A Framework for Quantifying the Uncertainty in Upscaling Evapotranspiration From Homogeneous to Heterogeneous Underlying Surface

The uncertainty of ground truth values at the pixel scale obtained via upscaling directly affects the credibility of remote sensing product validation. This article provides an in-depth analysis of the sources of uncertainty in ground truth evapotranspiration (ET) at the pixel scale. This uncertainty is quantitatively evaluated, and the methods for its control are discussed. The results indicate that the uncertainty from upscaling methods is highest, followed by that from auxiliary data, with that from instrument measurements being the smallest. The relative accuracy of the ground truth ET at the pixel scale for the LAS1–LAS4 (LAS5–LAS7) regions is 89.15%–90.16% (81.56%–82.58%). The accuracy for homogeneous surfaces is relatively high at approximately 90%–93%, whereas for moderately and highly heterogeneous surfaces, it is lower, varying from approximately 81% to 92%. To control uncertainty, precise instrument calibration, strategic positioning, the use of diverse constraints, and robust modeling are recommended to increase measurement accuracy and prediction reliability. This uncertainty study includes the analyses of different sources of uncertainty and quantitatively evaluates the uncertainty of ground truth values over different heterogeneous underlying surfaces. The results can be used to objectively evaluate the accuracy of remote sensing products, thus advancing studies of the uncertainty of ground truth values at the pixel scale and enhancing the scientific, reliable, and systematic validation of remote sensing products. This approach can greatly promote the validation of remote sensing ET products over heterogeneous surfaces.