| Currently, agricultural non-point source pollution is becoming one of the main factors affecting thequality of the water environment. Enhancing the prevention and control of agricultural non-point sourcepollution is very important to improve water quality. In recent years, the water quality of Erhai Lake,which is known as the Pearl in Yunnan Plateau, showed a gradual deterioration as affected byagricultural non-point source pollution. Fengyu River is located in the northern region of Erhai Lakebasin, which suffered from severe agricultural non-point source pollution. Fengyu River is one of themajor sources of inflow of Erhai Lake, and it thus plays an important role in influencing volume andquality of the water in Erhai Lake. Taking Fengyu River basin as the case study area, the work in thisdissertation investigated generation, migration and output of agricultural non-point source pollutants aswell as their temporal and spatial variation distribution characteristics, and identified the critical areasand periods that control agricultural non-point source pollution, using various methods including fieldmonitoring, SWAT model, phosphorus index and export efficient model. The research results providethe basis for improving fertilizer use efficiency, reducing agricultural non-point source pollution andprotecting water environment of Erhai Lake. In addtion, this work also has an important reference valueto assess nitrogen and phosphorus load and control agricultural non-point sources pollution in otherwatersheds. The main conclusions are as follows:(1) The analysis on the monitoring data suggests that rain flow and concentrations of nitrogen andphosphorus pollutants showed a seasonal variation, with differences between different forms of nitrogenand phosphorus.90.2%of the rainfall,60.8%of base flow,91.2%of surface flow and66.9%of totalflow happened in flood season (June to November). The highest concentration of dissolved nitrogen wasobserved in June while the highest concentration of particulate nitrogen was in August. Compared withnitrogen, the concentrations of dissolved and particulate phosphorus were generally low, except that theconcentrations were a bit higher in August because of heavy rains. The output pollutants are closelyrelated to surface runoff and soil erosion. There is a significantly positive correlation between theconcentration of particulate nitrogen, dissolved phosphorus, particulate phosphorus in total runoff andsediment or surface runoff. There is a significantly negative correlation between nitrate, dissolvednitrogen and base flow. Moreover, there is a significantly positive correlation between total runoff,surface runoff and sediment.(2) The output of nitrogen and phosphorus loads also showed obvious seasonal variation. Annualoutput load of total nitrogen is77.63t,76.0%of which is in the dissolved form; total phosphorus loadis19.07t, of which the particle form accounted for74%in the basin. The flood season (June toNovember) is a critical period for the loss of nitrogen and phosphorus, mainly by runoff.58.1%of thenitrate,63.0%of dissolved nitrogen,83.6%of particulate nitrogen,68.0%of total nitrogen,88.6%ofthe dissolved phosphorus,90.4%of the particulate phosphorus and90.0%of total phosphorus of theannual pollutant load happened in flood season, mainly in July to September, especially in August. In flood season, the surface runoff accounted for only18.2%of the total annual runoff, but it contributedby42.5%of the particulate nitrogen,46.9%of the dissolved phosphorus,60.9%of the particulatephosphorus and57.3%of the total phosphorus load of the annual output load.(3) The calibration and validation of SWAT model established by the attribute and spatial data ofFengyu River are finished based on monitoring data of hydrology and water quality from basin outletfor two years. The results showed that the simulated values are satisfactory and the model is appropritefor the study area. Both the R2and Enshave met accuracy requirements for calibration and modeling.(4) Spatial distribution of the generation, output of nitrogen and phosphorus pollutants and theirmigration characteristics in the river was simulated by using SWAT model. There are some differencesin the generation and output in unit area of total nitrogen, total phosphorus in different sub-basins. Thegeneration of total nitrogen per unit area in the basin is between0and17.69kg/ha, and output wasbetween0and14.55kg/ha; the discharge of total phosphorus per unit area is between0.01and1.82kg/ha, and output was between0.01and1.62kg/ha. The migration processes of nitrogen andphosphorus are quite different in different sub-basins, and the migration pathways of nitrogen andphosphorus also differed within the same sub-basin.(5) Characteristics of pollution sources analysis based on discharge and export coefficient showedthat animal production sources contributed more than70%of the emissions load of total nitrogen andphosphorus, which was followed by crop production source and rural life source.(6) A Yunnan plateau agricultural watershed P index evaluation system is proposed based on theIowa PI after modifications according to plateau agricultural watershed characteristics and referring tosome other similar PI evaluation systems. The new P index evaluation system is to assess risks of lossesof dissolved and particulate P. The results showed that higher and the highest risk areas of the twoforms of phosphorus loss are located within the distance of100meters away from both river banks.Moreover, higher and the highest risk areas of dissolved phosphorus loss are mainly the farmland areain the middle and downstream of the river channels. However, higher and the highest risk areas ofparticulate phosphorus loss focus on the farmland areas in middle and downstream river and on themeadows in upstreamriver.(7) The critical period and area to control agricultural non-point source pollution is determined in theFengyu River basin by integrating the analysis result of measured data on hydrology and water qualityand SWAT simulation results. The flood season is identified as the general critical period. Especially, thestorm period (August), when carries the most of rainfall, runoff and pollutant output load, is the controlcritical period; and pre-flood season (June), when carries the high concentration of pollutant but lowpollutant output loads, is the preventive critical period. Sub-basins3,5and15are the critical areas forprevention. Combined with results evaluated with phosphorus index, the farmlands with100meters onboth sides of the river are identified as the critical areas in these sub-basins. |
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