Assessing the Impacts of Interbasin Water Transfer Reservoir on Streamflow

Abstract

The hydraulic structures constructed for the benefit of society can cause perturbations in streamflows. Such disruptions to natural conditions may affect river ecosystems. The spatial scale of the impacts is further magnified in the case of interbasin water transfer (IBWT) projects, which can cause changes in the ecohydrological regime of donor as well as recipient watersheds. Assessment of the changes in the hydrological regime due to artificial structures requires a comparison of existing and counterfactual scenarios. In this paper, a methodological framework is proposed and applied to the Umiam Watershed to generate natural streamflow using postimpact data when preimpact data are not available. The Soil and Water Assessment Tool (SWAT) model is applied for simulating streamflow in the presence of a reservoir and water transfer out of the watershed. The Indicators of Hydrologic Alteration (IHA) method was used for the analysis of changes in streamflow. The results reveal a reduction in the monthly median flow rate by at least 21% in all months during the five-decade period. Large floods and small floods are found to be reduced in the presence of the reservoir, whereas it also causes more frequent low flows, which last longer. The extreme annual minimum flow conditions (1-, 3-, 7-, 30-, and 90-day minima) show low alteration, while the corresponding yearly maximum flow conditions show medium alteration. The frequency and duration of high and low pulses are greatly affected by the reservoir. The rising rate, falling rate, and number of reversals show a significantly decreasing trend. The results indicate a high overall degree of alteration in streamflow.