Future hydrological regimes and glacier cover in the Everest region: The case study of the upper Dudh Koshi basin

Assessment of future water resources under climate change is required in the Himalayas, where hydrological cycle is poorly studied and little understood. This study focuses on the upper Dudh Koshi river of Nepal (151km2, 4200-8848ma.s.l.) at the toe of Mt. Everest, nesting the debris covered Khumbu, and Khangri Nup glaciers (62 km2). New data gathered during three years of field campaigns (2012-2014) were used to set up a glacio-hydrologicalmodel describing stream flows, snow and ice melt, ice cover thickness and glaciers' flow dynamics. The modelwas validated, and used to assess changes of the hydrological cycle until 2100. Climate projections are used from three Global Climate Models used in the recent IPCC AR5 under RCP2.6, RCP4.5 and RCP8.5. Flow statistics are estimated for two reference decades 2045-2054, and 2090-2099, and compared against control run CR, 2012-2014. During CR we found a contribution of ice melt to stream flows of 55% yearly, with snow melt contributing for 19%. Future flows are predicted to increase inmonsoon season, but to decrease yearly (-4% vs CR on average) at 2045-2054. At the end of century large reduction would occur in all seasons, i.e.-26% vs CR on average at 2090-2099. At half century yearly contribution of icemeltwould be on average 45%, and snowmelt 28%. At the end of century ice meltwould be 31%, and snowcontribution 39%. Glaciers in the area are projected to thin largely up to 6500ma.s.l. until 2100, reducing their volume by-50% ormore, and their ice covered area by -30% ormore. According to our results, in the future water resources in the upper Dudh Koshi would decrease, and depend largely upon snow melt and rainfall, so that adaptation measures to modified water availability will be required.