Assessing the Effects of Snowmelt Dynamics on Streamflow and Water Balance Components in an Eastern Himalayan River Basin Using SWAT Model

The Soil and Water Assessment Tool (SWAT) model was configured to simulate streamflow under data scarcity scenario for a snow-dominated, forested, mountainous Mago River basin located in the Eastern Himalayan region of India. To account for the role played by snowmelt dynamics on streamflow and its influence on water balance components of the basin, two separate projects were set up: a Reference project in which only hydrological parameters related to streamflow were considered without regard to snow-related parameters, elevation bands, and associated lapse rates and another project referred to as Elevation Band project in which 10 elevation bands were set up for six snow-dominated headwater sub-basins. Sensitivity analysis and calibration was performed using SWAT Calibration Uncertainty Programs/Sequential Uncertainty Fitting 2 for both the projects. The result of the sensitivity analysis showed that curve number (CN 2) was found to be most sensitive. The comparison between the two calibrated projects revealed that inclusion of snow-related parameters, elevation bands, and the associated lapse rates in the snow-dominated headwater catchments improved the hydrological simulation performance and this improvement propagated downstream towards the outlet of the basin. This enhancement highlighted the ability of SWAT to capture the snowmelt dynamics prevailing in the Eastern Himalayan region. Analysis of the water balance components showed that water yield and percolation increased, while evapotranspiration decreased under the effect of snow cover. The contribution of snowmelt runoff to the annual streamflow of the basin was about 8%, and the maximum contribution was made during the pre-monsoon period. This study also highlighted the importance of the presence of snow cover to sustain the ecosystem of the region.