Impact Of Xin'anjiang Riverine Inputs Induced By Different Intensitive Rainfalls On Water Quality Of Lake Qiandao

Lake Qiandao,also named Xin'anjiang Reservoir,has been used for many purposes like tourism,aquaculture,flood control,electricity generation,etc.Lake Qiandao is the water conservation area in Yangtze River Delta.It plays an important role in the social and economic development of Chun'an and Hangzhou city.Due to human social activities and unreasonable exploitation of resources,the pollution loadings have increased gradually and the water quality has degradated in Lake Qiandao,resulting in seasonal algal blooms.During heavy rainfall,the inflow and pollution loadings of rivers into the lake increased significantly,which had a certain impact on the water quality of the lake.As the largest tributary,Xin'anjiang has remarkable impacts on water capacity and water quality in Lake Qiandao.To explore the impact of Xin'anjiang riverine inputs induced by different intensitive rainfalls on water quality of Lake Qiandao,we analyzed the variation characteristics of external loadings including total nitrogen?TN?,total phosphorus?TP?,ammonia?NH₄-N?and chemical oxygen demand(COD_Mn)in Xin'anjiang during 2006to 2016,and analyzed the relationship between external loadings and the water quality of Lake Qiandao.The one-dimensional water quality coupling model DYRESM-CAEDYM and the three-dimensional hydrodynamic coupling model ELCOM-CAEDYM were used to simulate and analyze the water quality under different rainfall conditions in Lake Qiandao.We hope to provide a scientific basis for the comprehensive management of eutrophication in Lake Qiandao and Xin'anjiang.The main focus and conclusions of this study are as follows:?1?The key influencing factors of nutrient in Xin'anjiang and its influence on the water quality of Lake Qiandao were discussed.There are 25 tributaries carrying significant water and nutrients into Lake Qiandao.In order to provide substantial information for the exact annual amount of water and nutrients from Xin'anjiang,the inflow volumes during 2006²016 were calculated based on a catchment model-Xin'anjiang Model.The annual external loadings including TN,TP,NH₄-N and COD_Mnn were further calculated with monthly-monitored TN,TP,NH₄-N and COD_Mn at Xin'anjiang in the same period.For investigating the impact factors of external loading from Xin'anjiang,statistical analysis was conducted with the external loadings correlating the water quality parameters,inflow volume at Xin'anjiang,the population,GDP,economic development and land use at the catchment.The results show that Xin'anjiang is carrying about 51.4%of the total inputted lake water including surface water,ground water and precipitation,accounting for 67.4%of the total water input by25 tributaries.The annual loadings of COD_Mn,NH₄-N,TN and TP from Xin'anjiang are11,458.44 t/a,756.50 t/a,7,649.16 t/a,and 214.91 t/a,which account for 50.7%,4.3%,63.7%and 48.4%of the total annual catchment loading respectively.During 2006²016,there was an increasing trend of inputted COD_Mn,NH₄-N,TN and TP by Xin'anjiang.The external loadings from Xin'anjiang in spring and summer were higher than those in autumn and winter for all parameters.Statistical analysis shows that the COD_Mn,TN and TP loadings are positively correlated with the catchment GDP.The non-point sources from agriculture in Huangshan City have a significant impact on the TN loading.The riverine inputs of TP,TN and NH₄-N from Xin'anjiang are having remarkable influence on water quality,which might cause a high potential risk of seasonal algal bloom in Lake Qiandao.?2?Theone-dimensionalhydrodynamic-waterqualitymodel?DYRESM-CAEDYM?of Lake Qiandao was constructed and the parameters were calibrated and verified.The DYRESM-CAEDYM model was established by the water quality,hydrological data and meteorological data of Lake Qiandao.The Mote Carlo sampling method was used to auto-calibration the model parameter.And the parameters were calibrated based on the water quality data including water temperature,DO,TN,NH₄-N,NO₃-N and TP from 2012 to 2014.The water quality data in 2016 was used to verify.Using the correlation coefficient?R?to evaluate the accuracy of the model.The results show that the surface water temperature?0.5 m?and DO of Lake Qiandao can be well simulated in both calibration and validation periods,because the R value were above 0.5.By comparing the data of automatically monitored with the profiles simulated in 2016,it is found that the model can well simulate the changes of thermocline and DO stratification in Lake Qiandao.The surface TN,NH₄-N,and NO₃-N of Lake Qiandao can be well simulated in both calibration and validation periods,because the R value were above 0.37.The simulated surface TP values in calibration periods were well with the measured values,but the simulated results are not satisfactory during the validation period.The simulation results of the middle layer?5²0 m?and deep layer?30⁵0 m?of NH₄-N and TP in calibration periods were better,and R were above 0.3.The simulation trends were consistent with the measured values.The R value of middle and deep NO₃-N in calibration periods were 0.27 and 0.26,respectively,which results were not satisfactory.The R value of middle and deep TN in calibration periods were greater than 0.3,but the simulated value of deep TN was affected by NO₃-N,which is different from the measured value.All above,the simulated values are in good agreement with the measured values,and the trends is consistent.The parameters obtained can be used to construct the Lake Qiandao model.?3?The impact of Xin'anjiang riverine inputs induced by different intensitive rainfalls on water quality of Lake Qiandao were simulated.The parameter that get from the auto-calibration,build the ELCOM-CADYM model of Lake Qiandao to simulation the variation of nutrient migration in the conditions of different rainfall.Different rainfall scenarios were set:40 mm for one day?scenario 2?,40 mm for three days?scenario 3?,75 mm for one day?scenario 4?and 75 mm for three days?scenario 5?for one day,respectively,and using average flow?scenario 1?as blank control,to study the effects of Xin'anjiang inflow and pollutants on the flow field and the concentration of TN and TP in Lake Qiandao.The results show that scenario 1,scenario 2,scenario 3and scenario 4 did not produce obvious circulation,mainly due to the interaction of wind field and huff and puff flow.The flow path from Jiekou to Xiaojinshan to Santandao was very obvious in scenario 5,mainly affected by the huff and puff flow.When scenario 1,it takes 7 days,13 days and 19 days for tracer to diffuse to Xiaojinshan,Santandao and Daba,respectively.With the increase of rainfall,the rate of tracer diffusion will be accelerated and the period of water exchange will be shortened.Because of the self-purification of the lake,the concentration of nutrient will gradually decrease,and the diffusion range will be reduced.The diffusion time of tracer is different from that of nutrient diffusion.The simulation of rainfall-runoff impact shows that the impact is limited to a small area near the entrance after one day of rainfall.With the passage of time,the pollutants gradually diffuse to the lake center.When the rainfall is 75 mm for three consecutive days,the nutrient flux of Xin'anjiang will far exceed the self-purification capacity of the lake,and the concentration of pollutants significantly increased in the lake.During the rainstorm,the runoff increased the pollutants concentration in the northwest region firstly,followed by the central region.In scenarios1,2,3 and 4,the highest TN concentration in Santandao increased by 1.2%,3.2%,4.1%and 27.2%respectively;the TN concentration of dam only scenario 5 increased significantly.So we need to focus on the eutrophication risk in the northwest region during rainfall,and we also should strengthen water quality management of the entire area to preventing the occurrence of eutrophication.