Quantification of hydrological processes and water requirements of aquatic ecosystems is required for many projects related to environmental security and efficient water use in agriculture. This also applies to the Asian monsoon region. This paper focuses on the Walawe River basin, located in a semi-arid zone of southern Sri Lanka. The two major reservoirs in the upstream and middle reaches of the river with a total capacity of 486 million cubic meters (MCM) have significantly affected the hydrology of the river, with associated adverse environmental and social consequences.