Response characteristics of water quality to the seasonal thermal stratification in Jin-pen reservoir along the Heihe river,Xi'an city in China

By taking Jin-pen reservoir of Xi'an City in China as the main research object,vertical water -quality monitoring was carried out monthly in order to investigate the correlation between thermal stratification and seasonal water-quality variance.The results show that during thermally stratified period from March to December,the steady temperature gradient could effectively restrict oxygen transfer and exchange among the vertical water layers,which accelerates water at the bottom to anaerobic state.The average concentration of dissolved oxygen(DO) in the hypolimnion shows a linear decline with time,and the average oxygen consumption rate of water here is as high as 0.0356～0.0372mg/(L·d)(O_2),which has caused accelerating release of pollutants such as nitrogen,phosphorus and iron from sediments to overlying water and deterioration of water pollution.During the isothermally mixed period,the understable failure of stratification structure occurs.The whole reservoir is in aerobic(DO8mg/L) and slight alkaline(pH:7.1～7.3) state, moreover,the water quality becomes homogeneous under the influence of convective mixing among vertical water layers.During the whole monitoring period,the steady chlorophyll-a content(based on Lois Elle's steady state assumption) has negative exponential correlation with mixing depth and varies in unimodal form.The existence of thermal stratification in Jin-pen reservoir is considered a promotion factor for increasing water eutrophication.In addition,with the running time extension of reservoir,the risk of sudden water pollution caused by understable failure of stratification would increase.