Multi-model and multi-sensor estimations of evapotranspiration over the Volta Basin, West Africa

The estimation of large-scale evapotranspiration (ET) is complex, and typically relies on the outputs of land surface models (LSMs) or remote sensing observations. However, over some regions of Africa, inconsistencies exist between different estimations of ET fluxes, which should be investigated. In this study, we evaluate and combine different ET estimates from moderate-resolution imaging spectroradiometer (MODIS), Global Land Data Assimilation System (GLDAS) and terrestrial water budget (TWB) approaches over the Volta Basin, West Africa. ET estimates from water balance equation are obtained as residuals from monthly terrestrial water-storage (TWS) changes derived from Gravity Recovery and Climate Experiment (GRACE), Tropical Rainfall Measurement Mission (TRMM)'s rainfall data, and in situ discharge from Akosombo Dam (Ghana). An averaged estimation of ET time series is derived from all the ET estimations under study, while taking into account their uncertainties. The resulting ensemble-averaged ET was then used to assess each of the individual ET estimates. Overall, out of the seven investigated ET estimates (two from the water balance approach of which one considers water storage using GRACE-derived TWS and the other ignoring it, four from GLDAS and one from MODIS), only MODIS (28.12mmmonth(-1)), GLDAS-NOAH (32.74mmmonth(-1)) and TWB (32.84mmmonth(-1)) were found to represent the range of variability close to the computed averaged reference ET (30.25mmmonth(-1)). ET estimations inferred from MODIS were also found to represent relatively lower magnitude of uncertainties, that is, 3.99mmmonth(-1) over the Volta Basin (cf. 7.06 and 18.85mmmonth(-1) for GLDAS-NOAH and TWB-based ET estimations, respectively).