Real time crop coefficient from SEBAL method for estimating the evapotranspiration

Abstract

Crop coefficients (Kc) are defined as the ratio of actual crop evapotranspiration to the evapotranspiration (Eto) of a grass reference crop, often taken from Penman/Monteith"s methodology. They are used to estimate theoretical crop evapotranspiration (Etc). Actual evapotranspiration is measured in field lysimeters but this lacks acceptability when applied to the field or large irrigation schemes where conditions are very variable. This paper uses the SEBAL method (Bastiaanssen et al., 1998) to determine actual evapotranspiration from satellite images. This is compared with estimated Eto from both Penman/Monteiths methodology and Eto estimated from a SEBAL estimation of reference crop evaporation within a project. The range of Kc values for the cotton crop were then calculated. Crop coefficient Kc maps were made for two irrigation projects. The methodology was applied on a large cotton irrigation scheme (Starikan) southern Kazakhstan. The estimated mean real time Kc value of 1.16 (±1-3% error) was higher than the standard Kc value of 1.1 identified by FAO-56. The methodology was validated on a different date and different irrigation schemes (Chardara) 200 km south Starikan. The methodology is discussed