Ground-based validation is crucial for ensuring the accuracy of remotely sensed evapotranspiration (RS-ET) and extending its application. This paper proposes an innovative validation method based on multisource evapotranspiration (ET) from ground measurements, with the validation results including the accuracy assessment, error source analysis, and uncertainty analysis of the validation process. It is a potentially useful approach to evaluate the accuracy and analyze the spatiotemporal properties of RS-ET at both the basin and local scales, and is appropriate to validate RS-ET in diverse resolutions at different time-scales. An independent RS-ET validation using such a method was presented over the Hai River Basin in 2002-2009, China. In general, validation at the basin scale showed good agreements between the 1km annual RS-ET and the validation data such as the water balance ET (root-mean-square error (RMSE): 50.73mm), MODIS ET products (RMSE: 79.84mm), precipitation, and land use types. At the local scale, multiscale ET measurements from large aperture scintillometer (LAS) and eddy covariance system (EC) with a footprint model were used for validation over three typical landscapes. In most cases, the 1km RS-ET resulted in slight overestimation with the LAS measurements (RMSE: 10.75mm for monthly results, 0.78mm for daily results), while the 30m RS-ET was underestimated compared to the EC measurements (RMSE: 16.28mm for monthly results, 0.99mm for daily results). Furthermore, error sources of RS-ET and uncertainties of the validation process were investigated in detail. The results showed that the proposed validation method was reasonable and feasible.
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