Identification of the spatial variability of runoff coefficients of three wet zone watersheds of Sri Lanka

Abstract

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 stream flow determination as a function of land US£r soil and ilope from developed CIS model 7 his study deveioped a method to estimate runoff ctwfficient as a function of land use, soil and slope within the wet zone basins of Sri Lanka.

Three Wet Zone basins, Ka!u Canga, Keiani Ganga and Attanagalu Oya were seated for the study. Regression analysis showed that the computed runoff agreed with the observed runoff with R- values of 0.80, Q.78 and 0.83 for K.ilu Ganga, Kelani Ganga and Attanagalu Oya basin respectively. Averaged runoff coefficients, for basins with the spatial variation were calculated as 0.52, 049 and 0.51 for Kelani Ganga, Kaiu Ganga and Altanagalu 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.