Research On The Water And Heat Exchange Of The Sediment-water Interface In Fushanqian Reservoir

The quantification of exchange rate between reservoir water and groundwater is not only important for exploring reservoir water balance,but also the basis of exploring the exchange of solute at the water-sediment interface.The exchange of reservoir water and groundwater is accompanied by the transfer of energy,so the temperature distribution of sediment vertical profile provides effective information for the exchange of reservoir water and groundwater.The research of water and heat exchange between reservoir water and sediment can provide theoretical basis for reservoir construction and management.To investigate characteristics of water and heat exchange at the reservoir water-sediment interface,field observations were carried out on the northwest,north and southeast sides of the Fushanqian Reservoir from Sep.5th to 17 th in 2018,and in the local area located in the northwest corner of the reservoir from Oct.8th to 9th in 2019.The temperature of vertical profile of sediment was continuously monitored.Based on the monitoring data,we determined the vertical exchange velocity of reservoir water-groundwater by numerical method,explored the spatial variation of reservoir water-groundwater exchange,and discussed the heat exchange at sediment-water interface.The results showed that:(1)Temperature distribution of the reservoir sediment had obvious spatial variability.The temperature fluctuation near the sediment-water interface was large and decreased with the increase of sediment depth.The time corresponding to the peak and valley in the temperature fluctuation curves lagged in turn with the increase of depth.The temperature fluctuation amplitude of the southeast sampling site was larger than that of northwest and north sampling sites.In the local area,the temperature at the water-sediment interface was different at different sampling sites.The difference decreased from the sediment-water interface down to about 0.24 m.In the vertical profile of sediment consisting of No.2,4 and 5 sampling site,the starting time for temperature rise and fall at No.5 was the earliest.These distribution characteristics could be explained by the differences of velocity obtained from subsequent inversion among sampling sites.(2)Piecewise continuous linear functions were used to characterize the irregular boundary conditions.To reduce the error caused by neglecting the thermal dispersivity(TD)or selecting the reference value of TD coefficient,the TD coefficient was also reversed when reversing the exchange rate.The results showed that these treatments effectively improved the inversion accuracy.If TD coefficient was assigned improperly according to reference values,the influence on the determined exchange rate was greater than that of neglecting TD.Synchronous inversion of exchange rate and TD coefficient is also helpful for determining whether it is necessary to consider the effect of TD on the heat conduction flux at the sediment-water interface.(3)The exchange rate of groundwater-reservoir water in the Fushanqian Reservoir showed a significant spatial change.The vertical exchange rates were different not only in magnitude but also in direction in different areas of reservoir and in local areas.Reservoir water was recharged by groundwater at the northwest and north sampling sites during the monitoring period,and the vertical exchange rate was 0.002 m/h and 0.007 m/h respectively.The groundwater was recharged by reservoir water at the southeast sampling site,and the vertical exchange rate was 0.12 m/h.In the local area located in the northwest corner of the reservoir,reservoir water was recharged by groundwater at the No.3 sampling site,where may had a close hydraulic connection to the western hillside,and the vertical exchange rate was 0.0004 m/h.Groundwater was recharged by reservoir water at the No.2,4 and 5 sampling sites,where may had a close hydraulic connection to the northern hillside,and the vertical exchange rate was 0.0003 m/h,0.0005 m/h and 0.0009 m/h respectively.The magnitude of exchange rate was different at different sampling sites,which may be caused by the difference of sediment permeability.(4)The daily fluctuation peak of heat conduction flux calculated by numerical method was higher than that calculated by using temperature gradient between the sediment-water interface and the depth of 0.12 m.There was obvious daily fluctuation of heat conduction flux at the sampling site where reservoir water was recharged by groundwater.Heat conduction flux calculated by numerical method could reflect the change of heat conduction direction at the interface in time.Due to the influence of temperature fluctuation and the relationship between reservoir water and groundwater recharge,the heat conduction at the sediment-water interface changed in time and space during the monitoring period.Heat mainly transferred from reservoir water to sediment at the sampling site where reservoir water was recharged by groundwater.Heat mainly transferred from sediment to reservoir water at the sampling site where groundwater was recharged by reservoir water.