Seasonal source water and flow path insights from a year of sampling in the Chamkhar Chhu basin of Central Bhutan

Hydrologic processes that control river flow in Bhutan's Chamkhar Chhu basin are important for understanding water supply vulnerability to downstream populations in a changing climate. Seasonal source waters and flow paths of streamflow of the basin were determined using isotopic and geochemical tracers for water year 2016. Samples including surface water, groundwater, glacier meltwater, and precipitation were collected in premonsoon, monsoon, and postmonsoon seasons along an elevation transect from 2,538 to 5,158 m. Solute trends in surface waters demonstrate the major influence that tributaries can have on main stem hydrochemistry and the increasingly important role of groundwater below 3,500 m. Groundwater's role in flow is supported by a two-component hydrograph separation using SO42- as a tracer and shows that groundwater is especially important to river flow in the premonsoon. Source waters to river flow were calculated using delta O-18 as a tracer, indicating that rain and meltwater are more evenly important across the elevation transect in the early monsoon period than in the postmonsoon period when ice melt contributions rapidly wane with distance from the glacier.