Establishment of dry weather flow in Kalu ganga under climate change scenarios

Abstract

Kalu Ganga basin is one of the most important river basins in Sri Lanka which covers a major portion of the wet zone and carries the highest discharge volume into the sea annually. Therefore it has been identified that, Kalu Ganga is the main source of water for potable water supply schemes especially for the Greater Colombo area. It is the dry weather flow which determines its potential as a source of water for potable water supply schemes with no major ‘at the source storage’. This study is focused on the analysis of low flow due to climate change. The objective of the study is to establish low flow conditions in the Kalu Ganga basin under different climate change scenarios. Effect of the predicted climate change scenarios on the low flows can be taken into account by using Statistical downscaling with emission scenario consideration. Statistical Downscaling Model (SDSM) is used for the downscaling of GCM data. Downscaling of GCM data using SDSM Global Circulation Model data of Hardley Center Coupled Model 3 (HadCM3) is used under both A2 and B2 emission scenario as the raw data. A variation of future rainfall is analyzed with observed data. Catchment runoff is predicted using a MIKE11 NAM based hydrological model for 50 years. Frequency analysis is conducted for measured and predicted flow data to establish low flow values due to climate change impacts. Reduction of low flow in Ellagawa station about 6% and Millakanda Station about 4% in 50 year return periods. It is about 1% for other return periods in both stations. Impact of climatic change is high for the events with high recurrence interval. Hence it is not recommended to extract water during dry period. Therefore, having an appropriate storage system to cater the required demand during dry season is needed.