关键词: 水体理化特征;分层特性;热分层稳定指数;真光层深度;调度运行;梯级水库;金沙江下游

Abstract: Damming and reservoir impoundment significantly alter riverine hydrological conditions, leading to reduced flow velocity and elevated water levels in upstream areas, subsequently influencing material cycling patterns in reservoir zones.However, the vertical variations in physicochemical characteristics of water bodies under cascade reservoir operation in the lower Jinsha River remain poorly understood.This study investigated these variations through stratified sampling of vertical water bodies at the front of three reservoirs(Wudongde, Xiluodu, and Xiangjiaba) during winter(January) and summer(July)2022.We analyzed the distribution patterns of water temperature and dissolved oxygen concentrations along vertical gradients, and explored the driving mechanism of seasonal differences in vertical light, thermal, and oxygen characteristics.The results revealed two key findings:(1) Vertical light, thermal, and oxygen characteristics exhibited distinct seasonal patterns, primarily controlled by reservoir inflow and outflow rates.The waters didn′t show obvious vertical temperature gradients in summer, and stable thermocline formations weren′t formed in all three reservoirs.The water was mixed evenly in the vertical direction, and the depth of mixing lay was almost equivalent to the whole water depth.In winter, the water exhibited stable thermal stratification predominantly observed in surface waters(0～10m),influenced by water level fluctuations and reservoir flow exchange.The thermal stratification stability index(RWCS/H) was substantially higher in summer compared to winter.Surface waters during summer showed dissolved oxygen concentrations exceeding theoretical saturation levels, accompanied by oxycline formation, reflecting the combined influence of water temperature and phytoplankton activity.(2) The concentration of chlorophyll-a was found to be significantly positively correlated with water temperature, dissolved oxygen, pH, conductivity, total nitrogen, and light conditions(water transparency and euphotic depth) in the summer, but only with water transparency in the winter, according to correlation analyses.According to these correlations, the right water temperature along with enough light and nutrients significantly boosted phytoplankton development, which raised saturated dissolved oxygen concentrations and increased oxycline thickness.As a result, summertime had substantially higher dissolved oxygen concentrations and thicker oxygen layers than wintertime.The euphotic depth in winter was significantly greater than that in summer.Through correlation analysis, it was found that the euphotic depth was mainly affected by the transparency of the surface water and was closely related to the sediment content and storm runoff in the upstream water.These findings can provide valuable insights for implementing adaptive reservoir ecological operation strategies.

Key words: water physicochemical characteristics ; stratification characteristics ; thermal stratification stability index ; euphotic zone depth ; scheduling operation ; cascade reservoirs ; lower reaches of Jinsha River