Evaluation of Urbanization Impacts on Flood Generation in the Adigrat Watershed, Northern Ethiopia

Abstract Rapid urbanization alters watershed hydrological processes by increasing impervious surfaces and modifying runoff generation. This study assessed the impacts of urbanization on flood generation in the Adigrat watershed, northern Ethiopia, using the Hydrologic Engineering Center–Hydrologic Modeling System (HEC-HMS) integrated with GIS-based spatial analysis. Land-use maps for 2000 and 2018, soil data, and a 30 m × 30 m digital elevation model (DEM) were processed using ArcGIS, Arc Hydro, and HEC-GeoHMS to derive watershed characteristics and Curve Number (CN) grids. Owing to the absence of a streamflow gauging station within the watershed, discharge data were transferred from the nearby Sluh station using the area-ratio method. The HEC-HMS model was calibrated for the period 1998–2012 and validated for 2013–2017 using the Nash–Sutcliffe Efficiency (NSE) and the coefficient of determination (R²). Urbanization increased the average CN and impervious surface area while reducing lag time and initial abstraction. Consequently, peak discharge at the watershed outlet increased from 6.1 m³ s⁻¹ in 2000 to 8.8 m³ s⁻¹ in 2018, representing a 30.68% increase. Similarly, flood volume increased from 166.49 × 10⁶ m³ to 215.09 × 10⁶ m³ , corresponding to a 29.19% increase. These findings demonstrate that urban expansion has substantially intensified flood magnitude and accelerated watershed response, highlighting the need to incorporate hydrological considerations into urban planning and flood risk management strategies in Adigrat and other rapidly urbanizing watersheds.