Impact assessment of Hathnur reservoir on hydrological regimes of Tapi River, India

The reservoir regulations exhibit varied impacts on streamflow regimes depending upon their operation policy, geographical location, hydro-climatology and localized changes within the catchment. In the present study, degree of hydrologic alterations in flow regimes of Middle Tapi River in the downstream of Hathnur reservoir is investigated by analyzing statistically 23 hydrologic indices representing the magnitude of mean flow, duration, timing, frequency and rate of change of extreme flow conditions. The results indicated considerable decrease (≈-21.4%) in mean annual runoff in the downstream channel during the post-dam period (1983–2004) vis-à-vis pre-dam period (1973–1982). The flow duration curves for daily streamflows exhibited significant deviations at discharge values lesser than 100 m3/s in the downstream channel during post-dam period. The number of high/low pulses observed in the downstream channel reported decrease/significantly increase during post-dam period. The significant decrease in timing of low flows during the post-dam period (from early May to mid-January) implied early drying up of the downstream channel. The overall degree of hydrologic alteration has been found to be moderate (≈62.0%) in the downstream channel. Thus, the Hathnur reservoir exercises significant influence on low streamflow regimes, while minimal influence is observed on high streamflow regimes in the downstream channel during post-dam period. © 2019 Indian Society for Hydraulics.