Determination of crop water requirements (CU/ETc) is one of the basic needs for planning of any irrigation project. Methods for determining crop water requirements depend on the desired level of accuracy, availability of equipment and technical know how.
More commonly mentioned methods are:
Direct measurements of consumptive use (ETC):
1. Transpiration ratio method
2. Depth-interval-yield approach
3. Lysimeter experiments
4. Soil moisture depletion studies
5. Field experimentation
6. Water balance method.
Calculation of consumptive use from climatic data:
1. Correlation with evaporation devises
2. Estimation of water loss based on climatic parameters.
Direct Measurements of Consumptive Use:
Transpiration ratio method:
The amount of water transpired by a crop to produce a unit quantity of dry matter is called transpiration ratio. In spite of several drawbacks, it was the usual method in earlier days. Transpiration ratio measured in green house with pot culture studies differ under actual conditions. Hence, it is of little value for scheduling crop irrigation.
Depth-interval-yield approach:
Irrigations are scheduled with different depths of water at different intervals between irrigations. Total amount of water used for highest yield is taken as crop water requirement. It gives an idea on crop water needs, but not reliable estimate because of differences in climate in different seasons.
Lysimeter experiments:
Use of lysimeter for estimating ET.
Soil Moisture depletion studies:
Consumptive use (CU) of water (ETC) can be determined by estimating changes in soil moisture from root zone depth at periodical intervals during the crop season. It is usually employed in fairly uniform soils, where the groundwater table is deeper enough to influence soil moisture in the root zone depth.
where, CU = Water use from root zone between two successive sampling periods (mm)
n = Number of soil layers considered in the root zone depth D
M1i = Soil moisture percentage at first sampling in ith layer
M2i = Soil moisture percentage at second sampling in the ith layer
BDi = Bulk density of ith soil layer (g cm-3)
Di = Depth of ith soil layer (mm).
Effective precipitation, groundwater contribution and deep percolation losses, if any are to be considered for reliable estimates of crop water requirements. Seasonal consumptive use can be obtained by adding CU values of each sampling interval.
Field experimentation:
Crops are grown with recommended management practices, including an ideal irrigation schedule for optimum yields. Irrigation water applied, effective precipitation and soil profile moisture contributions are measured.
Seasonal water requirements are computed by adding these components:
where, WR = Water requirement of crop (mm)
IR = Amount of irrigation water applied (mm)
ER = Effective rainfall during the season (mm)
n = Number of soil layers considered in root zone depth D
Mbi = Soil moisture percentage at the start of season in the ith layer
Mei = Soil moisture percentage at the end of season in the ith layer
BDi = Soil bulk density in the ith layer (g cm-3)
Di = Depth of the ith layer in root zone depth (mm).
Water balance method:
The water balance method depends on the following hydrological equation:
Precipitation = Evapotranspiration + surface runoff + subsurface drainage changes in soil water content.
If all the factors, except ET are measured, then it can be computed. There are, however, many difficulties associated with measurement of different components of water balance method.
Calculation of Crop Water Requirements Based on Climatic Parameters:
Correlation with evaporation devices:
It is assumed that conditions that affect crop water requirements (ETC) will also affect evaporation from free water surface (ETo) in a similar manner. It is then necessary to obtain a crop coefficient (kc) to estimate ETcrop (crop water requirement or WR).
ETc = ETo x kc
Actual crop water requirements, in addition to climate include the effect of crop characteristics and other crop production practices. Crop coefficient (kc) is used to account or all these variations.
Estimation of crop water requirements based on climatic parameters:
Empirical equations are used for estimating crop water requirements.
Variation in Crop Water Requirements:
There is considerable variation in crop water requirements from region to region depending upon climatic and management practices (Table 6.1). The differences are largely due to the length of crop growing season and incoming solar radiation during the crop growing season. Irrigation requirements, rarely, depend on soil type and rainfall during the crop period.
