Modified SCS-CN and Green-Ampt Methods in Surface Runoff Modelling for the Kundahpallam Watershed, Nilgiris, Western Ghats, India

Abstract

Kundapallam watershed is located in Nilgiris District of south India. This ungauged watershed has an areal extension of 14.37 km2. There are increasing numbers of rainfall induced landslide occurrences in this area. The causes of rainfall induced landslides require a thorough understanding of runoff characteristics of the watershed. In this respect, the runoff of the study area is estimated with two approaches such as modified Soil Conservation Curve Number (SCS-CN) and Green-Ampt loss method. By these approaches, the related thematic factors on runoff characteristics are prepared with satellite images by using ARCGIS and IDRISI image processing software. The catchment delineation is generated for the Kundapallam watershed to get watershed parameters by using HEC-GeoHMS extension in ARCGIS. Curve number is assigned based on Soil Wetness Index (SWI). Sixteen soil samples were collected from different localities for the determination of hydraulic conductivity, moisture content, wetting front capillary pressure, bulk density, porosity etc. The infiltration rate of sample locations is determined by using double ring infiltrometer. Then all the parameters are assigned to models applied in this study to determine runoff characteristics of the study area. The accuracy and goodness of fit of the model are tested by determining the correlation coefficient (SCS-CN = 0.979 and Green-Ampt = 0.985) and coefficient determination (SCS-CN = 0.958 and Green-Ampt = 0.97) and it indicates statically positive correlations and perfect fit between the results of the study. The results showed that the modified SCS-CN provides low runoff estimation compared than Green-Ampt loss method.