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The Oum Er-Rbia river basin, located in the Midwest of Morocco, is one of the largest watersheds in the country. Owing to its strategic importance, it has attracted important investments in hydraulic infrastructures. However, the variability of the meteorological and hydrological regimes leads to water scarcity in this region. Therefore, it is essential to analyze drought characteristics at the river basin scale. This study intends to analyze a large data set obtained from 25 weather stations during a period of 42 years (1970-2011) and to estimate the regional drought characteristics. We have analyzed precipitation data by linear regression and vector methods to develop a complete and homogeneous rainfall database. Next, we have estimated the drought characteristics, frequency, and duration, using the standardized precipitation index. Results indicate the occurrence of dry periods during the following time intervals 1980-1987, 1991-1995, 1997-2002, and 2006-2008 in the Oum Er-Rbia river basin. They exhibit a relatively high frequency with intensity ranging between moderate and severe. The spatial extent of droughts in the basin has increased over time. This could have impacts on water resource availability.