汛期水位波动对香溪河库湾热分层特性及水质的影响

Effect of water level fluctuation in flood season on thermal stratification and water quality in Xiangxi Bay

本文以三峡水库典型支流——香溪河为研究对象,研究汛期水位波动对热分层特性及其对水质的影响。于2019年6月5日—8月30日对香溪河中游进行定点气象、垂向水温及水质监测,并同步获取香溪河上游及干流水文数据。结果表明:香溪河热分层时期日循环特征显著,垂向水温差异明显,混合层深度在0~19.37 m之间变化,水位与混合层深度呈显著负相关关系(P=-0.468**)。底层水温增长缓慢,且受上游低温来流影响显著。垂向对流作用及异重流侵入改变水层掺混强度,水体不存在稳定的温跃层,溶解氧饱和度在69%~175%之间,垂向溶解氧差异明显,且与垂向叶绿素a浓度显著相关,表明浮游植物是影响溶解氧垂向差异的原因;水位波动过程对混合层深度的影响关键取决于是否改变倒灌异重流潜入方式,中上层倒灌有利于表层水温分层加剧,进而有利于水华爆发;利用水位波动(水库调度)抑制支流水华取决于水位波动过程是否能改变库湾水环流模式并有效增大混合层深度。本研究为有效利用水位波动(水库调度)改善三峡水库支流水质提供了新的认识。

In this study,we have conducted field measurements in Xiangxi River,a typical tributary of the Three Gorges reservoir,to collect the meteorological and hydrological data of daily variations for the period of June 5 to 30 August,2019,including water temperature and water quality measurements.This paper focuses on an analysis of the effects of water level fluctuation in Xiangxi Bay on its thermal stratification and water quality in flood season.The results show the daily cycle of an upper warm layer that led to sub-daily variations in the mixed layer depth in a range of 0.0-19.4 m,but in long term this depth was significantly negatively correlated with water level fluctuations(P=-0.47).The bottom water temperature increased slowly and this increase was impacted by the cold inflows at the bay inlets significantly.No stable thermocline was observed due to strong mixing driven by the intrusion of density currents and vertical convection.Significant correlations of dissolved oxygen content and saturation degree with Chl.a concentration indicate the variations in dissolved oxygen can be caused by phytoplankton distribution.Further analysis reveals that large water level fluctuation could vary the plunge depth of density currents,and a shift from the overflow to interflow mode of the currents will result in altered mixing regime in the tributary bay,i.e.,reducing the surface mixed layer and increasing vertical stratification,both significantly different from those of the reservoir filling period.This study sheds light on new understanding for effective utilization of water level fluctuation(through reservoir regulation)to improve water quality in the tributaries of Three Gorges Reservoir.