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

Abstract

AbstractThe FAO basal crop coefficient Kcb and the soil evaporation coefficient Ke 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 Kcb had a low value of 0.10 (Kcb, ini) during the initial period, increased rapidly to a maximum of 0.98 (Kcb, mid) at midseason, and decreased rapidly to 0.53 (Kcb, end) by the end of the growing season. The Kcb increased with increasing leaf area index (LAI) rapidly and slowly, respectively, below and above an LAI of 1.0. The Ke 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 (ETa) was estimated and compared to the ETa derived by the Bowen ratio energy budget (BREB) method. The root mean square error (RMSE) of the ETa obtained from the simulated crop coefficient to the ETa obtained by the BREB method was 0.84 mm day−1. Thus, the adjusted relationship of Kcb and Ke allowed a good estimation of the evapotranspiration in an irrigated field with sparse vegetation cover. Copyright © 2006 John Wiley & Sons, Ltd.