Hydrogen and oxygen isotope indicated thermal stratification and accumulation effects of cascade reservoirs in Jinsha River

梯级大坝的建设对河流水文循环和水生态环境产生了深远的影响。本文对金沙江下游4个梯级深大水库（水深＞150 m），依次为乌东德、白鹤滩、溪洛渡和向家坝，在丰水期热分层期间的氢、氧同位素值（δD和δO）进行垂向测定，探究同位素指示下水源、气象及水库建设的分层动力学等对河流水循环的作用。通过MixSIAR模型对研究区水体来源进行定量分析，得到偏枯年份（2023年）的丰水期大气降水和冰川融水为金沙江下游的主要水源补给，占比分别为89.8%和10.2%。水源的占比在丰、枯水年，以及年内的丰、枯水期内发生变化，表现为丰水期水库入流的氢氧同位素值相对高于枯水期。热分层期间，受蒸发、地下水和支流等因素的综合影响，库区同位素值在垂向剖面上呈现复杂的差异性。层化现象导致水体垂向交换减弱，通过垂向二维数值模型反演，表温层水域流动性快，携带丰水期水体信息，而深层的滞温层水体表现为混合期冬、春季节枯水期水体的滞留特性。上游乌东德库区位于干热河谷区，表层水体的氘盈余均值低至-26.40‰，δO均值为-11.15‰，表层蒸发效应显著高于其余3个水库。此外，部分垂线底层水体δD值的变化反映了乌东德水库坝前、白鹤滩近库尾及溪洛渡坝前的底层水体均受地下水入渗影响。大坝拦截增加了水体的水力停留时间，加强了蒸发富集作用，从库尾至坝前，乌东德、白鹤滩和向家坝水库的δO值均具有逐渐上升的趋势。梯级水库的累积效应主要表现为表层水体的δD均值从上游（-115.62‰）至下游（-100.66‰）逐渐升高，δO均值呈先降低后升高的波动增长趋势，乌东德水库的δO均值最高，而白鹤滩水库的δO均值（-15.36‰）最低。本研究可为流域系统调度管理提供理论参考。;The construction of cascade dams had profound impacts on the riverine hydrological cycle and ecological environment. In this study, vertical measurements of hydrogen (δD) and oxygen (δO) isotope were carried out in 4 deep reservoirs (water depth ＞150 m) in the lower Jinsha River, named Wudongde Reservoir (WDD), Baihetan Reservoir (BHT), Xiluodu Reservoir (XLD), and Xiangjiaba Reservoir (XJB), during the wet season with thermal stratification, to investigate the roles of water sources, stratification dynamics, and meteorology, as indicated by the isotope values. Based on the quantitative analysis of water sources area by using the MixSIAR model, the results showed that in the dry year (2023), atmospheric precipitation and ice-melt were the main water sources of the lower reaches of the Jinsha River, accounting for 89.8% and 10.2%, respectively. The proportions of these two sources changed in the wet and dry years, as well as the dry and wet seasons in a year. δO and δD in runoff were typically higher during wet seasons compared to dry seasons. The vertical profiles of these values showed complex variability influenced by thermal stratification, evaporation, groundwater inflow, and tributaries. Thermal stratification impaired vertical exchanges. The 2D numerical model results indicated that the rapid flow in the middle and lower epilimnion carried the information of the wet period, while water in the hypolimnion reflected stagnant conditions of the previous months during dry seasons. WDD Reservoir, situated in a dry-hot valley, experienced significant surface evaporation effects. The average deuterium excess of surface water was low at -26.40‰, and the average δO was -11.15‰. The surface evaporation effect was significantly higher than other reservoirs. In addition, changes in vertical δD indicated influence from groundwater infiltration in bottom waters near WDD, reservoir tail of BHT, and XLD. Dam interception increased hydraulic retention time and enhanced evaporation. δO of WDD, BHT and XJB gradually increased downstream from the reservoir end to the dam front. Cascade reservoirs cumulatively caused surface water δD to increase from -115.62‰ of upstream to -100.66‰ of downstream, while average δO initially decreased and then raised. WDD had the highest average δO, with BHT the lowest (-15.36‰). This study can provide theoretical references for the basin management of the reservoir regulations.