In the first part, the daily water requirements for a cropped area is calculated throughout one year. The program calculates the potential evapotranspiration. The potential evapotranspiration is the maximum amount of water that plants can evapotranspire when there is a full supply of water to the plant during its growth. The potential evapotranspiration is obtained by multiplying a reference evapotranspiration with a crop coefficient. The reference evapotranspiration is calculated from meteorological data of the area, according to one of three methods available in CRIWAR; preferably the Penman-Monteith method. The crop coefficients depend on the type of crop and the growth stage of the crop and are known from field studies and in literature. The program can handle 40 different crops in a cropping pattern (or cropping calendar). Calculated this way, the crop water requirement is the same as the potential evapotranspiration. When part of the rainfall which falls on the same area is effective to meet the plants’ evapotranspiration, the irrigation water requirement is obtained by subtracting the effective rainfall from the potential evapotranspiration. The output of the model is the amount of water in mm water depth, or volume in m3, of water to divert to the cropped area on a daily basis.
By reviewing alternative cropping patterns, the daily water demand of the cropped area can be calculated and optimized.