The spatial variability of actual evapotranspiration across the Amazon River Basin based on remote sensing products validated with flux towers

Actual evapotranspiration (ET) is a major component of the water balance. While several international flux measurement programs have been executed in the tropical rain forest of the Amazon, those measurements represent the evaporative process at a few selected sites only. The aim of this study is to obtain the spatial distribution of ET, using remote sensing techniques, across the entire Amazon River Basin. Results from six global ET products based on remote sensing techniques (GLEAM, SEBS, ALEXI, CMRSET, MOD16, and SSEBop) were merged to obtain an ensemble prediction of the ET rates for the complex and in-accessible environment of the Amazon at a spatial resolution of 250 m. The study shows that the basin-wide average ET is 1316 mm/year with a standard deviation of 192 mm/year. This new ET-Amazon product was validated against seven different historic flux tower measurements. The energy balance closure of the in situ measurements varied between 86 and 116%. Only months with more than 70% completeness of in situ measurements were considered for validation. Different procedures for closure correction were included in the analyses. The correlation between measured and remotely sensed ET is good (R-2 > 0.97 for consecutive periods of 2 to 12months), and the bias correction is negligible for the energy balance residual method, which seemed most favorable. Monthly ET values have more uncertainty. The monthly RMSE values vary between 7.4 and 27.8 mm/month (the average RMSE is 22.2mm/month), and the coefficient of determination (R-2) varies between 0.48 and 0.87 (the average R-2 is 0.53). The ET from the water balance is 1380 mm/year, being - 64 mm/year difference and 4.6% less than ET derived from the water balance. The evaporation from the Amazon basin inside Brazil is 5063 km(3)/year, followed by Peru with 1165 km(3)/year. ET- Amazon shows more spatial details and accuracy than alternative global ET products such as LandFlux- EVAL, Model Tree Ensemble (MTE), and WACMOS- ET. This justifies the development of new regional ET products.