Estimating Daily Evapotranspiration From Remotely Sensed Instantaneous Observations With Simplified Derivations of a Theoretical Model

Abstract

AbstractSurface evapotranspiration (ET) is one of the key components in global hydrological cycle and energy budget on Earth. This paper designs a theoretical relationship between daily and instantaneous ETs with a multiplication of multiple fractions through a mathematical derivation of the physics‐based Penman‐Monteith equation and further develops five methods for converting remotely sensed instantaneous ET to daily values, one of which is equivalent to the conventional constant evaporative fraction (EF) method. The five methods are then evaluated and intercompared using long‐term ground‐based eddy covariance system‐measured half‐hourly latent heat flux (LE) and three groups of Moderate Resolution Imaging Spectroradiometer‐based instantaneous LE data sets collected from April 2009 to late October 2011 at the Yucheng station. Overall, the constant decoupling factor (Ω) method, the constant surface resistance (Rc) method, and the constant ratio of surface resistance to aerodynamic resistance (Rc/Ra) method could produce daily LE estimates that are in reasonably good agreement with the ground‐based eddy covariance measurements, whereas the constant EF method and the constant Priestley‐Taylor parameter (α) method underestimate the daily LE with larger biases and root‐mean‐square errors. The former three methods are of more solid physical foundation and can effectively capture the effect of temporally variable meteorological factors on the diurnal pattern of surface ET. They provide good alternatives to the nowadays commonly applied methods for the conversion of remotely sensed instantaneous ET to daily values.