The divergence of energy- and water-balance evapotranspiration estimates in humid regions

Evapotranspiration (ET) calculated as the residual of catchment water balance (ET _WB) has often been used as a benchmark to evaluate satellite-based ET retrievals that use the energy-balance approach (ET _EB). However, errors from water balance components will accrue in ET _WB, leading to considerable disparities with ET _EB. In this study, we set out to investigate whether ET _EB from multiple sources (MOD16, GLEAM, PT-JPL, and PT-hybrid) can capture the spatiotemporal variability of ET _WB across 53 catchments in central-western Europe with a humid climate. Using ET retrievals from the Budyko framework that accounts for the control of energy demand on water supply and upscaled ET from FLUXCOM as references, we explored the causes of discrepancies between ET _WB and ET _EB at long-term, annual, and monthly scales. We found that (1) ET _EB significantly diverged from ET _WB at the mean annual scale (r = 0.35), particularly for energy-limited catchments, but Budyko-simulated ET considering energy limit correlated well with ET _EB (r > 0.86); (2) neither ET _EB nor upscaled ET can reproduce annual ET _WB time series (r < 0.40), and the closure errors in water budgets closely follow excess precipitation beyond energy demand; (3) monthly ET _WB exhibited better correspondences with ET _EB (r = 0.73), presumably because of similarity in seasonal patterns. Our results demonstrate that errors from precipitation and terrestrial water storage anomalies introduce large uncertainties in ET _WB, thereby complicating water balance validation in humid regions across multiple timesteps. To improve the application of ET _WB for benchmarking ET _EB in humid regions, high-quality input data should be used or – like the Budyko framework – energy constraints should be considered.