A Comparison Study of MODIS and ASCE Alfalfa Evapotranspiration in a Semiarid Climate

Abstract

Estimation of evapotranspiration (ET) is needed for many applications in diverse disciplines such as agriculture, hydrology and meteorology. The objective of this study was to compare two methods for estimating ET in a semiarid environment. The first method used ALARM and the dimensionless temperature procedure, while the second method used the ASCE reference ET. The use of the ASCE reference ET approach has been recommended worldwide because it provides reasonable results under a wide range of climatic conditions. Standard meteorological data from a weather station were used with both methods, and MODIS-based leaf area index, surface temperature, and albedo were obtained to estimate ET using the former method. The daily estimates of ET were computed for the summer and fall, 2006 for a 1-km pixel that contains 20 ha alfalfa field in Jordan Valley, Jordan. The root mean square difference between the ASCE and ALARM daily ET was 1.3 mm/day. Generally, the ASCE and ALARM daily ET were comparable although ALARM ET was slightly higher than ASCE ET during the summer. The result of this study suggested that remote sensing combined with ALARM and the dimensionless temperature procedure can be useful and reasonably accurate in estimating daily ET for alfalfa. Future studies are recommended to apply the dimensionless temperature approach for other crops in semiarid environment and other climates. In addition, accurate estimates of ET from remotely sensing data of MODIS can be used for water resources management as well as for drought monitoring.