Spatial-temporal Distribution Pattern Of Agricultural Non-point Source Pollution In Huangqian Watershed Based On SWAT Model

The Huangqian Watershed belongs to the Dawen River Basin of the Yellow River,and it is also an important water source in Tai’an City.In recent years,it is faced with serious non-point source pollution problems of high nitrogen and phosphorus.Mastering the spatial and temporal distribution pattern of nitrogen and phosphorus non-point source pollution in the Huangqian Watershed is an important basis for the protection,rational utilization and management of water resources.It can also serve the ecological protection and high-quality development of the Yellow River Basin.In this study,the Huangqian Watershed as the research object.Based on SWAT model and scenario simulation method,the temporal and spatial distribution pattern of non-point source pollution in Huangqian watershed was clarified,the key source areas were identified,and the management measures of agricultural non-point source pollution were discussed.It provides theoretical and practical reference for non-point source pollution control and water environment restoration in the Huangqian Watershed The main research results and conclusions are as follows:（1）Based on remote sensing images and human-computer interaction interpretation,the land use distribution characteristics of Huangqian Watershed were mastered.Land use types in Huangqian Watershed can be divided into 7 categories:cultivated land,garden land,forest land,rural residential area,urban land,water area and unused land.The land use structure is dominated by woodland and garden,accounting for 50.14%and 40.37%of the total basin area,respectively.Forest land mainly distributed in≥30°area,accounting for 78.40%of the forest area.Garden mainly distributed in≥30°,accounting for 55.66%of the total area.（2）Based on DEM,land use,meteorology,fertilization and soil data,the local database was established,and the sub-basins and hydrological response units were divided.The applicability of the model was verified by the measured data.Using Arc GIS,SPAW,SWATWeather and other software to process basic data,calculate and establish local database.SWAT model was used to divide 28 sub-basins and 329 hydrological response units.According to the measured data,the applicability analysis shows that R~2>0.6,NSE>0.5,Re<±25%of each water system meet the requirements of model operation.SWAT Model for Agricultural Non-point Source Pollution in Huangqian Watershed（3）Based on the calibrated SWAT model,the spatial and temporal distribution pattern of nitrogen and phosphorus non-point source pollution in the Yellow River Basin was mastered.From the time-month scale,the non-point source pollution load is mainly concentrated in June–August,which is closely related to the monthly average precipitation,daily runoff and sediment load.From the spatial distribution,nitrogen and phosphorus non-point source pollution has high consistency,mainly distributed in the northeast of Huangqian Watershed.From the point of view of water system,the high load area is mainly distributed in Shiwen River,and the contribution rate of nitrogen and phosphorus load is more than 50%,which is the main area of nitrogen and phosphorus load in the watershed.（4）Based on spatial overlay analysis,the non-point source pollution load patterns of different land use types,slopes and sub-basins were discussed,and the key source areas of non-point source pollution were identified.From different land use types,garden land is the main source of nitrogen and phosphorus pollution load in the watershed.The organic nitrogen,nitrate nitrogen,organic phosphorus,mineral phosphorus and soluble phosphorus load accounted for 71.36%,71.25%,71.27%,75.76%and 47.70%of the total nitrogen and phosphorus load,respectively.Organic nitrogen,nitrate nitrogen,organic phosphorus and mineral phosphorus were positively correlated with slope gradient.≥20°is the main area where nitrogen and phosphorus pollution load occurs,especially≥30°is the key area where nitrogen and phosphorus pollution load is concerned.From the perspective of sub-basins,sub-basins No.8,10,11,13,14,16 and 18 are the key sources of pollution.（5）Based on the division results of key source areas,effective watershed management measures were proposed by adopting scenario simulation of reducing fertilizer application rate and returning farmland to forest.When the amount of fertilizer was reduced by 10%and20%,the reduction efficiency of key source areas was 0.53～1.36%and 1.07～2.71%,respectively.It shows that reducing fertilizer application has certain effect on reducing nitrogen and phosphorus pollution load.With the decrease of fertilizer application rate,the reduction efficiency of key source area increases,and the load per unit area decreases gradually.After changing the≥30°and≥20°gardens to forest land,the nitrogen and phosphorus load reduction efficiency in the key source area ranged from 44.24 to 58.96%and62.35 to 76.19%.It shows that the effect of returning garden to forest on reducing non-point source pollution load is remarkable.With the increase of forest area,the reduction efficiency increases and the unit area load decreases.Reducing fertilization amount can be used as a way to control non-point source pollution in the watershed.The way of returning garden to forest is suitable for gradual progress,which can be used in steps and regions.