Prediction Of Non-point Source Pollution In Erhai Basin Based On Scenario Simulation

Surface source pollution,also known as non-point source pollution,is an important factor contributing to the deterioration of the water environment.Because of its complex formation mechanism,wide distribution and the difficulty of pinpointing the source of pollution,it has been a hot spot for scholarly research.However,most of the existing studies focus on the analysis of the current situation and history of surface source pollution,and relatively little on the prediction of future changes.Therefore,this paper takes the Erhai watershed as the study area and analyses the spatial and temporal variation characteristics of nitrogen and phosphorus loads of source pollutants in the Erhai watershed from 1990 to 2020 and in different scenarios for the future 2025 and 2030,with a view to providing scientific support for the management of surface source pollution in the Erhai watershed and providing reference for the prevention and control of surface source pollution in other similar lake watersheds.The main research contents and conclusions are as follows.(1)Based on the long time series multi-source data with a five-year cycle from 1990 to2020,the soil loss equation and sediment load estimation model were used to estimate the nitrogen and phosphorus loads of sorbed pollutants in the Erhai watershed.The results show that the nitrogen and phosphorus loads show an increasing trend from 1990 to 2000 and reach a peak in 2000 with 6024.44 t and 265.33 t respectively;they decrease in 2005 and the total phosphorus load drops to a minimum value of 184.25t;and then continue to increase from2005 to 2020,with the minimum value of total nitrogen load occurring in In terms of spatial distribution,the total load of nitrogen and phosphorus from sorbed pollutants in each basin is from the north,west,south and east of the basin in descending order;the load of nitrogen and phosphorus from dissolved pollutants is positively correlated with the area of the region,and the load is from the north,south,west and east of the basin in descending order.(2)Based on the improved output coefficient model,the nitrogen and phosphorus loads of dissolved pollutants in the Erhai basin were estimated for the period 1990-2020.The results show that: in terms of temporal variation,the load of nitrogen and phosphorus in the Erhai basin shows an increasing trend between 1990 and 2000,reaching a peak in 2000 with6151.47 t and 291.44 t respectively;afterwards,it decreases continuously in 2005 and 2010,and drops to a minimum in 2010 with 4210.59 t and 207.10 t respectively.The spatial distribution of pollutants is consistent with the trend of changes in sorbed state pollutants;the highest load of nitrogen and phosphorus per unit area for both sorbed and dissolved state pollutants is in the west,which needs to be focused on.(3)In order to explore the future risk of surface pollution in the Erhai basin,this paper modelled the future trends of land use,rainfall and population,which are the core factors affecting surface pollution.In terms of land use,a coupled Logistic-CA-Markov model is used to simulate the land use in 2025 and 2030 under two scenarios: development and conservation.The results show that under both scenarios,the area of cultivated land,grassland,unused land and construction land will decrease in 2025 and 2030,while the area of forest land and water will increase.Under the development scenario,the area of built-up land and forest land increases and the area of arable grassland decreases in 2030 compared to2025.Under the conservation scenario,there is little change in the area of each category,and only a small increase in the area of watershed and arable land.Based on the grey prediction model simulating the population and rainfall data in 2025 and 2030,the conclusions are as follows: the total population of the basin increases by 69,200 and 22,800 in 2025 and 2030respectively;the rainfall fluctuates in a small range in 2025 and 2030,except for a decrease in the annual rainfall at the Dali site in 2025,which shows a slow growth trend overall.(4)Based on the simulation results,the nitrogen and phosphorus loads of sorbed and dissolved pollutants in the Erhai basin in 2025 and 2030 were estimated.The results show that:in terms of temporal changes,the nitrogen and phosphorus loads of sorbed and dissolved pollutants show a decreasing trend under both development and protection scenarios;in terms of spatial distribution,the total amount of sorbed pollutants is the largest in the north under both development and protection scenarios;in terms of total nitrogen and phosphorus loads per unit area,the largest is in the west;in terms of dissolved pollutants,the nitrogen and phosphorus content of pollutants decreases in 2025 under the development scenario.However,with the exception of the eastern region,all other regions show a slight increase in 2030 compared to 2025.The reason for this is that the increase in population and livestock numbers leads to an increase in pollutant loads.Under the conservation scenario,the dissolved pollutant nitrogen and phosphorus loads show a decreasing trend in both 2025 and 2030.Based on the results of the two scenarios of surface source pollution simulations and the current situation,countermeasure suggestions for the management of surface source pollution in the Erhai basin are targeted according to the characteristics of surface source pollution formation and development plans in different regions.