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 δ¹⁸O 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.

控制不丹昌卡尔曲（Chamkhar Chhu）流域河川径流的水文过程，对于理解气候变化背景下下游人口的供水脆弱性具有重要研究价值。本研究针对2016水文年，采用同位素与地球化学示踪技术，明确了该流域的季节性水源组成与径流路径。研究团队在季风前、季风期与季风后三个季节，沿海拔2538米至5158米的高程样带采集了地表水、地下水、冰川融水及降水样品。地表水的溶质浓度变化特征表明，支流对干流的水化学组成具有显著调控作用，且海拔3500米以下区域地下水的水文贡献愈发重要。以硫酸根离子(SO₄²⁻)为示踪剂开展的双组分流量过程线分割分析，验证了地下水在径流中的关键作用，结果显示地下水在季风前的河川径流中贡献尤为突出。本研究以δ¹⁸O（氧-18稳定同位素）为示踪剂估算河川径流的水源组成，结果表明：相较于季风后期冰川融水贡献随距冰川距离增加而快速衰减的特征，季风早期各高程样带的降雨与融水贡献相对更为均衡。