The study aims to determine the runoff depth using Soil Conservation Service Curve Number (SCS-CN) method in Geographical Information System (GIS) environment. For River Asa Watershed, the SCS Curve Number method has been adopted for estimating the runoff depth using Rainfall data from 1987-2018. Land use and change cover map were used for the classification of soil type, in order to determine the Hydrological Soil Group (HSG) using ArcGIS. The runoff was estimated from the rainfall runoff equation. The Antecedent Moisture Condition (AMC), potential maximum retention(s) and initial abstraction were computed. Thematic maps such as Soil and Land Use / Land cover and have been used in conjunction with hydrological data for determining hydrological soil Group (HSG) and Curve Number (CN) for land used and change cover classes over the watershed. The values of Hydrological Soil Group, Curve Number and Annual runoff depth varied (6.28-914.22) km2, (65-100) and (16.50-144.89) × 106 m3. The study shows that the high runoff depth was observed in Hydrological soil group (HSG), when compared with curve number (CN), this is due to dense vegetation cover.