Validation of MODIS 16 Global Terrestrial Evapotranspiration Products in Various Climates and Land Cover Types in Asia

Evapotranspiration (ET), or the sum of water released to the atmosphere from ground surfaces, intercepts canopy precipitation through evaporation and plant transpiration and is one of the most significant components in the water cycle. In this study, Moderate Resolution Imaging Spectroradiometer (MODIS) 16 global terrestrial ET products were validated at 17 flux tower locations in Asia. Overall, overestimations due to energy balance misclosure distorted the trend of the data at nine locations [r: 0.27-0.82; bias: -21.41-2.38 mm 8-d(-1); Root Mean Square Error (RMSE): 6.12-21.81 mm 8-d(-1)]. Regardless of variation in the scattering patterns, good agreements between MODIS-based ET and ET measured at the flux towers were observed at five locations (r: 0.50-0.76; bias: -1.42-1.99 mm 8-d(-1); RMSE: 1.99-8.96 mm 8-d(-1)). Underestimation at one site (r = 0.28, bias = -17.00 mm 8-d(-1), RMSE = 17.41 mm 8-d(-1)) was accompanied by mismatches at two sites (r = 0.12-0.18; bias = -4.19 - -0.04 mm 8-d(-1), RMSE = 5.76-7.66 mm 8-d(-1)). The best performances of the MOD16 ET algorithm were observed at sites with forested land cover, but no substantial differences were found under a variety of climate conditions. This study is the first comprehensive trial to validate global terrestrial MODIS ET in Asia, showing that a MODIS global terrestrial ET product can estimate actual ET with reasonable accuracy. We believe that our results can be used as baseline ET values for satellite image-based ET mapping research in South Korea.