Identification of the Spatial Variability of Runoff Coefficients of Three Wet Zone Watersheds of Sri Lanka

College Assistant Professor of Civil Engineering, Department of Civil Engineering, Eng. (Dr.) (Mrs.), B.Sc. Eng. (Moratuwa), M.Phil. (Moratuwa), MS(USA), Ph.D. (USA), AMIE(Sri Lanka), Senior Professor of Civil Engineering, Department of Civil Engineering, Eng. (Prof.), B.Sc. Eng. (Sri Lanka), C. Eng., FIE(Sri Lanka), MICE(UK), PG Dip Hyd Structures (Moratuwa), M. Eng. (Tokyo), D. Eng. (Tokyo). Runoff estimation from rainfall records in the absence of stream gauge records is essential in Sri Lanka, because most of the watersheds are ungauged. Since runoff depends on the catchment characteristics in addition to the rainfall, this study focuses on streamflow determination as a function of land use, soil and slope from developed GIS model. This study developed a method to estimate runoff coefficient as a function of land use, soil and slope within the wet zone basins of Sri Lanka. Three Wet Zone basins, Kalu Ganga, Kelani Ganga and Attanagalu Oya were selected for the study. 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 basin respectively. 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 basin respectively. Study revealed that credible runoff coefficient will not be represented simply by the ratio between runoff and rainfall where runoff depends highly on catchment characteristics.