Refined Simulation Of Non-point-source Pollution And Identifying Multilevel Priority Management Areas At The Watershed Scale

As the second largest inland freshwater lake in Yunnan province,Lake Erhai plays an important role in regulating regional climate,maintaining ecosystem balance and supporting social and economic development.It is called "Mother Lake" by the people of Dali.Similar to the evolution law of many inland lakes,Lake Erhai is at the key turning point of the initial eutrophication state.For the present time,the water quality of Lake Erhai is generally Class ?water,which is lower than its target water quality Class ?,and the deterioration trend of the water quality is very significant.The Lake Erhai deteriorated from the relatively stable Class ?water in the 1980s to the current Class ? water,and the algal bloom phenomenon has also erupted.More serious regional cyanobacteria blooms broke out in 1996,2003 and 2013.Along with the change of water quality and nutrient status of the Lake Erhai,the large-scale emergent plants gradually declined,and the aquatic biological structure also changed accordingly,resulting in the stability of the water ecosystem t'ending to decline.In addition,with the rapid development of social econom y in the Lake Erhai basin in recent years,the high yield of agricultural planting and the high in.come of tourism not only bring huge economic benefits to local residents,but also bring huge pollution load into the lake.Excessive input of pollutants not only has a huge negative impact on the lake water quality,but also provides the necessary material basis for the occurrence of eutrophication of the lake.Judging from the management concept of "Mountains,water,forests,fields,and lakes are a community of life" proposed in the current work of ecological civilization construction in China,how to curb the further deterioration of water quality in the Lake Erhai?'How to fully understand the changing characteristics of pollutant input in the Lake Erhai?How to establish an effective non-point source pollution control plan for the improvement of lake water quality?This is an urgent question to be answered in the current protection work in Lake Erhai.In this context,this study is based on multidisciplinary theories such as watershed hydrology,water resources and water environment,with multivariate data of geography,meteorology,hydrology,water quality and socio-economics as research support.Combining on-site monitoring,mechanism model simulation and data mining analysis as research method,and the problem diagnosis,m odel simulation,and law reveal the layered progressive way as the logical relationship structure.Focusing on the study of non-point source pollution characteristics and priority management area identification in the Erhai Lake Basin,the watershed partitioning method system based on Monte Carlo AHP algorithm,and watershed non-point source pollution simulation framework based on partition parameter migration method are constructed.Based on the spatial Markov chain model and the lake water quality-pollution source response relationship model,the calculation method of the pollution contribution load into the lake is constructed,and the black box empirical model of flow and non-point source pollution load is established based on statistical analysis.Based on the grey system theory and the watershed water quality target management system,a multi-level priority management area identification method system for watershed non-point source pollution was established.The main findings and conclusions of this paper are as follows:(1)Objectively identify the main causes of the deterioration of the Lake Erhai's water environment.In the variation of the inter-annual,the water quality in the dry season of the Lake Erhai is better than that in the wet season.In the spatial distribution characteristics,the water quality in the northern lake region is better than that in the southern lake region in the dry season,and the water quality in the southern lake region is better than that in the northern lake region in the wet season.The input of excessive nitrogen and phosphorus pollution load is the main cause of the deterioration of the seawater environment.This study summarizes the three main problems of the current deterioration of the water environment in the Lake Erhai:The continuous increase of pollution load from the basin is the material basis for the change of eutrophication status,which increases the risk of eutrophication in the Lake Erhai.The peak of nitrogen and phosphorus nutrient concentration in the lake during high-risk period and and the change of physical and chemical environment are the predisposing factors of the algal bloom in the Lake Erhai.With the accumulation of nutrients in the lake and the degradation of aquatic ecosystems,the Lake Erhai has gradually changed from a phosphorus-controlled lake in the whole year to a nitrogen-phosphorus dual-controlled lake in the wet season.The pressure on lake eutrophication treatment in the Lake Erhai will be further increased.(2)Finely simulated the characteristics of non-point source pollution in different regions of the Lake Erhai Basin.This research breaks through the limitation of the non-point source pollution simulation using the uniform parameters of the whole basin.The Monte Carlo AHP algorithm is used to propose the Lake Erhai basin zoning method system.Based on the zoning method system,this study divides the Lake Erhai basin into the northern Eryuan plain river region,the western Dali alpine river region.the southern Fengyi middle-altitude river region and eastern Haidong low-altitude river region.On the basis of the partition results,the typical small watershed of each region is selected.Based on the SWAT model,the partition parameter migration method is used to simulate the hydrological and nutrient cycling in a typical small watershed.By comparing the simulation results of the partition parameters with the simulation results of the whole basin parameters,it is found that the partition parameter migration method significantly improves the simulation accuracy(Yangxi:the Ens of the partition simulation is 0.88,the Ens of the whole basin simulation is 0.43;Boluo River:the Ens of the partition simulation is 0.87,the Ens of the whole basin simulation is 0.62),which further verifies the necessity and applicability of the method system in the Lake Erhai Basin.(3)A set of calculation method system for the contribution of pollution load to the lake for the lake basin was constructed in detail.Based on the spatial Markov model and the lake water quality-pollution source response relationship model,this study calculates the contribution load into the lake of the Lake Erhai.The study found that the high-value areas of total nitrogen and total phosphorus in Lake Erhai are concentrated in the middle and lower reaches of the northern Eryuan plain river region and the southern Fengyi middle-altitude river region,and in the western Dali alpine river region.The water pollution sharing rate in the estuary mixed area of each sub-basin is not only related to the amount of water entering the lake and the amount of pollution output,but also related to the spatial distance between the sub-basin and the estuary mixed area.(4)Quantitatively studied the effects of dif ferent underlying surface conditions on the non-point source pollution transport in the basin.The study found the forestland has the largest contribution load due to the area ratio,and the farmland has the highest contribution intensity due to artificial fertilization,Paddy soil is the soil type with the highest contribution load and intensity of total nitrogen and phosphorus pollution into the lake.The contribution load intensity of agricultural land(dry land and paddy field)has a quadratic function relationship with the slope.Th,e 18° slope zone is the key topographic threshold for agricultural planting in the Lake Erhai Basin.The contribution load intensity of forestland and grassland has a logarithmic function relationship with slope,that is.as the slope increases,the contribution load intensity decreases.The contribution load intensity of different land use types has a logarithmic function relationship with geographic elevation.In areas with a geographical elevation of less than 2000 m,the contribution of nitrogen and phosphorus into the lake is relatively large.(5)Summarize the experience model of black box for flow and non-point source pollution load in different regions of the Lake Erhai.The study found that under the premise that the basin hydrological characteristics and the underlying surface conditions tend to be stable,the pollution load and the flow show a relatively good power function relationship,and the correlation coefficients are all above 0.70 and passed the confidence test of a=0.05.Through further verification and analysis,the flow and non-point source pollution load black box model in different regions of the Lake Erhai Basin has good stability,and the simulated annual scale pollution load relative error is controlled within 30%,and the correlation coefficient of monthly scale pollution load has passed the confidence test of=0.05.(6)Accurately locate the priority management area for non-point source pollution in the Lake Erhai Basin.Based on the grey system theory and the watershed water quality target management system,a multi-level priority management area identification system for non-point source pollution in the basin is constructed.The study found that the 80%of the nitrogen and phosphorus pollution loads into the lake are concentrated in 30%of the basin area.The priority management area recognition results of different evaluation points are quite different.For the average water quality of the lake,it is economical to increase the guarantee rate of water quality from 10%to 80%for total nitrogen(control 9.33%of the basin area)and from 10%to 50%for total phosphorus(control 8.74%of the basin area).For the water quality of the northern region,it is economical to increase the water quality assurance rate from 10%to 50%for the total nitrogen and phosphorus pollution(control 8.36%and 15.48%of the basin area,respectively).The identification of priority management areas for non-point source pollution based on region water quality is the development direction of refined water environment management.