Streamflow variability under climate change scenarios in Kelani river basin, Sri Lanka

Abstract

In recent years, the downstream floodplain of the Kelani River Basin has been suffering from frequent floods. With the current climate change trend, flood-related damages are expected to amplify in future. Therefore, understanding how such extreme events behave in the future is quite essential for the basin planners and managers. Using the output from three global climate models (CNRM-CM6-1, MRI-ESM2-0, EC-Earth3-CC, and their ensemble) from the Sixth Phase of Coupled Model Intercomparison Project (CMIP6), variation of streamflow was evaluated from historical (1985-2014) to mid-century (2030-2059) and late-century (20702100) periods under Shared Socioeconomic Pathways (SSPs 2-4.5 and 5-8.5).

Bias-corrected

precipitation and temperature

through linear scaling and power transformation, respectively were fed into the HEC-HMS model to project future river discharge. Based on the ensemble model, changes in average annual discharge indicates an increasing trend in future periods under both scenarios considered. Significantly high changes were projected in the late century (25-40 m 3 /s) compared to the mid-century (15-25 m 3 /s) under both scenarios. Projected changes in monthly streamflow indicate an increasing trend for wet months (June-December) while a decreasing trend for dry months (January-May). Further, the highest changes in streamflow were identified in the monthly changes (-5 to 60 and -5 to 100 m /s under SSP24.5 and SSP5-8.5, respectively). Comparison of seasonal changes shows the highest increase

for Southwest Monsoon (40-60 m 3 /s under both scenarios and future periods) while the highest decrease for First-Inter monsoon (<-10 m 3 /s under both scenarios and future periods). Moreover, changes in high, median, and low flows (5%, 50%, and 95% percentiles of flow duration curve, respectively) indicate significant changes in high flows compared to the median and low flows. The findings of this study suggest a significant increase in high flows during the Southwest Monsoon that can further threaten the basin with the devastating floods. The results provide important insight for planners and other stakeholders dealing with water resources management in the basin. 3