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

K R J Perera,N T S Wijesekera

doi:10.4038/engineer.v44i3.6960

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 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.

Highlights

Estimating runoff from rainfall records in the absence of stream gauge records is extremely important in water resources development
Runoff coefficients enable the estimation of runoff for practical applications such as water resource management and river basin planning
Determining runoff coefficient and its variation with the major parameters is important for water resources assessments giving due consideration to the soil, slope and land use variations

Summary

Introduction

Estimating runoff from rainfall records in the absence of stream gauge records is extremely important in water resources development. It is more so in Sri Lanka where most of the watersheds are ungauged. Abulohom et al [1] have developed a rainfall runoff model based on water balance equations where inputs to the model include precipitation and potential evapotranspiration on monthly basis which in turn give simulated runoff at watershed outlet. Determining runoff coefficient and its variation with the major parameters is important for water resources assessments giving due consideration to the soil, slope and land use variations. Eng., FIE(Sri Lanka), MICE(UK), PG Dip Hyd Structures (Moratuwa), M. Senior Professor of Civil Engineering, Department of Civil Engineering, University of Moratuwa, Sri Lanka

Study Area

Kilometers

Data and Methodology

Cultivation

Results and Discussion

Conclusions