Satellite-Based Estimation of Actual Evapotranspiration in an Arid Oasis Using the ALARM Model

Accurate estimates of actual evapotranspiration (ETa) are essential for irrigation management in arid environments but remains challenging in heterogeneous perennial tree systems. This study evaluated the Analytical Land–Atmosphere Radiometer Model (ALARM) for estimating ETa by comparing model estimates with measurements from an eddy covariance (EC) flux tower in the Palmyra Oasis, Syria, during two growing seasons. A sensitivity analysis was first conducted to identify the most influential model parameters, followed by calibration using 2005 MODIS observations and meteorological data as inputs. The calibrated parameter set was then applied without further adjustment to the 2004 EC dataset. Satellite-derived radiation and modelled sensible heat flux were also evaluated against EC measurements to assess the reliability of key model inputs and components. ALARM reproduced the seasonal dynamics of ETa across the study area, with RMSE from 0.72 to 0.73 mm d−1, NRMSE from 27.11% to 28.16%, and PBIAS from 2.24% to 12.40%. The model overestimated ETa during the early growing season and slightly underestimated it during the peak season, consistent with seasonal changes in available energy and surface thermal conditions. Overall, the results demonstrate the potential of ALARM for assessment of crop water use in arid perennial tree systems, while highlighting limitations associated with short-term variability.