The primary goal of this research is the hydrological modeling of rainfall-runoff in the Watir Valley Basin using the HEC-HMS Model in a GIS environment. This will allow for the calculation of peak discharges, direct runoff volume, and the creation of hydrographs in the return periods (25–50–100 years) of the sub-basins in the Watir Valley Basin. In this regards, the result of the modeling in the return periods of the sub-basins in the Watir Valley Basin where the total peak discharge in return period 100 years is 5257.15 (m3/s) and sub-basin 22 is the largest peak discharge in return period 100 years where reached 630.62 (m3/s) and the total discharge volume in return period 100 years is 122855 (1000 m3) and sub-basin 22 is the largest discharge volume in return period 100 years where reached 20048.79 (1000 m3). The majority of research concluded that HEC-HMS rainfall-runoff modelling was effective and trustworthy for accurately projecting runoff in different river basins.