Evaluation of Climate Elasticity of Runoff based on Observed Rainfall, Streamflow and Simulated Future Streamflow using SWAT Model in Kelani Ganga Basin

Abstract

Kelani Ganga basin is the 7 th largest watershed in Sri Lanka, spanning over 2,292 km 2 and annually discharging 4,225 MCM flow to the sea. The basin currently hosts over 19% of the country's population and is the primary source of drinking water to over 4 million people living in Greater Colombo. Hence, the present study was undertaken to evaluate the Climate Elasticity of runoff based on observed rainfall/ streamflow data and simulated future streamflow using SWAT Model in Kelani Ganga basin, targeting sustainable management of basin water resources in future. The runoff elasticity (ε) is assessed by two methods for current and 2040. All selected three hydrometric gauging stations exhibit significant downward trends for the period of 1980 to 2016. An 80% of the rain gauges in the middle and upper basin show significant decreasing trends for high to low rainfall totals for Yala season as Innovative Trend Analysis (ITA) for the period of 1980 to 2016. Mass balance performance Error (Er), Nash-Sutcliffe efficiency (NSE) and coefficient of determination (R 2) are used as multi-objective functions and 8.90%, 0.65, 0.72 and 9.10%, 0.69, 0.69 are obtained, respectively for the calibration and validation periods of 1970 to 1980 and 1982 to 1992, respectively, for SWAT model. 1 ⁰C of temperature increase causes 6.9% and 7.4% runoff decrease for current scenario and 0.4% increase and 1.5% decrease of runoff for Future Pessimistic Climate Change Scenario as evaluated by two methods, respectively. 1% of rainfall increase causes runoff increase of 0.002% and 0.370% for the current scenario and runoff increase of 0.005% and 0.360% for 2040 as evaluated by two methods, respectively. It is recommended to further analyse the water allocation model for better results with practical implementations by considering the identified trend after 1995 in future researches for planning and management of water resources in future.