A worldwide evaluation of basin-scale evapotranspiration estimates against the water balance method

Evapotranspiration (ET) plays a critical role in linking the water and energy cycles but is difficult to estimate at regional and basin scales. In this study, we present a worldwide evaluation of nine ET products (three diagnostic products, three land surface model (LSM) simulations and three reanalysis-based products) against reference ET (ETwb) calculated using the water balance method corrected for the water storage change at an annual time scale over the period 1983–2006 for 35 global river basins. The results indicated that there was no significant intra-category discrepancy in the annual ET estimates for the 35 basins calculated using the different products in 35 basins, but some products performed better than others, such as the Global Land surface Evaporation estimated using the Amsterdam Methodology (GLEAME) in the diagnostic products, ET obtained from the Global Land Data Assimilation System version 1 (GLDAS 1) with the Community Land Model scheme (GCLME) in LSM simulations, and ET from the National Aeronautics and Space Administration (NASA) Modern Era Retrospective-analysis for Research and Applications reanalysis dataset (MERRAE) in the reanalysis-based products. Almost all ET products (except MERRAE) reasonably estimated the annual means (especially in the dry basins) but systematically underestimated the inter-annual variability (except for MERRAE, GCLME and ET simulation from the GLDAS 1 with the MOSAIC scheme – GMOSE) and could not adequately estimate the trends (e. g. GCLME and MERRAE) of ETwb (especially in the energy-limited wet basins). The uncertainties in nine ET products may be primarily attributed to the discrepancies in the forcing datasets and model structural limitations. The enhancements of global forcing data (meteorological data, solar radiation, soil moisture stress and water storage changes) and model physics (reasonable consideration of the water and energy balance and vegetation processes such as canopy interception loss) will undoubtedly improve the estimation of global ET in the future.