Simulation Study Of Non-point Source Nitrogen And Phosphorus Pollution Load In Reservoir-type Water Sources Based On AnnAGNPS Model

Reservoir-type water source is a very important source of drinking water for residents,and water quality has been the focus of national and people’s attention,water quality safety and residents’health and social stability are closely related.In recent years,with the improvement of the control of point source pollution around the water source,non-point source pollution problems are increasingly prominent.The paper takes the reservoir-type water source in the loess plateau area of northwest China-Gaoer Plateau Reservoir drinking water source as the study area,and conducts the analysis and research on the mechanism of non-point source pollution,in order to provide a reference basis for the control of non-point source pollution of reservoir-type water sources in the semi-arid soil and rocky mountainous areas of north China.The drinking water source of Gaoer Plateau Reservoir is one of the important water sources for production and living as well as agricultural irrigation in Yaozhou District of Tongchuan City.However,the current total phosphorus and total nitrogen content of the water source is higher than the surface water category III standard,affecting water quality safety.In order to quantify the output input loadings of non-point source pollution and the weakening measures of load control,in this paper,based on the review of relevant literature,the anthropogenic net nitrogen and phosphorus input models NANI and NAPIwere used to estimate the input of nitrogen and phosphorus to the water source,and the Ann AGNPS non-point source pollution model was developed for the water source of Gaoer Plateau Reservoir based on field conditions.The migration and export processes of total phosphorus and total nitrogen in water sources were simulated to identify key risk areas,and the constructed Ann AGNPS model was used to analyze scenarios of best management practices（BMPs）for nitrogen and phosphorus pollution control in water sources.The study concluded the following:（1）The average value of NANI from 2009 to 2019 in the water source of Gaoer Plateau Reservoir is 4256.72kg·km^-2·a^-1,and the mean value of NAPIis 1018.98kg·km^-2·a^-1.All interannual variations showed an increasing trend followed by a decreasing trend.The spatial distribution shows an overall pattern of high in the southwest and northwest parts,followed by the central and southern parts,and the weakest in the north.The main source of total N input was the amount of N fertilizer applied(N_fer,70%),the main source of total phosphorus input was phosphorus fertilizer application(P_fer,93.92%).Specific analysis of the correlation of components revealed that controlling nitrogen fertilizer output and increasing livestock breeding output are effective measures to control nitrogen input to water sources,and controlling phosphorus fertilizer output is an effective measure to control phosphorus input to water sources.（2）The Ann AGNPS non-point source pollution model has good applicability in the water source of Gaoer Plateau Reservoir,and the annual runoff simulations have the highest accuracy,with Nash coefficients E and correlation coefficients R² above 0.8 for the rate period and validation period.The accuracy of the monthly-scale runoff simulations is slightly lower（E>0.6,R²>0.7）;the accuracy of the daily-scale runoff simulation is the lowest,with the correlation coefficient R² around 0.5,and the error is within an acceptable range.The model’s simulation accuracy（R²=0.6942）for the total nitrogen output of the Gaoer Plateau Reservoir water source is better than that for total phosphorus（R²=0.3741）and can be used for the assessment of nonpoint source pollution of the water source.（3）The output of total phosphorus and total nitrogen in the water source generally showed the highest interannual variation in the year of abundance and the lowest in the year of drought.In terms of intra-year variation,rainfall,total phosphorus and total nitrogen all gradually increased before August and then gradually decreased after reaching the peak in August.The percentage of total phosphorus and total nitrogen in August was 66.23%and57.30%respectively.Therefore,August is also a critical period for controlling nitrogen and phosphorus pollution.The spatial distribution shows a general distribution of low in the north and high in the south.Among the different land types,cropland and construction land are the most polluted land types causing nitrogen and phosphorus export loads.The key source area is the village of Lishu,waist ridge village,etc.The area with higher output intensity is the area on both sides of Lishu village and the river of the reservoir.（4）Using a scenario analysis approach,we analyzed best management practices（BMPs）consistent with actual water source pollutant output using a well-defined Ann AGNPS nonpoint source pollution model.The results show that the best measure to reduce pollutants in the water source area of Gaoer Plateau Reservoir is to return farmland to forest,with a reduction rate of 38.72%and 36.53%in the key source area.Considering the cost and operational difficulty of each measure,it is recommended that no-till cultivation measures be taken to reduce nitrogen and phosphorus load in the water source of Gaoer Plateau Reservoir according to the actual situation.