Evapotranspiration

Evapotranspiration (ET) is an important term in the water balance of a cropped area. Irrigation engineers need to know how much of the applied irrigation water is consumed by the crop; only then can they calculate, or estimate, the remaining components of the water balance. Agriculturists, on the other hand, need to know the specific water requirements of a crop so that they can obtain a satisfactory yield; they also need to know whether these water requirements are being met under the prevailing irrigation practice. Reduction in ET due to plant-water stress, caused by water shortage, is associated with reduced plant yield, since both ET and photosynthesis are functions of stomatal regulation. Figure 2.1 shows the impact of the actual ET on above-ground biomass production (and thus on the yield) of a crop

The method to estimate reference crop evapotranspiration (for cool season clipped grass) depends on the availability of accurate meteorological data and on local practices. As shown in Table 2.1, the FAO Modified Penman Method and the Penman-Monteith Method need a full range of accurate meteorological data (Penman 1948; Doorenbos and Pruitt 1977; Monteith 1965). Of these two methods the FAO Modified Penman method of Doorenbos and Pruitt 1977 has been widely used in the past. However, as will be discussed in Section 2.5, the Penman-Monteith method is the currently recommended “default” method (Burt et al. 2002; Allen et al. 1998). If, however, data on humidity, wind speed and radiation are missing the Hargreaves-Samani (1985) Method is recommended. This method gives a relatively good estimate of the reference ET (Section 2.4). How the theory on evapotranspira-tion is applied in practice is explained in Sections 2.6, 2.8, and 2.9