An Improved Method for Deriving Daily Evapotranspiration Estimates From Satellite Estimates on Cloud-Free Days

An improved method, based on the daily surface resistance, is proposed to extend satellite evapotranspiration (ET) on a clear day into ET for each and every day. Alternative climatic variables such as soil moisture, wind speed, and net radiation are explored for estimating daily surface resistance using a Penman-Monteith (P-M) formulation. The study was carried out for the Yingke (YK) oasis plains area (maize cropland) and the Arou (AR) alpine meadow area (grassland) located in the midstream and upstream, respectively, of the Heihe River Basin of northwestern China. Statistical results show that the proposed method performs well for estimating daily ET for both study areas, with results slightly superior in the midstream, cropland area where the coefficient of determination (R
²
) was 0.9249 and the index of agreement (d) was 0.978. In the upstream alpine meadow area, the coefficient of determination (R
²
) was 0.9074, and the index of agreement (d) was 0.961. The proposed method provides an enhanced approach for estimating daily ET in the ETWatch model. Future work will focus on scaling this improved method to the estimation of regional daily ET map.