The Oum Er Rbia basin is one of the watersheds with the largest number of hydraulic infra structures in Morocco. These hydraulic structures supply water for drinking, industrial and agricultural uses. The Ahmed El Hansali dam is a 740 Mm³ reservoir located near Zaouyat Cheikh and have an active storage of473 Mm³. The succession of dry years in the Oum Er Rbia basin has had a negative impact on the water resource and has caused a remarkable decrease in the reservoir of the Ahmed el Hansali dam. In this paper, the MNDWI (Modified Normalized Difference Water Index) from Landsat 5-TM, Landsat 7-ETM, and Landsat 8-OLI satellite images was used to estimate the spatial and temporal fluctuations of the volumes of water stored in the reservoir between hydrological years 2002-03 and 2018-19. Results show that the volumes estimated by remote sensing reasonably match the volumes estimated by the Oum Er Rbia Hydraulic Basin Agency (OERHBA) using recorded water levels and reservoir storage curve for years 2002-03 and 2013-14; the determination coefficient R² exceeds 0.90. The mapping of the extent of the dam's impoundment has shown a very significant decreasein the flooded area level during dry years.