| In order to explore the formation and evolution of seasonal thermal stratification, seasonal change of water quality and the impact of rainfall runoff on Xi’an Jin-pen reservoir, The water temperature, turbidity, dissolved oxygen, p H, conductivity, nitrogen and phosphorus nutrient and total organic carbon from the riverine zone to the reservoir areas were monitored continuously, The results show that:(1) The thermal stratification period of Jin-pen reservoir(from March to October) is longer than mixed period in other months, which shows a mixed single-cycle mixed pattern. The atmospheric temperature change is the key factor for the formation and evolution of seasonal thermal stratification in the reservoir, there is a significant linear correlation between the local atmospheric temperature and the surface water temperature, which is not related to the bottom water temperature.(2) Both the seasonal thermal stratification and the oxygen consumption of the sediment has made a significant contribution to seasonal anaerobic condition in the bottom layer of the reservoir. The bottom water of the reservoir is in different extent of anaerobic condition from April to October in the thermal stratification period, the thickness of anaerobic layer is reach maximum 16.1m at the end of July of the year, the aerobic layer thickness and RWCS/H index exist significant linear correlation and the thermocline distribution characteristics have played a dominant role in variation of the anaerobic layer. Sediment Oxygen Demand(SOD) rate is 0.387g/(m2×d) through in-situ test, which account for 81.5% of the total oxygen consumption rate.(3) The thermal stratification of water body plays a key role in the change of the water environment system, The anaerobic environment contribute to the release of numerous pollutants from sediment of the reservoir in the thermal stratification period, the concentration of total phosphorus is up to 0.062mg/L at the end of July. the conductivity increased and p H value decreased in the bottom water of reservoir along with internal pollutants release at the anaerobic condition.(4) Rainfall runoff in flood season is the main source of external pollution in the Hei-he river, which has a significant impact on the water environment of the reservoir. Rainfall runoff, containing a large amount of particulate nutrients and sediments, entered the reservoir in the form of density current in September, which changed from underflow to interflow between transition zones and lacustrine zones, and the thickness of interflow was 20 m during the early period of current intrusion. The concentrations of TP, TN, TOC increased significantly due to the intrusion of density currents in the middle and lower of water, with the highest concentration of TN, TP, TOC and turbidity was 0.2, 2, 0.6 and 16 times higher than that in normal condition, resulting in serious water body pollution in short time. The intrusion of density currents result in that bottom water temperature rise, surface temperature decreased and vertical water turbulence strengthen, which has weakened the thermal stratification stability, naturally mixing process is enhanced. The decomposition rate of particulate pollutants settled in sediments increased due to the rise of bottom water temperature, leading to a continuous increase of the endogenous nutrient loads in the reservoir. On the one hand, density current carrying dissolved oxygen can increase the dissolved oxygen concentration of previous water, on the other,it can accelerate the consumption of the dissolved oxygen by carrying a large amount of reducing matters, the variations of dissolved oxygen in the water body of the reservoir is decided jointly by the two means.(5) Through analyzing the correlation between characters of rainfall runoff and undercurrent and building a plunge location prediction model of density current, managers can take water from different layers of the reservoir and discharge high-turbidity current to guarantee the high quality drinking water supply in flood season. |
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