Study On The Temporal-Spatial Evolvement Of Runoff And Sediment Of The Dongting Lake And Its Impact On Water Security

The Dongting Lake is converged with Four Water basin of Hunan province in the south and Three Outlets of Jingjiang River in the north.As an important lake for flood storage and water source of the middle reaches of the Yangtze River,the Dongting Lake carries adjusting the balance of the Yangtze River and lake wetland ecosystems,and providing support for socio-economic development and other complex functions.However,due to the dual effects of nature and human activities,Dongting Lake water ecological environment has undergone dramatic changes driven by human activities nearly 100 years.Several major changes have taken place on conditions of runoff and sediment and the relationship between rivers and lakes successively;and flow and sediment of the Three Outlets have been diverted.The volume and area of the Dongting Lake have shown a substantial decrease,and the flood situation of the Dongting Lake remains grim.Deterioration of the aquatic environment,the destruction of wetland resources,growing water shortages,biodiversity decline,and schistosomiasis harm worsened severely restricted the development of the regional economic and social development and people's living standards improve.The water and sediment evolution of the Dongting Lake is the key factor causing changes in the ecological environment of Dongting Lake,the dramatic changes of runoff and sediment in Three Outlets of Jingjiang River is the driving force of Dongting Lake,especially after the Three Gorges Reservoir operation.Thus,a comprehensive study on the variations of the runoff and sediment in Dongting Lake,and its impact on water resources safety after the Three Gorges Reservoir operation are the cutting-edge study topics of the Dongting Lake.Comprehensively analyzing on the evolution and variation of runoff and sediment law of the Dongting Lake,focusing on the effect of the new runoff and sediment discharge conditions on the Dongting Lake area after Three Gorges Project operation,and discussing the flood control measures and other measures of Dongting Lake,these can provide new ideas and technical support for the management of Dongting Lake.Based on the basic theories of hydrology,ecology,physical geography,water resources and environmental science and engineering hydrology and other disciplines basic theory and the diagnostic system on variations of runoff and sediment were proposed.With adoption of time series analysis method,Mann-Kendall trends and mutation test method,linear regression trend analysis method,double cumulative curve method,the cumulative filter method,R/S analysis,wavelet analysis method,the diagnosis on trend changes,cyclical changes and mutations of runoff and sediment in the Dongting Lake was carried out.The paper adhered to the combination of the qualitative analysis and quantitative calculations,and assisted with computer technology,and fully applied to the statistics analysis software,namely Spss,DPS,scientific computing software Matlab and other related software,using ArcGis,MapInfo and CorelDraw.The main research contents and results are as follows:(1)The new adjustment process of river-lake relationship will happen.River system of Dongting Lake is developed and river-lake relationship is complex.Variations of runoff and sediment from Yangtze River into Dongting Lake and evolution of the downstream river bed will cause a chain reaction of Dongting Lake,and it will also impact on the storage and discharge capacity,ecosystem stability,wetland function of rivers and lakes.After the operation of the Three Gorges reservoir,great changes have taken place in three outlets.The new situation of flood storage capacity and sediment deposition has taken place.(2)Based on the basic theories of time series variation detection,using a variety of time-series statistical analysis,evolution and variation of the amount of runoff and sediment in and out the Dongting Lake area were analyzed and temporal and spatial variation of the erosion and siltation in Dongting Lake were discussed.(I)Variation of runoff and sediment of Four Water in Hunan Province.The runoff of Four Water presents a weak attenuation trend except Xiangjiang.The sediment of Four Water presents a significant attenuation trend.The runoff and sediment of Four Water are in the low runoff(sediment)period.The variation of runoff and sediment show the mutation characteristics.The runoff of Four Water has periods of 23a,23a,24a and 33a respectively,and the sediment of Four Water has periods of 21a,20a,22a and 13a respectively.Distributions of annual runoff and sediment of Four Water has obviously inhomogeneity characteristics,runoff concentration in May and July,and sediment concentration in June and July.(?)Variation of runoff and sediment of Three Outlets.The runoff and sediment of Three Outlets present a significant attenuation trend.The runoff and sediment of Three Outlets are in the low runoff(sediment)period.The runoff of Three Outlets has periods of 16a?31a and 31a respectively,and the sediment of Three Outlets has periods of 33a?31a and 31a respectively.Runoff and sediment transport are concentrated in the year July,and dry days of Three Outlets increase significantly.(?)Variation of runoff and sediment of Chenglingji.The runoff and sediment out Dongting Lake present a significant attenuation trend.Runoff and sediment of Chenglingji have mutations in 1971 and 1982 respectively.The runoff and sediment have periods of 35a and 31a respectively.Runoff and sediment concentrate in July and April respectively.(3)Variations of runoff and sediment of Dongting Lake area were diagnosed,and the impact mechanism was analyzed.Due to the dual effects of nature and human activities,several major changes have taken place on conditions of runoff and sediment and the relationship between rivers and lakes successively.The procedure and method of recognition and diagnostic system on variation of runoff and sediment were proposed.With adoption of cumulative filter,double mass curve,R/S test,Mann-Kendall method and wavelet analysis,the diagnosis on trend changes,cyclical changes and mutations of runoff and sediment in the Dongting Lake were carried out,and the factors and impact mechanism of the changes of runoff and sediment were analyzed.(?)Tendency:The runoff and sediment of Three Outlets,Four Water and Chenglingji present an attenuation trend,and sediment attenuation trend is more obvious.(?)mutation:Runoff and sediment of three outlets have mutations in 1971 and 1982 respectively.Runoff of Four Water is not occur,sediment mutation of Four Water happens in 1996.(?)Continuity:The runoff and sediment of Three Outlets,Four Water and Chenglingji have a straight continuity.(?)periodicity:The first main period of runoff series are 31a?23a and 35a;The first main period of sediment series are 31 a?21 a and 31a.(4)The runoff and sediment factors and boundary conditions of Dongting Lake and middle reach of Yangtze River are mutually dependent.The changes of the Three Outlets in Jingjiang River directly affect the proportion and its structure of the runoff and sediment in the Dongting Lake area.Based on generalization of river network structure in the Jingjiang River and the Dongting Lake and analysis of interactive characteristics of runoff and sediment in the Dongting Lake,through the establishment of mathematical model of joint runoff and sediment,the simulation and coupling process of runoff and sediment in Dongting Lake area were proposed.Four non-linear simulations models of interactive relationship between runoff and sediment were established,i.e.Multivariate Auto-Regression Model(MARV),Artificial Neural Network model(ANN),Projection Pursuit Regression model(PPR),and Support Vector Machine(SVM).By model comparison and test,the SVM model was shown with higher simulation accuracy,high reliability and better validity and credibility,which could be used to predict the inflow and outflow amount of runoff and sediment in the Dongting Lake as well as regional sediment deposition.By using the SVM model to predict the runoff and sediment out Dongting Lake,the maximum error percentage of runoff simulation is 2.84%,and the remaining error percentage is less than 2%.The maximum error percentage of sediment simulation is 15.22%,and the remaining error percentage is less than 2%.Better result has been achieved by simulated model,and to predict the runoff and sediment out Dongting Lake in 2012-2060.(5)The influence on Dongting Lake after the operation of Three Gorges Project and the optimized allocation of water resources were researched.As a water control project in the complex giant system of the Yangtze river and Dongting Lake,the operation of Three Gorges Project is bound to change the natural water resources allocation pattern of Dongting Lake,and also has a significant impact on flood control,water environment,exploitation and utilization of water resources and water resources system management of Dongting Lake area.Water security condition of Dongting Lake was analyzed from the flood,water pollution,water safety and the safety of drinking water,and the factors to cause the water resources security were discussed in this paper.The most destructive disasters of the Dongting Lake area is floods,the losses due to floods are increased year by year.There are dual effects of nature and human activities,also both historical reasons and practical reasons,which constraint and influence each other between different factors.(I)Effects of water quantity security:Water quantity security includes two aspects,namely flood control security and water scarce security.Situation of flood control in Dongting Lake improved after operation of the Three Gorges Project,but the Three Gorges can not completely solve the problem of flood control security of Dongting Lake.Situation of flood control in Dongting Lake is still grim.In addition,with the decrease of runoff and sediment from three outlets,water scarce security situation of Dongting Lake has become more and more serious,must cause enough attention.(a)Effect of flood control security:?The reservoir has a peak clipping effect on the upper reaches of flood of the Yangtze River after the operation of the Three Gorges reservoir in 2003,and the pressure of flood control in Dongting Lake ease.?Three Gorges reservoir are not able to completely solve the problem of the flood disaster in Dongting Lake area.Dongting Lake area is one of the most serious flood disaster affected by flood,so the function of flood control of Dongting Lake itself still occupy the irreplaceable position.?The running of the Three Gorges Reservoir reduces the sediment of three outlets into the lake,and it can reduce the sedimentation into Dongting Lake.(b)Effect of water scarce security:The Three Gorges Reservoir intercepts large sediment,the dry season was prolonged,the flow decreases,low water level decreased significantly.Not conducive to the protection of the ecological environment of Dongting Lake.The lake has resulted in seasonal water shortage;dry days of three outlets increased year by year.Dongting Lake continued low water level in recent years,causing serious water shortage situation in Dongting Lake area.(II)Effect of water quality security:After operation of Three Gorges reservoir,great changes have taken place of Three Outlets into the lake,self purification ability of the lake water also greatly affected.? Water pollution in the lake increased gradually,and the concentration of TN and TP is increasing.Water pollution index showed a decreasing trend from entrance to exit in Dongting Lake,which reflect the natural degradation ability.The lake water quality of the East Dongting Lake is inferior to South Dongting Lake;South Dongting Lake is inferior to the West Dongting Lake.?The proportion of ? type water section in Dongting Lake present a declining trend overall before 2007,and ? type water section proportion is raising.After the 2007 the proportion of ? type water section in Dongting Lake present an upward trend overall,and ? type water section proportion is declining.The proportion of ?-? water quality section was 100%each year,overall,the water quality condition is better.? The E TLI index in Dongting Lake increased steadily,and eutrophication in Dongting Lake increased obviously.The general state of eutrophication is in mid-nutrition from 1991-2007 to mild-nutrition in 2008?2010.After the operation of the Three Gorges Project,it will bring comprehensive effect to the eutrophication of Dongting Lake:one side,the water self purification ability in Dongting Lake reduces because of the decrease of the runoff volume into the Lake.On the other hand,the decrease of sediment resulted in reduced SS,and then result in an increase in SD and Chla.?By establishing the eutrophication prediction model of the Dongting Lake using support vector machine,TN,TP,CODMn,Chla and SD prediction model can meet the demand of forecasts.Prediction results show that:the pollutant concentration rising trend is obvious.?The average water environmental capacity of CODMn,TN,TP in Dongting Lake are 2963740t,699935t,50073t respectively.November(in dry season):monthly mean water environmental capacity of CODMn,TN,TP attenuate 10757t,289t and 24t respectively.The water environmental capacity of January to April,and also in December,is almost no change before and after the impoundment.Water environment capacity has the greatest change in October of the flood season.?Dongting Lake water pollution and eutrophication is formed by the joint actions of point source pollution and non-point source pollution.The point source pollution mainly refers to industrial pollution;non-point source pollution mainly includes agricultural pollution,surface runoff pollution and domestic pollution etc.N,P elements exceed the standard in Dongting Lake is mainly caused by the non-point source pollution.(?)Effect of drinking water security:The water pollution in the Lake is mainly caused by the industrial wastewater,sewage and pesticide and Fertilizer.The water quality shortage situation becomes more and more serious.The population whose drinking water security must be solved has reached 3503000 people,accounting for 24.5%of the province.?The surface water and groundwater of Dongting Lake is polluted Seriously,and drinking water security is threatened.At the same time,runoff of three outlets into Dongting Lake is reducing,precipitation slants continuously little,and these also aggravate the drought conditions in Dongting Lake.In addition,improvement of drinking water has many difficulties is the reality of rural drinking water.?After the running of Three Georges Project,runoff discharge decrease 7890m3/s less than the average runoff for many years in October each year.The runoff increase by 1170?1760m3/s in the dry season 1?3 month,reduces by 370m3/s in April,increased by 3760m3/s in May,little changes in 6?9 month during flood period.A significant decrease in runoff of three outlets into the Dongting Lake,in particular,water level decreased significantly in the dry season.?Finally,put forward measures to solve the drinking water security.We should optimize the allocation of water resources,strengthen the protection of water resources,and combine the water use and pollution control,increase the construction of the drinking water project,strengthen the management of drinking water security.