Determination of a crop coefficient for evapotranspiration in a sparse sorghum field

The FAO basal crop coefficient K cb and the soil evaporation coefficient K e for a sparse sorghum field in one growing season were estimated using the compartment model (Shuttleworth–Wallace model; S–W model), which separately estimates transpiration from crops and evaporation from the soil. The K cb had a low value of 0.10 ( K cb , ini ) during the initial period, increased rapidly to a maximum of 0.98 ( K cb , mid ) at midseason, and decreased rapidly to 0.53 ( K cb, end ) by the end of the growing season. The K cb increased with increasing leaf area index (LAI) rapidly and slowly, respectively, below and above an LAI of 1.0. The K e showed a positive relationship to increasing soil water content ( SWC ) at a depth of −15.0 cm, but the soil water content indicated a negative relationship to increasing LAI. Using the dual crop coefficients derived by their relationships to the LAI and soil water content, the actual evapotranspiration (ET a ) was estimated and compared to the ET a derived by the Bowen ratio energy budget (BREB) method. The root mean square error (RMSE) of the ET a obtained from the simulated crop coefficient to the ET a obtained by the BREB method was 0.84 mm day −1 . Thus, the adjusted relationship of K cb and K e allowed a good estimation of the evapotranspiration in an irrigated field with sparse vegetation cover.