Study On The Influence Of Dongting Lake On The Water Sediment Relationship Of The Yangtze River And The Replenishment Of Nutrients

At present,the research on Dongting Lake mostly focuses on the impact of rivers on the lake under the operation of the TGR.However,the"sink source"relationship between rivers and lakes will change each other under different conditions,and the hydrological and water quality of Dongting Lake also directly affects the strategic security of water resources of the Yangtze River（YR）.Therefore,the research on the impact of Dongting Lake on the YR will not only help to understand the relationship between rivers and lakes more comprehensively,but also provide reference for the water environment management of Dongting Lake and the lower reaches of Jingjiang River（JR）.In this paper,Dongting Lake area is selected as the research object,and JR is used as the control to analyze the variation characteristics of hydrology and water quality in the lower reaches of JR after Dongting Lake flows into JR.Based on the data of water level,discharge and sediment,the response relationship between the discharge of Dongting Lake and the hydrology and water sediment of the YR is analyzed;Through the determination of environmental factors and indoor experiments,the distribution characteristics and influencing factors of nutrients were analyzed.In addition,the nutrient flux was calculated and the isotopic tracer method was used to explore the replenishment effect of Dongting Lake on the water and nutrients of the YR in different periods.The main conclusions are as follows:（1）Through the analysis of hydrological data,it is found that The water level changes of Luoshan（LS）-Chenglingji（CLJ）-Xiangyin are basically the same.And the flow in LS is mainly affected by CLJ during the Dry period（D）and Rise period（R）,and CLJ is mainly affected by the Four Rivers（FR）;the flow in LS is mainly affected by JR in the period of Wet period（W）and Back period（B）.The inflow of Dongting Lake did not change the water level discharge relationship of the main stream of the YR.Due to the increase of the jacking effect of the main stream of the YR during the W-B,the water level discharge relationship of CLJ was disordered.（2）By analyzing the water sediment relationship,it is found that（1）the sediment concentration in CLJ is the lowest in the W and the highest in the R.The sediment concentration of CLJ is higher than that of JR during D-R,which is the main sediment source in the lower reaches of the YR;In the W,the sediment concentration of Dongting Lake is significantly lower than that of JR,which may dilute the sediment in the lower reaches of the lake mouth.JR is the main sediment source in the lower reaches of the YR.（2）There is an obvious inflection point in the flow sediment concentration relationship of JR.The flow sediment concentration relationship of CLJ can be divided into two types（flow<10000 m³/s;flow>10000 m³/s）.The slopes of the two types of sediment concentration increase with the increase of flow are different.After Dongting Lake flows in,there is no inflection point in the flow sediment concentration relationship downstream of the lake mouth,and the overall relationship is linear.（3）Through water environment monitoring and chemical analysis,it is found that the turbidity between Dongting Lake River and YR in different periods significant differences,the conductivity was lower than that of the YR,and the chlorophyll a increased in the period of W.the TP of the YR is generally in the W-B>D-R,the water quality in the W is class V,the other periods are class II～III,and the TP of FR and CLJ is in the R>B-W>D,and the water quality is in class II～III.CLJ and FR are dominated by TDP in D,TPP in other periods,TPP in JR W,TDP in other periods,and TPP in all periods in the lower reaches of lake mouth.The TN water quality of each water body is class III～V,and the COD_Mn water quality is class II.The size of TP is determined by TPP.Among the physical and chemical indexes,turbidity is the main factor affecting TP and TPP of each water body.Reservoir operation,Lake scouring,sand mining activities and YR jacking are important factors affecting the turbidity of the FR and the outlet of the lake.Self scouring and sediment from the upper reaches of the YR are the main factors affecting the turbidity of JR.（4）It is found that the fluxes of TP,TPP,TN and COD_Mn in the lake are the largest in the R.The TP flux out of the lake in the D-R is 474g/s and 3225.8 g/s respectively,which is 1.1and 3.6 times that of Jianli（JL）respectively.It is the main contribution source in the lower reaches of the lake mouth,and the contribution of the FR is greater than that of the Three mouths;The TP flux out of the lake in the period of W-B is 973.2g/s and 977.4 g/s respectively,which is 0.11 and 0.48 times higher than that of JL respectively.It is the main contribution source in the lower reaches of the lake mouth.The upper JR was lost in TDP transport,while the JL-LS was recharge in TDP transport（except in W）.The fluxes of TN and COD_Mn out of the lake in each period were 8.5×10³～25.5×10³ g/s and 34.2×10³～67.9×10³g/s.Isotope tracing shows that the hydrogen and oxygen isotope values of the Hukou section are closer to the FR in the D-R-W,closer to the YR in the B,and the hydrogen and oxygen isotopes of Dongting Lake and FR are more enriched than the YR in each period In the D-R-W,the water outflow from the lake is mainly Xiang river、Zi river and Yuan river,accounting for about 80%,in the D-R,the water outflow from Xiang river accounts for 43%and 74%respectively,and in the B,the water outflow from Li river-YR accounts for about 56%.The results are consistent with the characteristics of isotopic enrichment.