Southern Iraq, especially Wasit and Maysan governorates, is experiencing flash flooding and water shortages. Limited hydrological research and streamflow data limit runoff estimates, flood risk assessment, disaster preparation, and floodwater harvesting. Simulating the hydrological and hydraulic response of a 2,945.33 km² watershed during a March-May 2019 flood and modelling rainfall-runoff processes with 2019 daily rainfall data in ArcGIS, HEC-HMS, and 2D HEC-RAS highlights the potential for floodwater harvesting to address seasonal shortages. In the HEC-HMS software, the SCS-CN method for losses, the SCS Unit Hydrograph for transformation, and the Muskingum routing method were used to compute run-off in each watershed reach. The extracted discharge outputs were input as upstream boundary conditions to a 2D HEC-RAS model to estimate flood extent. Simulations revealed about 144.4 million m³ of runoff volume, with initial abstraction (la) having the greatest impact on volume. Qualitative comparison with Sentinel-2 satellite imagery from a similar flood occurrence indicated good spatial consistency for model validation. Because of their concentration on flood forecast accuracy, quantitative measures like CSI were evaluated but not employed as validation evidence. Although data is rare in ungauged arid basins, integrated modelling can improve water planning and flood risk management if monitored and regulated.