Assessing Uncertainty in Climate Change Effects on Design Hydrographs: A Case Study in the Kalu River Basin, Sri Lanka

Changes in precipitation patterns, hydrological regimes, and increasing frequency of extreme weather events introduce uncertainties in design hydrographs under changing climate conditions. This study aims to assess the uncertainties of design hydrographs attributed to future climate change in the Kalu River Basin in Sri Lanka focusing on the Ellagawa subbasin. The Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) rainfall-runoff model was calibrated and validated using four extreme events selected using a frequency analysis with a 100-year return period. Precipitation extremes and their future changes were assessed using three General Circulation Models (GCMs): CNRM-CM61, HadGEM3-GC31-LL, and MRI-ESM2-0 under two Shared Socioeconomic Pathways (SSP2 and SSP5) from the Coupled Model Intercomparison Project Phase 6 (CMIP6) outputs downscaled to local scale. Generalized Extreme Value, Weibull, and Gamma distributions compared the observed and projected annual maximum daily precipitation. The simulated streamflow closely matched the observed streamflow, with Nash-Sutcliffe coefficients of $0.79-0.85$ during calibration and verification. Projections show annual total rainfall increases from -8% to 40% (SSP2-4.5) and from - 10% to 36% (SSP5-8.5) compared to the baseline period 1980-2017. Design hydrograph peak flow increases from 3% to 106%. These findings highlight the importance of considering climate uncertainties in hydraulic designs to minimize the potential risks.