Climate Change Impacts on Low-Flows in Maha Oya River Basin, Sri Lanka: Implications for Domestic Water Security

Water security in small island developing countries like Sri Lanka is increasingly threatened by climate change and anthropogenic pressures. Maha Oya River, located in the intermediate climatic zone of Sri Lanka, is a vital water source for over four million people who face increasing water demand and hydrological stress. Despite its socio-economic centrality, the basin’s hydrological sensitivity to climate change remains inadequately quantified by scientific research. Therefore, this study assessed the climate-driven dynamics of low-flows in the Maha Oya River basin by a hydrologic modelling approach coupled with future rainfall projections to evaluate the adequacy of low flows for planned potable water extractions. HEC-HMS hydrological model was calibrated and validated using observed streamflow data at Giriulla and Badalgama from 2009 to 2018, with goodness-of-fit values falling within acceptable ranges. The study utilized NASA NEX-GDDP CMIP6 dataset and HadGEM3-GC31-LL and CESM2 Global Climate Models (GCMs), under SSP245 and SSP585 scenarios, to project future rainfall, with bias correction conducted using the Quantile Delta Mapping (QDM) method. The calibrated HEC-HMS model was used to simulate the future streamflow across near-future (2019–2049), mid-future (2050–2074), and far-future (2075–2099) periods. Low-flow trends, defined by the 90-100% flow exceedance percentiles, were identified across all projected time periods and evaluated against anticipated potable water abstraction demands. Results revealed severe reductions in low-flow conditions, with decreases of 90–100%, anticipated by the end of the 21st century under both scenarios. Key intake locations, including Mawanella-Kappagoda and Hemmathagama, faced persistent near-zero flows, jeopardizing planned extractions (333,230 m³/day by 2040). Deficits were intensified under SSP585, with the CESM2 projecting 100% loss of low flows by mid-century. While upstream zones demonstrated transient recovery, downstream regions exhibited irreversible drying. The proposed expansions in potable water extractions at Rambukkana (22,000 m³/day) and Hemmathagama (24,200 m³/day) significantly exceeded projected availability, risking supply-demand imbalances during critical dry periods. This study underscores the urgency of adaptive strategies such as managed aquifer recharge and demand governance to mitigate water insecurity. Furthermore, the study contributes to the development of a methodology that integrates CMIP6 projections into basin-scale hydrologic modelling, providing broadly applicable insights for assessing climate-impacted water availability in tropical catchments.