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Crop Water Requirement

Agriculture and Rural Development (Phase-I/II) Water Resource

Crop water requirements (CWR) are defined as the depth of water [mm] needed to meet the water consumed through evapotranspiration by a disease-free crop, growing in large fields under non-restricting soil conditions including soil water and fertility, and achieving full production potential under the given growing environment.

=>Crop water requirement is the water required by the plants for its survival, growth, development and to produce economic parts.

=>This requirement is applied either naturally by precipitation or artificially by irrigation.

Hence the crop water requirement includes all losses like:

a) Transpiration loss through leaves (T)
b) Evaporation loss through soil surface in cropped area (E)
c) Amount of weather used by plants (WP) for its metabolsm These three components cannot be separated so easily.

Hence the ET loss is taken as crop water use or crop water consumptive use.

=> d) Other application losses are conveyance loss, percolation loss, runoff loss, etc., (WL).

=> e) The water required for special purposes (WSP) like puddling operation, ploughing operation, land preparation, leaching, requirement, for the purpose of weeding, for dissolving fertilizer and chemical, etc.

Hence the water requirement is symbolically represented as:

WR = T + E + WP + WL + WSP

WR = IR + ER + S

WR = CU + WL + WSP

CU = E + T + WP IR

- Irrigation requirement; ER - Effective rainfall; S - Contribution from ground water table.

ESTIMATING THE WATER REQUIREMENTS OF THE CROP

ET0 represents the maximum, or potential, evapotranspiration rate that can occur. However, the water requirement of the crop is usually less than ET0, as there are factors of the crop itself that have to be taken into account.

These include the growth stage of the plant, the leaf coverage that provides shade to the ground, and other particulars of the crops that make them vary from each other. With these factors taken into account, ET0 is converted into ETc, through the crop-specific coefficient, Kc.

ETc represents the evapotranspiration rate of the crop under standard conditions (no stress conditions).

When calculating ETc, one must identify the growth stages of the crop, their duration and select the proper Kc coefficient that need to be used.

ETc = Kc*ET0

The crop water requirement varies from place to place, from crop to crop and depends on agro-ecological variation and crop characters.

The following features which mainly influence the crop water requirement are:

1) Crop factors

a) Variety
b) Growth stages
c) Duration
d) Plant population
e) Crop growing season

2) Soil factors

a) Structure
b) Texture
c) Depth
d) Topography
e) Soil chemical composition

3) Climatic factors

a) Temperature
b) Sunshine hours
c) Relative humidity
d) Wind velocity
e) Rainfall

4) Agronomic management factors

a) Irrigation methods used
b) Frequency of irrigation and its efficiency
c) Tillage and other cultural operations like weeding, mulching etc

Irrigation requirement

=> The field irrigation requirement of crops refers to water requirement of crops exclusive of effective rainfall and contribution from soil profile.

=> It may be given as follows

IR - WR – (ER + S)

IR - Irrigation requirement; WR - Water requirement; ER - Effective rainfall; S - Soil moisture contribution

Irrigation requirement depends upon the

a) Irrigation need of individual crop based on the area of crop
b) Losses in the farm water distribution system etc.

All the quantities are usually expressed in terms of water depth per unit of land area or unit of depth.

Net irrigation requirement

=> It is the actual quantity of water required in terms of depth to bring the soil to field capacity level to meet the ET demand of the crop.

d = Net irrigation water to be applied (cm)
Mfci = FC in ith layer (%)
Mbi = Moisture content before irrigation in ith layer (%)
Ai = Bulk density (g/cc)
Di = depth (cm)
n = number of soil layer

Gross irrigation requirement

=> The total quantity of water used for irrigation is termed gross irrigation requirement.

=> It includes net irrigation requirements and losses in water application and other losses.

=> The gross irrigation requirement can be determined for a field, for a farm, for an outlet command area, and for an irrigation project, depending on the need by considering the approximate losses at various stages of crop.

Irrigation frequency

=> Irrigation frequency is the interval between two consecutive irrigations during crop periods.

=> It depends upon the rate of uptake of water by plants and soil moisture supply capacity to plant and soil moisture available in the root zone.

=> Hence it is a function of crop, soil, and climate.

=> Normally, irrigation should be given at about 50 percent and not over 60 percent depletion.

Irrigation period

=> Irrigation period is the number of days that can be allowed for applying one irrigation to a given design area during peak consumptive use period of the crop.