Comparison of ALARM and SEBAL Evapotranspiration of Irrigated Alfalfa

Abstract

Estimating crop evapotranspiration (ETc) is essential for effective agricultural water management and water resources planning. The objective of this study was to compare two methods for estimating daily crop ET (ETc). The first method used the Analytical Land Atmosphere Radiometer Model (ALARM) and the dimensionless temperature (DT) (ALARM-D) procedure, while the second method used the Surface Energy Balance Algorithm for Land (SEBAL). A study was conducted on an irrigated alfalfa field in southern California in summer 2008. Standard meteorological data from a weather station and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Moderate Resolution Imaging Spectroradiometer (MODIS) surface temperature were used to estimate ETc. Observed hourly and daily ETc were determined using surface renewal meteorological method. On average, the relative error of the ASTER 90-m pixel hourly and daily ETc was 8 and 1.2 and -10.3 and -16.3 % for ALARM-D and SEBAL, respectively. For 1-km pixels, MODIS and ASTER (upscaled) ALARM-D daily ET estimates were higher than SEBAL and ASTER ALARM-D and SEBAL daily ETc was greater than MODIS ALARM-D and SEBAL daily ETc. These initial results suggest that the performance of ALARM-D and SEBAL was comparable in hourly ETc estimation while ALARM-D outperformed SEBAL in daily ETc.. More studies in diverse locations and environmental conditions are needed before solid conclusions can be drawn about how ALARM-D and SEBAL compare in hourly and daily ETc estimation.