Numerical Simulation Of The Phosphorus Loads And Evaluation Of The Reduction Measures In Typical Mountain Areas

With the point source pollution being gradually controlled and the composition of river basin pollution changed,the proportion of agricultural non-point source pollution load became larger.Presently,agricultural non-point source was still the main source of water pollution in China.The process of non-point source pollution involved the interdisciplinary intersection of agriculture,water conservancy,environment and ecology,etc.It was of great significance to carry out calculation of non-point source pollution load and to evaluate the effect of the relevant control measures.In this thesis,based on the study on accounting and reducing measures of non-point source phosphorus process in hilly and mountainous area,the characteristics of phosphorus pollution in Linjiang River Basin,a typical catchment in hilly and mountainous areas of the Yangtze River,was analyzed by collecting geographical data,hydrological and water quality data,and social and economic data.A process based model of non-point source phosphorus pollution in Linjiang River Basin was firstly further built,and then the characteristics of spatial and temporal distribution of phosphorus process and the main factors influencing the phosphorus process were summarized.Secondly,The contribution rate of phosphorus pollution sources was analyzed and the key source areas of phosphorus pollution were identified.Thirdly the effects of different phosphorus pollution reduction measures were evaluated.The main research contents are as follows:（1）The attributive database and spatial database required for SWAT model were constructed by collecting and sorting out the relevant data collection such as topography,river network distribution,land use,soil type,agricultural production and pollution sources of Linjiang River Basin.The whole watershed waas divided into 19sub-watersheds and 177 hydrological response units.The model was calibrated and validated based on runoff data from 2018 to 2021.Calibration and verification results showed that the determination coefficient R² is above 0.8 and the Nash coefficient E_nsis around 0.7,which shows that the model has good usability in this area.（2）According to the calculated results of SWAT model,the monthly average phosphorus load in Linjiang River Basin from 2018 to 2021 presents the characteristics of fluctuation distribution.The fluctuation increased from January to May,reached the highest value from June to September and gradually decreases from October to December.From the spatial distribution,the total phosphorus loss intensity was0.12-1.08 kg/ha.Overall,the load from upstream to downstream was decreasing,and the runoff intensity in the tributary river basin was significantly higher than that in the main river basin.（3）According to SWAT model calculated results,the inflow of total phosphorus pollution in Linjiang River Basin was as follows:phosphorus fertilizer（67%）>soil phosphorus pool（17%）>urban life（10%）>livestock and poultry breeding（6%）.The pollution sources of chemical fertilizer and livestock and poultry farming were mainly distributed in the middle and southwest of the watershed,while the pollution sources of urban life were mainly in the northeast of the watershed.The principal component analysis method was used to determine the key source area of phosphorus pollution control.The area of the key source area of phosphorus pollution loss in the catchment accounted for 34.43%of the whole catchment and contributed42.98%of the total phosphorus in the catchment.（4）The results of total phosphorus pollution control analysis using best management practices（BMPs）scenario program showed that terrace and contour tillage have high efficiency in reducing sediment and total phosphorus pollution,with an average reduction rate of more than 90%.The reduction rates of interception measures vary greatly on different HRU scales,reflecting the important influence of spatial distribution on the effectiveness of measures.From the watershed scale,the spatial distribution of the key source areas in the watershed had changed greatly after the arrangement of measures.Source control and process interception were often combined in non-point source pollution control of watershed.The combined BMPs evaluation results show that FR20+FS+TE and FR20+TE scenarios had good pollution reduction effects.Total phosphorus concentration at the outlet of watershed had been reduced by more than 90%,while that of FR20+FS was less than 40%.The scientific significance of this study was to calculate the phosphorus load in a typical hilly and mountainous small watershed without enogh measured data and to analyze the spatio-temporal variation process of phosphorus loss and contribution characteristics of pollution sources using the model.The model was applied to analyze the reduction of phosphorus load and the change trend of water quality under the artificial control measures such as landscape,forest,field and lake pattern and green agriculture.Further research works should be strengthened as:（1）The field measurement especially in the observation on the influence of agriculture management,ditches and ponds on phosphorus flux;（2）the mechanism and measurement research on the influence of sediment and crops process on the phosphorus morphology;（3）The distribution and load of main sources and their effects on the flux of control stations under the comprehensive action of various pollution sources;（4）Quantitative evaluation of the impact of agricultural cultivation,livestock and poultry breeding and different living sources respectivelyon key source areas.