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To decide possible cropping pattern of area

Effective use of available water

Plan and design an irrigation project

Plan water resource development in an area

Assess irrigation requirement of an area

Management of water supply from sources

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Crop water requirement (CWR): It is the total amount of water required by the crop in a given

period of time for normal growth, under field conditions.

It includes; evapotranspiration, water used by crops for metabolic growth, water lost during conveyance and application of water and water required for special operations such as land

preparation, tillage and salt leaching etc.

It is expressed as the surface depth of water in mm, cm or inches.

CWR = Consumptive use (Cu) +

Conveyance losses (Wu) +

Water required for special operation (Ws)3

Approximate daily water use and total growing season water use in millimetres (mm) for some commonly grown crops in

AlbertaSource: http://www1.agric.gov.ab.ca/%24department/deptdocs.nsf/all/agdex12726 4

The crop water requirement mainly depends on: the climate: in a sunny and hot climate, crops need more water

per day than in a cloudy and cool climate

the crop type: crops like maize (Makei) or sugarcane (ganna) need more water than crops like millet (Bajra) or sorghum

the growth stage of the crop: fully grown crops need more water than crops that have just been planted.

Moreover, there are short duration crops, e.g. peas, with a duration of the total growing season of 90-100 days and longer duration crops, e.g. melons, with a duration of the total growing season of 120-160 days

Climatic factor Crop water requirementHigh Low

Sunshine Sunny (no clouds) cloudy (no sun)Temperature hot cool

Humidity low (dry) high (humid)Wind speed windy little wind

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Conveyance Losses: These losses take place from diversion structure (barrage) to

the field (outlet). Major loss of water in an irrigation channel is due to absorption,

seepage or percolation and evaporation. In an earthen channels losses due to seepage are much more

than the losses due to evaporation. The absorption losses depend upon the:

Type of soil Subsoil water Age of canal Position of Full Supply Level w.r.t to Natural Surface Level Amount of Silt carried by canal Wetted perimeter

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 Irrigation water losses in the fieldIrrigation water losses in canals

1. Evaporation from the water surface, 2. Deep percolation to soil layers underneath the canals, 3. Seepage through the bunds of the canals, 4. Overtopping the bunds5. Bund breaks, 6. Runoff in the drain7. Rat holes in the canal bunds

1. Surface runoff, whereby water ends up in the drain2. Deep percolation to soil layers below the root zone

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In Pakistan the following formula can be used for obtaining the conveyance losses in earthen channels

K= 5.0Q0.625

K= absorption loss per million square feet of wetted perimeter

Q= Discharge in channel (cusecs).   According to Lacey

QA=0.0133 L Q0.5625

QA= Absorption loss L= Length of channel in thousand feet Q= discharge in channel (cusecs)

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Effective Precipitation (ER): It is that part of total precipitation which is used by crop as soil water

reserve. It is the precipitation falling during the growing period of a crop that

is available to meet the evapotranspiration needs of the crop. It is determined as:ER = Total rainfall (P) – Runoff (R) – deep percolation (PW)

Gross Irrigation requirements of crops (IRg): It refers to the amount of water applied to the field from the start of

land preparation to harvest of the crop together with the water lost through distributaries, field channels and during water application to the crop field.

IRg = CWR – (ER + ∆GW+∆SW ) Where Ground Water Contribution for Crop Use (∆GW) Soil Water Contribution for Crop Use (∆SW)

Ground Water Contribution for Crop Use (∆GW): It refers to the water used by crops due to capillary rise in case of

shallow water tables.

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Soil Water Contribution for Crop Use (∆SW): It refers to the difference in moisture content at the

time of sowing and harvesting of the crops that may be positive or negative. It is given as:

Where: ∆SW = soil water contribution in cm Msi = moisture content at the time of sowing in the ith layer, % Mhi = moisture content at the time of harvesting in the ith layer,

% Asi = Apparent specific gravity of soil (The specific gravity of a

porous solid when the volume used in the calculations is considered to exclude the permeable voids)

Di = depth of ith layer of the root zone soil, cm

Net Irrigation requirements It refers to the amount of water needed to replenish/fill soil

moisture deficit in the crop field.IRn = IRg x Efficiency of water

application = Cu – ER - ∆SW

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Consumptive use (CU): It is the amount of water required by a crop for its

vegetated growth to evapotranspiration and building of plant tissues plus evaporation from soils and intercepted precipitation.

It is expressed in terms of depth of water

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.

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Daily consumptive use: The amount of water consumptively used during 24-

hours.

It is usually estimated to record the peak period consumptive use rates to

formulate the cropping pattern and to decide the water supply from sources during different

periods of cropping.

Peak period consumptive use: It is the average daily consumptive use during a few

days (6 to 10 days) of highest consumptive use in a season.

It occurs when the vegetation is abundant, temperature is high and the crops are in flowering stage.

It is used in the planning of an irrigation system

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Seasonal consumptive use: It is the amount of water consumptively used by crops

during the entire cropping season/period. It is used to evaluate and decide the seasonal water

supply to a command area of an irrigation project.

Rabi Season (October to March):Crop Consumptive

Use (cm) Wheat 37 Gram 30 Barley 30 Potato 60-90 Sugar cane 90 Fodder 40 Oil seed 45 Berseen 70

Kharif Season (April to September):Crop Consumptive

Use (cm)Cotton 25-40Maize 45Rice 125-150Sugar Cane 90

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Evapotranspiration: It is defined as the water transpired by crop plants and

the water evaporated from the soil in the crop field and intercepted precipitation by areal parts of plants in any specified time period

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Potential/reference crop evapotranspiration (ETo): This is the evapotranspiration rate from a reference surface

(crop) which is not short of water.

The reference surface is a hypothetical grass reference crop with an assumed crop height of 0.12m, a fixed surface resistance of 70sm-1 and an albedo of 0.23. 

The reference surface closely resembles an extensive surface of green, well-watered grass of uniform height, actively growing and completely shading the ground.

Potential/reference crop evapotranspiration (ETo)

15Surface resistance describes the resistance of vapour flow through the transpiring crop and evaporating soil surface

Actual crop evapotranspiration (ETc): It is the rate of evapotranspiration by a particular crop

in a given period under prevailing soil water and atmospheric conditions.

It refers to the evapotranspiration from a disease free crop growing in a large field under optimal soil conditions with adequate water and fertility and giving full potential production under the given environment.

It is usually calculated by multiplying the Crop Coefficient (Kc) with ETo, thus:

ETc = Kc. ETo

Actual crop evapotranspiration (ETc) 16

Climatic factors:

Precipitation, with greater frequency and amount of rainfall, ET becomes higher.

Solar radiations, it supplies energy for ET processes.

With increasing day length or solar radiation, ET becomes more.

Temperature, Temperature of plant and soil rises because of more amount of solar radiation received from the sun and consequently increases ET.

Wind speed, ET from soil surface and plants occurs at a

higher rate on a windy day. The moist air in the immediate vicinity of a moist soil or leaf surface is swept away by wind and the dry air occupies the space.

Relative humidity, ET varies inversely with the atmospheric humidity

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Growing season: Length of growing season and the actual date of sowing

and maturing are important factors. The growing season of a crop coinciding with the hotter part of the year is expected to increase ET. Crops grown in different seasons have different ET.

Crop characteristics: Growth habit, canopy development, leaf area index,

plant density, duration and time of year when the growth is made, are important consideration to study the effect of crop characteristics on ET.

Soil characteristics: Hydraulic conductivity and water holding capacity of soil

affect ET.

Cultural Factors: Irrigation frequency, method of irrigation, depth of

irrigation, fertilizer application and mulching are the important cultural factors affecting ET. Mulching is covering of soil due to rotten vegetable matters

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Crop coefficient: It is the ratio b/w the actual crop evapotranspiration to

the reference crop evapotranspiration.

Kc = ETc / ETo

It is determined experimentally for various crops.

ETc is determined by Lysimeter technique and ETo is determined with USWB class A evaporation pan.

Kc is different for different crop and for different crop growth stages.

It is mainly affected by crop type, soil type and climate of the area.

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Water is essential to crop plants for their growth and development.

Amount of water required by the crops is influenced by the soil type.

Soil water plant relationship is a process that requires to be regulated for maximization of yields with a given unit of water.

An understanding of this relationship is essential in order that water management principles are applied to various climate, soil and cropping regions of both rain-fed and irrigated lands.

To understand this relationship, the concept of soil water/moisture and field capacity is essential.

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Gravitational moisture: When the water falls over the ground, a part of it gets

absorbed in the root zone, and the rest flows downwards under the action of gravity, and is called as gravitational moisture.

Field Capacity: Immediately after the rain or irrigation water application,

when all the gravity water has drained down, a certain amount of water is retained on the surface of soil grains by molecular attraction and by loose chemical bonds (adsorption). This water cannot be drained under the action of gravity and is called the field capacity.

Field capacity is very important because it is the water which is available in the soil for crop use

The total field capacity water is not used by the crops. The plants can extract water from the soil till the permanent wilting point is reached. 22

Fields capacity is further divided into two types: 1. Capillary moisture:

It is that moisture which is attached to the soil molecules by surface tension against gravitational forces and which can be extracted by crop through capillarity.

2. Hygroscopic moisture: It is that moisture which is attached to the soil

molecules by loose chemical bond and it is not available to the plants for use (adsorption).

Permanent wilting point: It is moisture content at which plant can no longer

extract sufficient water for its growth and wilts up.

Available moisture: It is the difference in moisture content between field

capacity and permanent wilting point.

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Moisture Content (%)

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Crop Period: It is the time normally in days that a crop takes from the

instance of its sowing to harvesting. Base period: It is the time between first watering of crops at the time of

its sowing and the last watering of crops before harvesting

Delta of crops: Total depth of water required by the crop in unit area during

base period. In other words it is the total depth of water required for maturing the crop.

Volume of water required by the crop = Delta x Area or

Delta = Volume (acre-ft) / Area (acres)

Duty of irrigation water: It is defined as the no. of hectares (acres) of land irrigated

for full growth of a given crop by supply of 1 m3/sec (1 ft3/sec) of water continuously during the entire base period.

Crop Total growing period (days)

Crop Total growing period (days)

Alfalfa 100-365 Melon 120-160

Barley/Oats/ Wheat

120-150 Millet 105-140

Bean, green 75-90 Onion, green 70-95

dry 95-110 dry 150-210

Citrus 240-365 Pepper 120-210

Cotton 180-195 Rice 90-150

Grain/small 150-165 Sorghum 120-130

Lentil 150-170 Soybean 135-150

Maize, sweet 80-110 Squash 95-120

grain 125-180 Sunflower 125-130

Let there be a crop of base period B days.

Now the volume of water applied to this crop during B days @ 1 m3/sec = V = 1 x 60 x 60 x 24 x B) m3 = 86400 B

By definition of duty, D, is the Area in hectare (10000m2) irrigated by 1m3/S

1 m3/S of water supplied for B days irrigates D hectares (104 m2) of land.

Therefore, total depth of water required by crop per unit area (Delta)

= Volume/Area = 86400B/104D

Hence,

Delta = ∆ = 8.64 B / D (meters)Delta = ∆ = 864 B / D (centimeters)

Example: find the delta of a crop when its duty is 864 hectare/cumecs with base period of 120 days.

Full Supply Factor/(Duty): The term duty is only used for existing or running projects,

but in a proposed project it is known as full supply factor.

Intensity of Irrigation: Percentage of culturable area irrigated during a base period

or annually

Cropping Intensity/cultivation intensity: It is to the %age of area of a particular crop with respect to

culturable command area

Cropping Pattern: It means how many crops and how much area for a crop is

being cultivated.

Water Allowance:  is antonymous of duty. It is the discharge in cusec required to irrigate 1000 Acres of an area and is expressed in cusec/1000 acres (or in cumec/100 ha) at outlet head, distributory head or main canal head

Water Conveyance Efficiency: It is the ratio of the water delivered to the farmer by conveyance system to the water introduced into the canal at source.

Gross Command Area (GCA): is the total amount of area which needs to be irrigated. It also includes the area which cannot be cultivated e.g., villages, roads, utility etc.

Culturable Command Area (CCA): is the effective area which is culturable or the area that is cultivated out of gross command area.

Non-Culturable Command Area (NCCA): It is the area which is not cultivated.

CCA=GCA – Non-Culturable Command Area

Find out the capacity of the reservoir if its culturable area is 65000 ha, from the following data

B=BASE PERIODD=DUTY

These methods are classified into three types: Direct methods

Lysimeter method Field experimentation method Soil water depletion method Inflow-outflow method

Pan evaporimeter method USWB class-A pan evaporimeter

Empirical methods Blaney-criddle method Penman method Modified penman method Radiation method Penman Monteith equation

Source: Irrigation water management: principles and practice By D. K. Majumdar

Lysimeter method: Used to measure ET and various components of water balance

It is a container (usually 0.5m – 2m in diameter) having an experimental soil separated from the surrounding soil in the crop field

Lysimeter are installed in fields with a large guard area having the same crop as in the lysimeter

Measurements of different components for water balance studies such as water added to lysimeter through precipitation and irrigation, change in soil water storage and water lost through evaporation, transpiration, runoff and deep percolation are made,

By recording the amount of precipitation that an area receives and the amount lost through the soil, the amount of water lost to evapotranspiration can be calculated.

The general relationship to estimate ET is :

Lysimeters are so constructed that measurements of deep percolation and surface runoff are possible or it is possible to avoid these losses

Both weighing and non weighing type lysimeters are used for measurement of ET

For very short period (daily or hourly) estimates of ET, weighing type lysimeter is used

Field experimentation method: Field experiments with varying level of irrigation are

carried out to estimate seasonal consumptive use of irrigated crops

Measurement of water supplied to the crops through effective rainfall and irrigation and changes in the soil moisture reserves during the growing season are made

The water, thus, supplied under varying levels of irrigation is then correlated with the yields obtained

The quantity of water used to produce most profitable yield is taken as CU

Field experimentation method:

Soil water depletion method: Soil water contents in different layers of root zone are

measured just before and after irrigation or rainfall and during the period between two successive irrigations as frequently as possible depending upon the degree of accuracy desired.

The soil water depletion during any short period is considered as the consumptive use for that period

The seasonal consumptive use is obtained by summing up soil water depletion or losses during the different periods of measurement in the growing season

Inflow-outflow method: It is used to estimate yearly consumptive use over

large area It is also called as water balance method

Change in soil water storage is considered negligible and it is assumed that the subsurface inflow into the area is same as subsurface outflow

USWB class-A pan evaporimeter:

There exist a close relationship between the rate of consumptive use by crop and the rate of evaporation from properly located pan evaporimeter.

Pan evaporation is the combined effect of all atmospheric factors and is independent of plant and soil factors

Crop evapotranspiration rates for various crops may be estimated from the pan evaporation rates multiplied by a factor known as crop factor (Kcrop) which varies with the stages of growth, extent of ground cover with foliage, climate and geographical locations

It is the most widely used evaporimeter for finding evaporation from the free water surface

The Class A Evaporation pan is circular, 120.7 cm in diameter and 25 cm deep. It is made of galvanized iron (22 gauge) with a stilling pan

The pan is mounted on a wooden open frame platform which is 15 cm above ground level to facilitate the circulation of air beneath the pan

Daily evaporation rate is given by the fall in water level measured in the stilling well by hook gauge

Adjustments are made to the evaporation values if rainfall occurs during a period of measurement

After measuring the drop in water level each time, water is added to the pan to bring back the water level to original position of pointer tip level

The relationship between potential evapotranspiration and pan evaporation is given as:

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Blaney criddle method

Penman method

Modified penman method

Radiation method

Penman Monteith equation