution pipes.

Sprayed water wets both the crop and the soil and, hence, has a refreshing effect.

Water is applied at a rate less than the intake rate of soil so that there is no runoff.

Measured quantity of water is applied to meet the soil water depletion.

Sprinkler irrigation is suited for most row, field and tree crops and water can be sprayed over or under the crop canopy.

Large sprinklers are not recommended for irrigation of delicate crops such as lettuce because the large water drops may damage the crop.

Suitable slopes Sprinkler irrigation is adaptable to any farmable

slope, whether uniform or undulating. Lateral pipes supplying water to the sprinklers

should always be laid out along land contour. This will minimize the pressure changes at the

sprinklers and provide a uniform irrigation.

Sprinklers are best suited to sandy soils with high infiltration rates although they are adaptable to most soils.

Application rate from the sprinklers (in mm/hour) is always chosen to be less than the basic infiltration rate of the soil - so that surface ponding and runoff can be avoided.

Sprinklers are not suitable for soils which easily form a crust.

A typical sprinkler irrigation system consists of the following components:

Pump unit Mainline Laterals Sprinklers   Suitable irrigation water A good clean supply of water, free of

suspended sediments, to avoid problems of sprinkler nozzle blockage and spoiling the crop by coating it with sediment.

Components of Sprinkler Irrigation Components of Sprinkler Irrigation

136

Sprinkler irrigation: Criteria

• Must permit cost recovery within one to two years (and double investment in a short time)

• Must be suitable for use on small and irregular shaped plots

• Must require only simple maintenance and tools

• Have a low risk of component failure

• Be simple to operate

• Be durable and reliable – able to withstand rough and frequent handling without serious damage

137

Sprinkler irrigation: System layout

138

Sprinkler irrigation

139

Sprinkler irrigation: Drag hose system

140

Sprinkler

141

Sprinkler irrigation: Spray pattern

142

Sprinkler irrigation: Spray pattern

143

Sprinkler Spray pattern: Variation in pressure

144

Sprinkler irrigation: Variation in pressure

145

Sprinkler irrigation: Hand move laterals

146

Sprinkler irrigation: Drag hose system

147

Sprinkler irrigation: Centre pivot system

148

Sprinkler irrigation: Centre pivot system

149

Sprinkler irrigation: Centre pivot system

150

Sprinkler irrigation: Linear move system

151

Sprinkler irrigation: Linear move system

152

Sprinkler irrigation: Linear move system

153

Sprinkler irrigation: Mobile Raingun

154

Sprinkler irrigation: Mobile Raingun

Raingun Irrigation SystemRaingun Irrigation System

Linear MoveLinear Move

Lateral DischargeLateral DischargeThe 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)

Pressure at Head of LateralPressure at Head of Lateral

The Pressure requirements (PL)where the Lateral joins the Mainline or Submain are determined as follows:

PL = Pa + 0.75 Pf + Pr For laterals laid on Flat land

PL = Pa + 0.75 (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.

Pumping RequirementsPumping 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.

DRIP OR TRICKLE IRRIGATIONDRIP 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 1 - 12 l/hr.

(vi) There is reduced evaporation, only potential transpiration is considered.

vii) There is no need for a drainage system.

Drip irrigation / trickle irrigation - involves dripping water onto the soil at very low rates (2-20 litres/hour)

-from a system of small diameter plastic pipes fitted with outlets called emitters or drippers.

Water is applied close to plants so that only part of the soil in which the roots grow is wetted (Figure 60 in Notes).

With drip irrigation water, applications are more frequent (usually every 1-3 days).

This provides a very favourable high moisture level in the soil in which plants can flourish.

162

Drip irrigation: Layout

163

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.

164

Micro irrigation: Root zone

165

Micro irrigation: Emitters

166

Micro irrigation: Thick walled drip hose

While drip irrigation may be the most expensive method of irrigation, it is also the most advanced and efficient method in respect to effective water use.

Usually used to irrigate fruits and vegetables System consists of perforated pipes that are placed by rows of

crops or buried along their root lines and emit water directly onto the crops that need it.

As a result, evaporation is drastically reduced and 25% irrigation water is conserved in comparison to flood irrigation.

Drip irrigation also allows the grower to customize an irrigation program most beneficial to each crop.

Fertigation is possible. Caution : Water high in salts / sediments should be filtered -

otherwise they may clog the emitters and create a local buildup of high salinity soil around the plants if the irrigation water contains soluble salts.

Drip irrigation is most suitable for row crops (vegetables, soft fruit), tree and vine crops where one or more emitters can be provided for each plant.

Generally only high value crops are considered because of the high capital costs of installing a drip system.

 

Drip irrigation is adaptable to any farmable slope.

Normally the crop would be planted along contour lines and the water supply pipes (laterals) would be laid along the contour also.

This is done to minimize changes in emitter discharge as a result of land elevation changes.

Drip irrigation is suitable for most soils.

On clay soils water must be applied slowly to avoid surface water ponding and runoff.

On sandy soils higher emitter discharge rates will be needed to ensure adequate lateral wetting of the soil.

One of the main problems with drip irrigation is blockage of the emitters.

All emitters have very small waterways ranging from 0.2-2.0 mm in diameter and these can become blocked if the water is not clean.

Thus it is essential for irrigation water to be free of sediments.

]If this is not so then filtration of the irrigation water will be needed.

Blockage may also occur if the water contains algae, fertilizer deposits and dissolved chemicals which precipitate such as Ca and Fe.

Filtration may remove some of the materials but the problem may be complex to solve and requires an experienced professional.

SOIL TYPE AND WATER MOVEMENT. THE APPLICATION OF WATER IS BY DRIPPERS

172

A typical drip irrigation system is shown in Figure 61 and consists of the following components:

Pump unitControl headMain lineLateralsEmitters or drippers. 

Pump unit takes water from the source and provides the right pressure for delivery into the pipe system.

The control head consists of valves to control the discharge and pressure in the entire system.

It may also have filters to clear the water. Common types of filter include screen filters and

graded sand filters which remove fine material suspended in the water.

Some control head units contain a fertilizer or nutrient tank.

These slowly add a measured dose of fertilizer into the water during irrigation.

This is one of the major advantages of drip irrigation over other methods.

Supply water from the control head into the fields. They are usually made from PVC or polyethylene hose

and should be buried below ground because they easily degrade when exposed to direct solar radiation.

Lateral pipes are usually 13-32 mm diameter. Emitters or drippers are devices used to control the

discharge of water from the lateral to the plants. They are usually spaced more than 1 metre apart with

one or more emitters used for a single plant such as a tree.

For row crops more closely spaced emitters may be used to wet a strip of soil.

Many different emitter designs have been produced in recent years.

The basis of design is to produce an emitter which will provide a specified constant discharge which does not vary much with pressure changes, and does not block easily.

The water savings that can be made using drip irrigation are the reductions in deep percolation, in surface runoff and in evaporation from the soil.

These savings, it must be remembered, depend as much on the user of the equipment as on the equipment itself.

Drip irrigation is not a substitute for other proven methods of irrigation.

It is just another way of applying water. It is best suited to areas where water quality is

marginal, land is steeply sloping or undulating and of poor quality, where water or labour are expensive, or where high value crops require frequent water applications.

Water Use for Trickle Irrigation Water Use for Trickle Irrigation SystemSystem Contd. 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.

EmittersEmitters

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.

Water Distribution from EmittersWater Distribution from Emitters

Emitter discharge variability is greater than that of sprinkler nozzles because of smaller openings(lower flow) and lower design pressures.

Eu = 1 - (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.

Pressure Head at Manifold InletPressure 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 (0.51 + 0.68) + Riser Height = Zero for Trickle since no risers

exist. + Elevation difference = Zero , since the field is

Level = 9.79 m

Solution ConcludedSolution Concluded Total Head for Pump = Manifold Pressure = 9.79 m + Pressure loss at Sub-main = 6.59 m + Pressure loss at Main = 2.90 m + Suction Lift = 20 m + Net Positive Suction head for pump = 4 m

(assumed) = 43.28 m i.e. The Pump must deliver 3.23 L/s at a head of

about 43 m.

SUB-SURFACE IRRIGATIONSUB-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 SUB-SURFACE IRRIGATION (Contd…)(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.