Assessment of Surface Runoff from Sub Basin of Kodayar using NRCS CN model with GIS

Objectives: To estimate the runoff from the sub basin of Kodayar, Tamilnadu using Natural Resources Conservation Service Curve Number (NRCS-CN) method with GIS and Remote sensing techniques. Methods: The GIS and remote sensing techniques facilitate accurate estimation of surface runoff from watershed. Thematic maps such as soil map, land use and hydrologic soil group (HSG) maps have been generated with GIS environment. Weighted CN value for the basin was obtained and the runoff for the selected rainfall event was estimated. Findings: In the HSG map, the region predominantly comprises of group B (56%) followed by group C (27.4%), group D (12.4%) and A (3.9%) respectively. In order to calculate the CN value of the basin, it is divided into 12 sub basins of which each sub basin has area less than 150km2 which gives more accurate result than considering the whole basin. The CN values for these 12 sub basins are ranging from 69 to 80. Composite curve number value for each sub-basin was calculated by multiplying weights according to the area occupied by each land use class and the corresponding curve number. Weighted CN value and the storage retention (S) for the entire basin are estimated as 75 and 85 respectively. Runoff values are estimated on four different rainfall events selected during the study period in this basin. Conclusion: The study ultimately leads to the development of the rainfall runoff model based on NRCS-CN-GIS technique. The runoff for any rainfall event occurred can be estimated by feeding the inputs into this corresponding model which would in turn be helpful on significant conservation of water in the study area.