Flow alterations due a constructed reservoir in the Menik Ganga basin, Sri Lanka

Abstract

Natural water flows and their ecosystems are altered due to manmade hydraulic structures like dams. However, limited research on hydrologic alteration has been carried out in developing countries. This research explores the hydrologic alterations which occurred in the Menik Ganga basin, Sri Lanka due to the Weheragala reservoir constructed in 2009 for inter basin transfer. The hydrologic variations of the flow regime due to the construction of the reservoir was analyzed using Indicators of Hydrologic Alterations (IHA). For this purpose, we compared the calculated IHAs for streamflow at the Kataragama station (located downstream of Weheragala) during pre-construction (1990–2009) and post construction (2010–2019) periods. Also, the IHAs of simulated flows from the Water Evaluation And Planning (WEAP) model during 2010–2019 was compared with observed gauge discharge during the same period. The monthly observed flows in the “Maha” rainfall season (September to March) demonstrated a decreasing trend in post dam period with respect to pre dam period (highest decrease of 77 m3/s during October), whilst it showed an increasing trend (highest increase of 5 m3/s during August) in the Yala season (May to August) in the post-reservoir construction scenario. This was further visualized by comparing the indicators of the simulated flows with observed for post reservoir period, in which highest percentage differences occurred in June (− 4000% in 2016) and November (− 300% in 2010) for Yala and Maha periods respectively. Large alterations of the river flow due to the impoundment depicted by higher percentage differences. These alterations are extensively examined by other indicators as well. The fluctuations of flows have been decreased due to the construction of the reservoir which resulted in reductions of low and high pulses. The results are highly appealing to the authorities who are in water resources management to reach sustainable goals.