Identification Of Critical Source Areas Of Non-point Source Pollution And Nutrient Transportation Rule In Watershed Based On Source-sink Landscape Pattern

Non-point source pollution is a global environmental pollution problem and the main cause of aquatic environmental pollution in my country.Therefore,the protection of water sources in the Danjiangkou Reservoir area has become the primary task at present.Based on the"source-sink"theory,this study combines GIS technology,RUSLE and"source-sink"landscape pattern index to identify the critical source areas of non-point source pollution in the small watershed.Based on the landscape pattern of"source–sink",the water quality of small watershed was evaluated and analyzed by WQI index and rank correlation analysis,etc.By analyzing the nutrient migration changes of"source–sink"landscape under different rainfall events,it lays the foundation for improving the regulation measures of water quality in small watershed.The main research results are as follows:（1）The main two types of land use in small watershed are forest land and cultivated land.Forest land is mainly distributed in the north and south of the small watershed,and most of the central and southern regions are residential land and cultivated land.The area of forest land and cultivated land is 27.69%and 19.06%,respectively,accounting for 46.75%of the Hujiashan small watershed.The LWLI value is 0.497.The maximum soil erosion modulus is 3394.21 t/（km²·a）by RUSLE model.Combined with land use factor,LWLI factor and RUSLE factor,the primary risk area is mainly in the south,and the area is 3.68 km².（2）During 2013–2019,most TN and NO3^--N concentrations exceeded the national standard class III standard,while most NH4⁺-N,TP and COD_Mnconcentrations met the standard.In spatial distribution,the concentrations of TN and NO3^--N showed obvious variability in dry season and wet season,and their concentrations were the highest in the southern cultivated land and residential land.WQI,TN and NO3^--N in the sub-basin scale and 50 m,100 m,150 m and 200 m buffer area of forest land and"sink"landscape proportion have more obvious relationship,but with NH4⁺-N,TP,COD_Mnrelationship is not obvious.WQI was positively correlated with the proportion of forest land and"sink"landscape at different scales,indicating that the proportion of forest land and"sink"landscape has an important impact on improving water quality.The proportion of"sink"landscape has a significant negative correlation with TN and NO3^--N at different scales,indicating that the increase of"sink"landscape proportion can significantly reduce the concentration of TN and NO3^--N.（3）In the two seasons,TN and"sink"landscape proportion has the best fitting effect in 200 m buffer zone.The fitting effect of NO3-N and"sink"landscape ratio is also optimal in 200 m buffer zone.Under the condition of ensuring normal agricultural production,the optimization of"source-sink"landscape proportion in 200 m buffer zone is beneficial to improve the water quality of the basin,so as to effectively control the occurrence of non-point source pollution.（4）In different rainfall events,"source-sink"landscape pattern and rainfall characteristics have an impact on nutrient migration.The"sink"landscape can effectively control the nutrient concentration in the runoff process,and the higher proportion of"sink"landscape can keep the nutrient concentration at a relatively low level.The super-osmotic runoff generated by short-term heavy rainfall type will promote the nutrient concentration in runoff to a high level.