Spatiai-Temporailanalysis Of Non-Point Source Pollution In Nanliu River Basin Based On SWAT Model

Based on the changes of the land surface system in the Nanliu River Basin,this paper uses a large number of literature reading and geographic information system（GIS）techniques.Analysis the pollution characteristics of the Nanliu River Basin,this paper studies the change of land use in the basin,quantitatively evaluates the soil erosion in the basin,and the relationship between the spatial distribution of soil erosion and the topography and land use type.Establish the SWAT non-point source pollution load model,the process of runoff in the Nanliu River Basin was analyzed,and the pollution load was simulated.The main sources,contributions and distributions of non-point source pollution in the Nanliu River Basin were identified,and the temporal and spatial distribution of ammonia nitrogen and total phosphorus（TP）were analyzed.Mainly achieved the following results:1.The paper analyzes the changes in the structure of land use quantity,statistics the dynamics of land use change and the characteristics of change and transfer over the past 15 years,and views the changes of various land types from the perspective of agricultural adjustment and economic strategic objectives.There are several causing reasons for this change.Firstly,the expansion of urbanization and the process of industrialization have changed the farmland around townships into construction land.Secondly,the large amount of labor exported to the city has indirectly led to desolation of the land which makes grassland increased slightly.Thirdly,the adjustment of agriculture,which makes the total amount of new land increased by 40.98%,while the total amount of forest land reduced by 59.41%in the same period.2.Based on the dynamic changes of land use,this paper analyzes the characteristics of soil erosion in the Nanliu River,makes a quantitative description on basic conditions of soil erosion,soil erosion characteristics and erosion intensity of several land types,and explores the relationship between spatial distribution of soil erosion and topography,and land use types and other factors.The conclusion points out that soil erosion in Nanliu River Basin is characterized by wide range,fragmentation and large amount of erosion,which is mainly concentrated in the low hills of the Liuwan Dashan and Bobai Basin,especially the southeast side of the Liuwan Dashan.These basins are mainly low-lying hills in recent years,with broken terrain,and as a large number of orchards are being reclaimed,human development and slash burning were strong,which results in strong soil erosion in these areas.The total annual average erosion of soil is 1,172,500 tons,the total erosion area is 474.63 km²,and the annual average erosion modulus is only 124.43 t/km²·a^-1,which illustrates from the side that the Nanliu River Basin has a wide range of erosion,but its intensity is not large,mainly concentrated in mild and moderate erosion.3.In order to have a further understanding on the non-point source pollution in each small watershed,combined with the preliminary research on the basic research of the basin,the SWAT model was used to simulate the non-point source load in the Nanliu River Basin.The results show that:（1）The runoff simulation values of the two sites of Bobai and Changle show the same trend as the measured values,and the fitting degree of flood season is higher than that of non-flood period.The extent of runoff dry season within the control basin range of Changle Station is obviously larger than that of the Baibai station,and the periodicity is more obvious,which also led to runoff simulation of Changle station in the verification period（2010-2012,2013-2015）too high,and the production flow in the basin was overestimated.（2）Due to the lack of more months of measured data,the simulated values of pollutants in the whole period are lower than the measured values,but the accuracy of pollutant simulation meets the model evaluation index,and the simulation results are relatively good,which can reflect the trend of pollutants as a whole.（3）From the time point of view,before 2011,the pollution load of ammonia nitrogen and total phosphorus（TP）fluctuated regularly with the runoff flood season and non-flood period.However,after2011,although the runoff entered the dry phase of the annual hydrological cycle,the ammonia nitrogen and total phosphorus（TP）pollution load showed a leaping fluctuation.In some low runoff months,the pollution load had an abnormal value,and the overall continued high,and did not show a downward trend.（4）From the space point of view,since the basin topography is from the basin to the low valley and then goes into the low hill and gentle slope area,the annual average ammonia nitrogen load is mainly distributed in the lower reaches of the basin from 2003 to 2015,especially follows the river channel from the county boundary of Bobai County to the mouth of the basin（Jiangkou bridge,south section）.The annual average total phosphorus（TP）load is mainly distributed in the basins of both sides along the river channel,such as Bobai Daling to Hepu Shikang.According to the survey,there are a large number of large-scale livestock farms along the coast where the contradiction between livestock manure and sewage efflux and limited land consumption capacity is prominent,and the total phosphorus（TP）load is greater than ammonia nitrogen to some extent.