Snow and Ice melt contribution in the daily discharge of langtang and modi rivers, Nepal

Abstract

This study has been undertaken with an objective of calculating the daily discharge from glacierized Langtang and Modi River basins in Nepal, using the modified Positive Degree-Day (PDD) Model. This model estimates snow and glacier ice melt from the debris free and debris covered areas based on the positive degree-days concept from daily mean temperature. The model is calibrated from May 2011 to December 2012 and validated in 2013 for the Langtang River. For the Modi River, it is calibrated from 1991 to 2000 and validated from 2001 to 2006. It satisfactorily simulates the observed discharge and hydrologic regime. The average discharges of Langtang River in the calibration and validation years are 9.1 m3 s-1 and 8.5 m3 s-1, respectively with Nash-Sutcliffe model efficiency (NSE) coefficient values and volume differences being 0.9 and 5.7%, respectively in the calibration years and 0.9 and -4.8%, respectively during the validation years. Similarly, the average discharge of the Modi River during the calibration and validation years were 55.3 m3 s-1 and 46.3 m3 s-1, respectively with NSE coefficient values and volume differences as 0.8 and -9.9%, respectively in calibration years and 0.7 and -8.9%, respectively in validation years. The average snow and ice melt contribution during calibration and validation years in Langtang River were 38.7%, whereas in Modi River, the contribution is 2.2%. The river discharge is also predicted for the period 2020-2099 using the future climate data derived from Cubic Conformal Atmospheric Model-Centre National de Recherches Meteorologiques (CCAM-CNRM) experiment of CORDEX with resolution 50 km in two different climate scenarios (RCPs 4.5 and 8.5). The Regional Climate Model (RCM) used for downscaling is Commonwealth Scientific and Industrial Research Organization (CSIRO)-CCAM and the driving GCM used is CNRM-CM5. The projected discharge of Langtang River shows a slight increase in both scenarios. However, RCP4.5 shows decreasing snow and ice melt and RCP8.5 shows increasing snow and ice melt contribution in the Langtang River. Similarly, the discharge projection of Modi River in both scenarios shows a slight increasing trend of river discharge whereas the snow and ice melt contribution is seen decreasing in both scenarios.