DETERMINATION OF POTENTIAL RUNOFF COEFFICIENT USING GIS; CASE STUDY: KALUTARA DISTRICT

Abstract

Runoff coefficient is a hydrologic parameter that depends on land use, soil type, and soil moisture to describe the storm water runoff capacity for drainage areas. The Geographic Information System (GIS) technology and available data for a Kalutara district were used to estimate the potential runoff coefficient (PRC). The estimated PRC was used to calculate the depth of runoff. The land use map was categorized mainly into 11 classes including seven impervious areas and four pervious areas. Impervious surfaces were categorized as commercial and industrial areas, built-up land, residential area, open water, rock area, wetland, transportation and communication utilities. Pervious surfaces categorized as forest, grass, bare soil and crop. The slope map was developed from the digital elevation model (DEM) with 30 m resolution. The Sri Lankan wet zone soil map was used to prepare the soil texture map. These maps were overlayed using GIS technology to create the PRC map. Annual runoff depth was derived based on the annual rainfall surplus and runoff coefficient per pixel using the raster calculator tool in ArcGIS. The kriging interpolation technique was used to estimate the rainfall of the study area in GIS environment. Obtained PRC value range varied from 0.13 to 1.0, and runoff depth values vary from 31.0 mm to 479.2 mm. Results indicate that the highest PRC values for frequently flooded areas; Bulathsinhala and Palindanuwara. These results can be useful for identifying floodplains, suitable locations for water harvesting, and improving water resources management programs.