Assessing the Impacts of Projected Climate Scenarios on Irrigation Reliability and Reservoir Sustainability in Sri Lanka’s Dry Zone

Climate change poses major challenges for irrigation water management in Sri Lanka’s dry zone. This study assesses the performance of the Nachchaduwa reservoir under historical and projected climate scenarios using multiple General Circulation Models (GCMs). Eight GCMs were ranked through compromise programming, with the top three models bias-corrected and downscaled to generate climate projections for the Shared Socioeconomic Pathways (SSP) scenarios: SSP2-4.5 and SSP5-8.5, under near-future (2015–2050) and mid-future (2051–2075) conditions. The ensemble GCM data used for HEC-HMS to simulate inflows, HEC-ResSim to reservoir water levels, and releases for these future scenarios. Current and future irrigation requirements were estimated using CROPWAT for the assessment of Reliability, Resilience, and Vulnerability (RRV) metrics, as well as the Reservoir Sustainability Index (RSI). Trend analysis indicates that precipitation is increasing by 2.6% -14.2%, particularly under the SSP5-8.5 scenario. Temperatures are rising, with minimums increasing by 3.9% -9.6% and maximums by 1.5% -5.4%. The HEC-HMS model suggests that annual flows will experience a slight decrease under SSP2-4.5 and an increase under SSP5-8.5. Mean annual irrigation demand is projected to rise by 40.5%- 43.6% in the near and mid-future across both SSP scenarios. Reliability remains near 60%, resilience varies 48–58%, indicating moderate recovery after deficits. Vulnerability remains below 30%, indicating inadequate irrigation flows, with RSI values ranging from 0.22 to 0.35. Combining RRV metrics with RSI shows the Nachchaduwa reservoir is moderately sustainable under future climate scenarios. However, adaptive strategies like optimised operations and management are vital to maintain irrigation reliability.