Identification of the spatial variability of runoff coefficient of three Wet Zone Watersheds of Sri Lanka for Efficient River Basin Planning

Increasing population and rapid economic and technical developments are threatening water resources all over the world. Careful management of natural resources has become one of the major concerns of watershed managers to protect and prevent depletion of resources for a sustainable future. As streamflow is a key consideration in water resources management, this study focuses on streamflow determination as a function of land use, soil, slope and rainfall from developed GIS model. Estimating runoff from rainfall records in the absence of stream gauge records is an important step in Sri Lanka, because most of the watersheds are ungauged. Even though catchment-specific studies have been carried out to identify the effect of rainfall over runoff, it is not addressed in wet zone basins of Sri Lanka. This study therefore developed a method to estimate runoff coefficient as a function of land use, soil and slope within the wet zone basins of Sri Lanka. Sub-basins of three rivers Kalu Ganga, Kelani Ganga and Attanagalu Oya (wet zone basins) were selected as the study areas. Spatial analysis of the above factors for the three subbasins was done by incorporating GIS into the system. Runoff rainfall ratio calculated for the basins under the assumption of homogeneity were 0.40, 0.72, and 0.70 for Attanagalu, Kelani and Kalu subcatchments at Karasnagala, Glencourse, and Putupaulla gauging stations, respectively. Regression analysis showed that the computed runoff agreed with the observed runoff with R2 values of 0.80, 0.78 and 0.83 for Kalu Ganga, Kelani ganga and Attanagalu Oya sub-basin respectively while overall R2 calculated to 0.73. Averaged runoff coefficients, for basins with the spatial variation were calculated as 0.52, 0.49 and 0.51 for Kelani Ganga, Kalu Ganga and Attanagalu Oya sub-basins