Estimating rainfall, surface runoff, and river discharge in the Shilabati river basin using an integrated RS and GIS based SCS CN approach

Accurate runoff data is scarce in tropical plateaus, creating a significant challenge for hydrologists. Determination of the surface runoff volume and depth in Shilabati River Basin (SRB) is very important for the need of efficient management of the lack of water resources and mobilization of flows of SRB. Soil conservation service-curve number (SCS-CN) method coupled with geospatial technology was applied to estimate the depth, volume, and discharge rate of surface runoff in SRB. Occupying an area of 3500 km2 and receiving an average annual rainfall 2106 mm, SRB is located on western hard rock plateau fringe of West Bengal, India. Various geo-hydraulic parameters viz., slope, hydrologic soil groups (HSG), land use / landcover (LULC), and average rainfall were developed in GIS domain and processed to develop the curve number (CN) and runoff maps. SRB was categorized into three HSGs i.e. A (71%), B (18.2%), and C (10.8%). The findings revealed the presence of six predominant categories of land use and five major slope classes in the SRB. Under standard conditions, the mean curve number is recorded at 80.12, whereas for dry conditions, it measures 64.12, and in wet conditions, it rises to 89.98. Furthermore, the analysis showed that CNI, CNII, and CNIII values range between 54 and 85, 72 to 92, 86 to 98 respectively in agricultural and habitat areas. Utilizing the SCS-CN method, the runoff depth for the SRB was determined to range from 1920.9 mm to 2184.8 mm. This analysis revealed total runoff volumes varying between 172 m³ and 236 m³, while the flow rates were observed to fluctuate from 0.5 m³/s to 179.1 m³/s. The analysis indicates a strong positive correlation between rainfall and runoff, with an (r) value of 0.97. This finding emphasizes the need for further exploration of the SCS-CN method when combined with RS and GIS to effectively estimate runoff in ungauged basins. This approach is crucial for promoting optimal water resource conservation and mitigating the risks associated with drought conditions.