Assessment of Seepage from the Grand Ethiopean Renaissance Dam: A Remote Sensing-Based Application

The Grand Ethiopian Renaissance Dam (GERD) was built over the Pan African highlands in western Ethiopia and upon completion, its reservoir will cover an area of 1,904 km2 and impound 74 km3. The reservoir encroaches over Precambrian metamorphic rock, syntectonic intrusives, late and post tectonic granitic intrusives, calcite, dolomite, and Tertiary basalts. The basement rocks are characterized by deep regolith (up to 60 m) and highly deformed texture, related to NE–SW extension associated with the breakup of Gondwana and NW–SE-directed extension and opening of the Main Ethiopian Rift. The extended and recurrent tectonic activities left behind a highly deformed landscape in western Ethiopia. The static (Copernicus Digital Elevation Model), and temporal satellite data analysis (GRACE and GRACE-FO, Sentinel-1, 2, MODIS, Radar altimetry) were used to monitor and quantify seepage from the GERD reservoir. Findings reveal the following. (1) using Sentinel-1 images that capture the variations in the reservoir storage, the impounded water in GERD in the first (2020) and second (2021) fillings were found to decline relative to the Initial Storage (IS Filling I: 3.7 Km2, IS Filling II: 9.3 Km2) by 29 and 37%, respectively. (2) using GRACE data that measures the water in the GERD Reservoir and its surroundings, we found that the GRACETWS values exceed the reservoir storage detected from Sentinel-1 data and is more comparable to the targeted filling volumes. (3) One interpretation of the discrepancies between the GRACE and Sentinel-1 data is losses to evaporation and infiltration.  The evaporation remained insignificant during the filling process (<0.5 Km3), whereas losses to infiltration along the highly fractured (faults, shear zones) and within the weathered basement rocks remain a plausible explanation that merits additional investgations.