Spatial-temporal patterns of land surface evapotranspiration from global products

Knowledge of spatio-temporal patterns of global land surface evapotranspiration (ET) is essential for understanding the exchanges of energy, water and carbon between the earth surface and atmosphere and responses to human activities, climate changes, and extreme weather events. This paper comprehensively reviewed accuracies and spatio-temporal patterns of 25 state-of-the-art global datasets of land surface ET from 2001 to 2018, where the global and/or local distributions of annual mean, interannual variability, and annual trend for both ET and its components [canopy transpiration (Ec), soil evaporation (Es), and interception evaporation (Ei)] were investigated. Overall, 23 out of the 25 ET products exhibited good consistency with monthly eddy covariance observations at 83 globally distributed sites, with the root mean square error (RMSE) ranging from similar to 19 mm/month to similar to 29 mm/month and the coefficient of determination (R-2) ranging from 0.45 to 0.76. Comparatively, almost all ET products were in good agreement with the water balance-based observations at 137 large basins worldwide, with the R-2 of >0.60 and the RMSE ranging from similar to 200 mm/yr to similar to 260 mm/yr. In general, remote sensing-based, machine learning-based, and hybrid-based ET products generally agreed better with site observations than reanalysis-based products. Global annual ET ranged from 530 mm/yr to 700 mm/yr with the ensemble mean of 628 +/- 44 mm/yr (excluding areas of the urban, barren, wetlands, permanent snow and ice, and water bodies). Most products captured consistent global spatial distributions of annual mean, interannual variability, and annual trends of land surface ET and its components. Both mean annual ET and its components generally decreased with increasing latitudes though with an increasing trend in time. Interannual variability also decreased with increasing latitudes and with an increasing trend in time, but only in the Northern Hemisphere; these signals were not clear in the Southern Hemisphere. Most of the 25 products presented unimodal shapes of frequency curves, and displayed a sequenced multi-year mean of seasonal ET of JJA > MAM > SON > DJF. The Ec exhibited the largest mean annual value, in most areas followed successively by the Es and Ei. The ET products displayed a relatively high consistency in the spatial distribution of the Ec/ET with higher estimates of Ec/ET in the Southern Hemisphere than in the Northern Hemisphere. This up-to-date review could benefit the understanding of the states and advances of global ET datasets, afford scientific instruction for the development and improvement of ET models and products, and serve as a basis for the choice of ET datasets for specific use.