The paper presents an application of NOAA-AVHRR and GRACE data for studying the regional hydrogeology of the Quaternary aquifer in the Lake Chad basin. The actual evapotranspiration (ET) over the phreatic aquifer is computed using the S-SEBI algorithm and then compared to the ET estimates derived from the GRACE land water solutions. Both the GRACE solutions and the S-SEBI results are similar in description of the actual ET over the Lake Chad basin. AVHRR data helped to overcome non-uniqueness in the modelling of the piezometry anomalies. Particularly, in regional groundwater models, the Harr piezometric dome appeared to be an area of relatively high effective recharge, while the Chari-Baguirmi and the Kadzell piezometric depressions and Bahr-el-Gahzal were probably areas of excess ET over recharge.