Multi-scale analysis of six evapotranspiration products across China: Accuracy, uncertainty and spatiotemporal pattern

Evapotranspiration (ET) is an essential variable in the global water cycle. With the development of remote sensing techniques, multiple large-scale ET products based on different algorithms have been developed to accurately estimate ET. However, the performance of these products suffers from various factors, including input datasets, algorithms, and environmental factors. It is critical to analyze the accuracy, uncertainty and spatiotemporal pattern of various ET products for selecting the optimal product and understanding the ET process. In this study, we systematically compared the performance of six ET products, including ERA5-LAND, GLASS, GLDAS, GLEAM, PMLV2, and SSEBop, from 2005 to 2020 across China. The comparison was conducted at the monthly scale, utilizing eddy covariance observations from eight flux tower stations for point-scale evaluation, and employing the water balance method to derive ET in 24 basins for basin-scale assessment. The threecornered hat (TCH) method was then utilized to quantify the uncertainty of these products at basin-scale. Furthermore, we analyzed the spatiotemporal distribution of ET and its seasonal variation across China. The results revealed that all products effectively captured the ET variations across China at point and basin scales, particularly in semi-humid and semi-arid climate regions covered by forest, but with significant variability in metrics among these products. Generally, GLEAM and PMLV2 demonstrated the best correlation coefficient (r) and root mean squared deviation (RMSD), outperforming the others. The uncertainty analysis indicated that GLASS achieved the lowest uncertainty at 5.53 mm/month while SSEBop showed the highest uncertainty at 11.45 mm/month. Regarding the spatiotemporal pattern of ET, these products consistently displayed an ascending trend from northwest to southeast, with the annual ET ranging from 395.18 mm in SSEBop to 504.04 mm in ERA5-LAND. However, substantial interannual and seasonal discrepancies of ET were observed widespread throughout China. This research provides a reference for selecting and applying the suitable ET product in China to facilitate the sustainable water resource management.