Global estimation of terrestrial evapotranspiration based on the atmospheric water balance approach

Quantifying global terrestrial evapotranspiration (ET) relies on models with different levels of complexity. The water balance method offers a straightforward approach for benchmarking complex ET models, as evidenced by the widely-used terrestrial water-balance-based ET (ET_TWB) data. However, deriving ET_TWB must rely on ground-observed runoff data, which is not feasible for ungauged or poorly-gauged regions. In this context, the atmospheric water balance (AWB) method offers an alternative for estimating ET, which can be applied to the entire global land area. Nevertheless, the accuracy of the AWB approach in estimating global ET remains poorly understood. In this study, we generated monthly atmospheric water-balance-based ET (ET_AWB) globally from 1983 to 2020 at a 0.25° resolution using multi-source data. Validations against the annual ET_TWB of 56 large river basins suggest that ET_AWB, estimated using the moisture convergence and atmospheric water vapor from the fifth generation of European Center for Medium-Range Weather Forecasts Reanalysis (ERA5) and the precipitation from four observation-based products, is overall accurate. Specifically, the AWB method yields Nash–Sutcliffe efficiency coefficient (NSE), root mean square error (RMSE), and relative bias (RB) of 0.88, 89.5 mm year⁻¹, and 2%, respectively. These statistical metrics indicate that the AWB method is generally on par with current mainstream ET models. However, the AWB approach still has certain challenges in capturing the trend in ET. The ensemble mean ET_AWB, estimated using the moisture convergence and atmospheric water vapor from ERA5 and four precipitation datasets, yields a global-averaged value of 619 ± 8 mm year⁻¹ (excluding Antarctica) and shows an increase of 2.1% from 1983 to 2020, with a trend of 0.35 mm year⁻¹. Tropical regions exhibit pronounced interannual variability in ET_AWB due to the internal climate variability influencing precipitation and moisture convergence. The current AWB approach  can potentially improve the understanding of regional and global ET processes, as it represents an independent approach to ET estimation, distinct from current remote sensing and land surface models.