Crop water need

A crop's water need is the amount of water it uses through evapotranspiration to grow without water stress. It depends on climate, crop type and growth stage. Engineers need it to size canals, wells and reservoirs and to plan irrigation schedules.

The standard approach starts from reference evapotranspiration ET₀, the water use of a well-watered reference grass, calculated from weather data (temperature, humidity, wind and sunshine) by the FAO Penman-Monteith method or estimated from pan evaporation. Crop evapotranspiration is ETc = Kc × ET₀, where the crop coefficient Kc is low at the initial stage, rises to a peak at mid-season when the canopy is full, and falls as the crop matures. Rice also needs water for land preparation, puddling and maintaining standing water, plus seepage and percolation losses.

Net irrigation requirement is crop need minus effective rainfall, the part of rain stored in the root zone and usable by the crop. Gross requirement divides the net value by the overall irrigation efficiency. Older terms still widely used in India are duty, the area irrigated by a unit discharge running continuously through the base period (hectares per cumec), and delta, the total depth of water applied over the base period. They are related through the base period. Soil water concepts guide scheduling: field capacity, permanent wilting point and the allowable depletion between irrigations.