Effect of Irrigation Reservoirs on Streamflow Simulation in Dry Zone, Sri Lanka

Advances in technology and its applications to hydrology and water resources engineering have created different approaches in modeling the hydrological response to climate change. However, hydrological modeling of a river basin where a major component of rainwater utilised for irrigation purposes through rainwater storage reservoirs and interconnected irrigation canal systems remains complex. This study attempted to simulate the hydrological processes of the Malwathu Oya basin, which is comprised of reservoir operations and inter-basin diversions using the HEC-HMS software package developed by the United States Army Corps of Engineers. The irrigation water requirements of major irrigation schemes in the Malwathu Oya basin during the Yala and Maha cultivation seasons were estimated using the CROPWAT model, and estimates were incorporated in hydrological modeling. Rainfall-runoff simulations were conducted for two scenarios (with and without reservoir operations) to analyse the impact of major reservoirs on river runoff. Average seasonal streamflow simulations at the Kappachchi river gauging station for with and without major reservoirs during the Yala season were 99.86 million m3 and 189.63 million m3 and during the Maha season, estimates were 359.09 million m3 and 686.88 million m3. Runoff coefficient estimates for two scenarios imply the importance of incorporating reservoir operations in comprehending the hydrological processes of a reservoir-rich river basin.