Validation of Remotely Sensed Evapotranspiration Products Using Optical–Microwave Scintillometer Flux Measurements Over the Heterogeneous Surfaces

Accurate estimates of terrestrial evapotranspiration (ET) are critical for regional drought monitoring and water resources management. Various remotely sensed evapotranspiration (RS_ET) products have been applied from regional to global regions, and their validations are often conducted using measurements from numerous eddy covariance (EC) sites, which do not well match the pixel sizes of most RS_ET products at coarse resolutions. Scintillometry provides flux measurements at a much larger scale of 0.5–10 km. This article presents a new perspective that the optical microwave scintillometer (OMS) measurements collected from 2019 to 2020 over two different surfaces (alpine meadow and irrigated cropland) in the Heihe River Basin of northwestern China were used to validate the commonly used RS_ET products. It is found that OMS could measure reliable area-averaged turbulent fluxes and ET over both homogeneous and heterogeneous underlying surfaces. Footprint analysis indicates that the larger spatial representativity of OMS is comparable to the pixel sizes of most satellite images. The OMS ET observations were taken as a reference to validate four kinds of 1-km resolution RS_ET products using the related evaluation indices. Validation results indicate that the RS_ET products demonstrated strong agreement with OMS measurements over two distinct underlying surfaces (Arou homogeneous grass and Daman heterogeneous cropland), with ETMonitor showing superior performance, followed by GLASS, DTD, and MOD16. The OMS measurements can effectively represent larger area averaged ET, particularly over heterogeneous underlying surfaces, thereby providing an outstanding opportunity for better validation of RS_ET products at moderate to coarse resolutions.