南方水库热分层消亡期混合层深度及缺氧区时空变化特征——以南宁市天雹水库为例

Spatial and temporal characteristics of mixed layer depth and anoxic zone during the extinction of thermal stratification in southern reservoirs in China--A case of Tianbao Reservoir,Nanning City

湖库热分层消亡引起的突发性水质恶化现象引起了广泛的关注,我国南方水库大多是暖单次混合型湖泊,每年混合一次,导致水库水质周期性下降,但目前对南方水库热分层消亡过程的高频监测研究较少。为探究我国南方水库热分层消亡期水体混合过程的时空变化规律及驱动因素,以广西南宁天雹水库为例,于冬季热分层消亡期(2019年11月—2020年2月)对水库多点位水体理化指标开展原位监测,并利用自建气象站获取气象水文数据。结果表明:(1)水库热分层消亡期间,过渡区水深较浅可在短期内达到完全混合状态且缺氧区同步消失;而湖泊区混合过程整体滞后于过渡区,混合层深度由6.85 m增加至13.65 m,缺氧区逐渐减小,缺氧指数(AI)由0.40减小至0.07,直至水体完全混合后缺氧区消失;水库过渡区较湖泊区提前约40 d达到完全混合状态。(2)气象因子是引起热分层结构变化的主要因素,气温(T)、辐射(R)与混合层深度(MLD)呈现显著负相关(R_(T)=0.927、R_(R)=0.925,P<0.01),风速(WS)与MLD呈现显著正相关(R_(WS)=0.728,P<0.01);气温变化是热分层结构破坏的关键因子。(3)热分层消亡期水体理化指标受到混合程度的影响较为显著,水温与混合层深度呈现显著负相关(R=0.868,P<0.01),DO、pH、ORP与MLD呈现显著正相关(R_(DO)=0.747,R_(pH)=0.761,R_(ORP)=0.747,P<0.01)。热分层消亡为突发性水质恶化提供了环境条件,应密切关注水库分层破坏过程中水体理化指标变化对整个库区水质的影响。

The phenomenon of sudden water quality degradation caused by thermal stratification extinction in reservoirs has attracted widespread attention.The reservoirs are mostly warm lakes in southern China that are mixed once a year,resulting in cyclical water quality degradation.However,research on high-frequency monitoring of thermal stratification decline is relatively limited.To investigate the spatial and temporal variation patterns and driving factors of water mixing processes during the thermal stratification extinction period of reservoirs in southern China,taking Tianbao Reservoir in Nanning City,Guangxi as an example.The physical and chemical indices of multi-point water bodies in the reservoir were monitored in situ during the winter thermal stratification extinction period(from November 2019 to February 2020),and meteorological and hydrological data were collected using self-designed weather stations.The results show:(1)During thermal stratification extinction,the transition zone can reach a full mixing state in a short time and the anoxic zone disappears synchronously.The transition zone of the reservoir reaches the full mixing state about 40 days earlier than that of the lake.The depth of the mixed layer increased from 6.85 m to 13.65 m,the anoxic zone gradually decreased and the anoxic index(AI)decreased from 0.40 to 0.07 until the anoxic zone disappeared after the water was completely mixed.(2)Meteorological factors are the main drivers of changes in the thermal stratification structure.Temperature(T)and radiation(R)show a significant negative correlation with mixed layer depth(MLD)(R_(T)=0.93,R_(R)=0.93,P<0.01).Wind speed shows a significant positive correlation with MLD(R_(WS)=0.73,P<0.01).Temperature change is the key factor for the failure of the thermal stratification structure.(3)The physical and chemical indicators of the water body during the period of thermal stratification extinction were significantly influenced by the degree of mixing.Water temperature shows a significant negative correlation with MLD(R=0.87,P<0.01).DO,pH and ORP show a significant positive correlation with MLD(R_(DO)=0.75,R_(pH)=0.76,R_(ORP)=0.75,P<0.01).Thermal stratification extinction provides the environmental conditions for sudden water quality deterioration.It is necessary to pay close attention to the influence of changes in physical and chemical indices on the water quality of the whole reservoir.