Analyzing the Hydrological Regime of Sunkoshi River Basin, Nepal, using a Glacio-hydrological Degree-day Model (GDM)
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I.O.E. Pulchowk Campus
Abstract
The Glacio-hydrological Degree-day Model (GDM), a distributed and gridded model in glacio-hydrology, uses a temperature index concept to calculate daily river discharge from snow and ice melt, rainfall and base flow. GDM calibration relies on factors like positive degree-days, snow and rain runoff coefficients and recession coefficient. The Sunkoshi River basin is a transboundary river between Nepal and Tibet. The study from 2000 to 2020 A.D includes calibration (2000-2009) and validation (2010-2020) phases in the model simulation. Its robust performance, reflected by Nash-Sutcliffe Efficiency (NSE) values between 0.79 to 0.77, volume difference below 10% and a strong R-squared (R2) value of 0.83 to 0.77, underscores its reliability. During calibration and validation, snowmelt contributes 9.68% to 11.38%, while clean ice and ice melt beneath debris account for 2.5% to 3% to the total discharge. Rainfall maintains substantial proportions at 48.26% to 50.15%, and baseflow ranges from 37.33% to 37.66% to the total discharge. In May to June's low-flow period, the study shows snow and ice melt greatly affect river discharge. Future analysis predicts a rise in ice melt's influence, especially during low-flow phases, impacting stream flow significantly. However, increasing temperatures reduce snowfall and glacier cover, to the point of diminishing ice and snow melt contributions, ultimately impacting future low-flow stream conditions.
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Future peak discharge shifts to July from August with variation in discharge under SSP24.5 and to July with increased discharge post 2030 in SSP58.5 scenarios from the baseline period (2010-2020). Future Projections indicate increased discharge under SSP58.5, notably 3.22 m3/s under EC-Earth3, contrasting with decreases projected under SSP24.5