HYDROGRAPH SEPARATION AND SNOWMELT MODELLING OF RIVER BHAGIRATHI AT HARSHIL

Abstract

Snowbound and glacier-bound catchments of the Himalayas are the prime water source for a wide range of the population. The population downstream relies on the streamflow from such catchments to fulfil their domestic, industrial, and agricultural water demands. Water from snow and glacier melt is a significant component of the annual streamflow apart from direct runoff and baseflow. With increasing global warming, there has been a significant loss in glacier and snow cover, which will increase river flow due to the increase of glacier melt and snowmelt. This brings a change in the runoff pattern and the contribution of different sources in runoff in the basin in both space and time. However, over time, due to the excessive retreat of glaciers, the snowmelt contribution would reduce, and the river discharge would reduce, causing a reduction in water availability downstream. Therefore, it becomes essential that the study for quantifying the contribution of different sources to the streamflow and modelling is done to understand the streamflow dynamics in a much better way. The current study aims to model the discharge on the snowmelt runoff model (2016 -2021) and also to quantify end members' contributions to the river Bhagirathi at Harshil for the winter and spring seasons (1 December 2021- to -9 April 2022). The hydrograph separation is done by application of isotope hydrology. A three-component mixing model is applied using EC and δ 18O as tracers. The average percentage of contribution of surface runoff, groundwater, and ice melt is 13.07 %, 44.66% and 42%, respectively. The groundwater was the major contributor before the ablation started, with an average percentage contribution of around 46.46 %. The ablation starts in second week of march, and ice melt contribution starts rising and peaks at 61.20 % at the end of the study period. For the snowmelt runoff modelling, the snow cover area and hydroclimatic data are used to model the discharge at Harshil. The model is calibrated from July 2016 to Dec 2018 and validated from 2019 to December 2021. The average Dv and NSE are -2.20% & 0.92 during calibration and -6.27 % & 0.89 during validation, respectively.