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

Abstract

Evapotranspiration (ET) calculated as the residual of catchment water balance (ETWB) has often been used as a benchmark to evaluate satellite-based ET retrievals that use the energy-balance approach (ETEB). However, errors from water balance components will accrue in ETWB, leading to considerable disparities with ETEB. In this study, we set out to investigate whether ETEB from multiple sources (MOD16, GLEAM, PT-JPL, and PT-hybrid) can capture the spatiotemporal variability of ETWB 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 ETWB and ETEB at long-term, annual, and monthly scales. We found that (1) ETEB significantly diverged from ETWB at the mean annual scale (r = 0.35), particularly for energy-limited catchments, but Budyko-simulated ET considering energy limit correlated well with ETEB (r > 0.86); (2) neither ETEB nor upscaled ET can reproduce annual ETWB time series (r < 0.40), and the closure errors in water budgets closely follow excess precipitation beyond energy demand; (3) monthly ETWB exhibited better correspondences with ETEB (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 ETWB, thereby complicating water balance validation in humid regions across multiple timesteps. To improve the application of ETWB for benchmarking ETEB in humid regions, high-quality input data should be used or – like the Budyko framework – energy constraints should be considered.