Occurrence of extreme hydrologic events in the kelani river basin and impact of climate change on river flow regime

Abstract

Climate change is a severe and growing issue with visible and disproportionate impacts in many parts of the world. Recent studies state that climate change is occurring at a relatively faster and even more critical rate than previously expected. Climate consequences such as harsher heat waves, longer droughts, more frequent floods, increased sea level rise, and storm surges are already experienced worldwide. Climate change is expected to impact Sri Lanka through changes in rainfall patterns, sea level rise, and increased temperature. Climate changerelated variations in temperature and rainfall patterns can result in more prolonged droughts and frequent floods. The IPCC's 6th Assessment Report on Climate Change states that the average global surface temperature is anticipated to increase, ranging from 1.1-1.8 °C, 2.3-3.6 °C, 3.3-4.7 °C, and 3.3-6.7 °C, 4.3-7.8 °C under SSP1, SSP2, SSP3, SSP4, and SSP5 scenarios, respectively, relative to 1986-2014 (IPCC, 2022). The Kelani River basin, which experiences yearly flooding, is one of Sri Lanka's most vulnerable basins (Dissanayaka & Rajapakse, 2019). The study aims to evaluate how climate change may affect river flow change and the occurrence of extreme hydrologic events in the Kelani River basin. This study uses a hydraulic model to analyse river flow and precipitation data to identify extreme hydrological events and simulate river flow while considering the effects of extreme rainfall. The study also aims to determine future trends in river flow variation caused by climate change. Observed rainfall, streamflow, and Simulated Future Streamflow using GCM data were used to identify future trends of river flow variation due to climate changes based on the HEC-HMS hydrological modeling process. The results revealed that the Hanwella sub-basin exhibits a variation in the range of -35.2% to 37.46% in the annual mean streamflow percentage under assumed climate change scenarios. This study also aims at developing quantitative estimates to handle the impacts of these extreme hydrological events, and flood events become more frequent when considered for SSP5 scenarios than SSP2 scenarios. The model results can be used to understand better catchment characteristics and its hydrologic response to rainfall under the impact of climate change scenarios. The return periods for floods in the Kelani River basin were calculated using the projected future data for both SSP2 and SSP5 scenarios. The results indicated a significant decrease in return periods, particularly in the SSP5 scenario, indicating that future flood events will occur more frequently. Applying the Gumbel distribution is appropriate for the frequency analysis of the Kelani River basin. This statistical method permits the analysis of extreme events and provides valuable insights into the frequency and magnitude of particular hydrological phenomena within the basin.