Calculation of Evapotranspiration and Crop Water Requirements

Reference ET (ET0) is defined as the ET of short grass with full soil cover, and an unlimited supply of water and nutrients. In the absence of water deficit, the ET of any crop may be calculated as the product Kc × ET0, where Kc is the crop coefficient, which depends on crop related factors (leaf area, roughness) and ET0, the reference ET (grass), which is a function of climatic variables (radiation, temperature, humidity, and wind speed). The main equation for calculating ET0 is the Penman–Monteith-FAO, although the Hargreaves equation can be used when only air temperature data are available. Kc is calculated by the method proposed by FAO that uses linear functions between the initial, maximum, and harvest dates of the cycle. The initial Kc depends on the frequency of soil wetting and ET0. The maximum values of Kc of annual crops and deciduous fruit trees are typically between 1.0 and 1.3 (median 1.2). The crop irrigation water requirement is the difference between ET and the sum of effective rainfall and soil water depletion. The latter may be an important contributor to meeting the ET demand and thus reduce the capacity (and investment) of irrigation networks.