Assessment of Environmental Flows and River Health in the Middle Regions of Ganga River and its Tributaries

Assessment of Environmental flow (E-flow) plays a vital role for achieving sustainable water resources management. This study intends to develop suitable methodologies for assessing E-flows in the middle reaches of the Ganga River. The methodology includes developing a hydrological model, hydrological alteration analysis, stream health assessment, and E-flow estimation using flow health indicators. First, a hydrological model has been simulated, calibrated and validated in northern and southern tributaries of Ganga River (namely Kosi, Gandak and others) of the study region. The outcomes of the model reveals that the monthly discharge for ungauged basins is promising and aids in completing the water balance analysis for the entire reach. Second, hydrological alternation analysis is carried out using Indicators of Hydrological Alternation tool in the tributaries and the main river. The analysis indicates that no significant changes occurred in the river and its tributaries' flow for the last four decades. However, the water balance flow chart shows notable variations in the interaction patterns between surface water and groundwater. Third, stream health conditions of the river are analysed by using Flow Health tool. The results represent the natural flow variation of the stream for the analysis period. Since the deviation between natural and observed flows reflects the stream's health condition, a detailed stream health analysis is carried out by considering various combinations. The analysis reveals that stream health and its temporal variation of upstream are good with the less temporal variation. At major tributary level, the stream health and its temporal variations are deteriorating from 1970s. At the main Ganga reach level river health and its temporal variations does not show any declining trend. Finally, E-flows are estimated in two methods; computing ten percent Mean Annual Flow and computation of monthly E-flows using Flow Health. The outcome of the study demonstrates, Flow Health tool can be applied for estimation of the variations in the monthly E-flows in the case of rivers like Ganga.