| After impoundment of Three Gorges Reservoir, the hydrodynamic situation of therelated rivers in the reservoir area had undergone fundamental changes, abnormalproliferation phenomenon of “algae bloom” in tributaries continuously breaks out. Thechemical fertilizers and pesticides in the agricultural production are the main reasons forthe destruction of water quality in tributaries; rural human and animal feces, sewage andgarbage are also the important reasons for the pollution of tributaries in the reservoirarea. According to incomplete statistics,52%-59%of total phosphorus (TP), totalnitrogen (TN) and organic pollutants in the secondary rivers of the Three GorgesReservoir Area are from agricultural non-point source pollution. Nitrogen, phosphorusand pesticide non-point source pollution in Three Gorges Reservoir Area has becomeone of the most serious ecological problems for the contamination threat of surfacewater and groundwater in the reservoir area. Therefore, the study of migration andtransformation of nitrogen, phosphorus and pesticide non-point source pollution in atypical area of the Three Gorges Reservoir Area has important theoretical significancesand practical values for the social, economic and environmental sustainability of theThree Gorges Reservoir Area, and for the promotion of ecological harmony and theprotection of the water environment security in the reservoir area.In this study, according to the characteristics of climate, topography, geology,geomorphology, land use types and well developed interflow in purple soil in ThreeGorges reservoir area, a distributed-dissolved non-point source nitrogen and phosphoruspollution model based on surface flow and interflow, a distributed-absorbed non-pointsource nitrogen and phosphorus pollution gray model based on the uncertainty theory,and a comprehensive migration and transformation model of pesticide pollution basedon the kinetic mechanism of non-point source pollution were built respectively, and thecalibration and validation of the model were carried out by observed data of runoff,sediment, and water quality from basin outlet. The Xiaojiang River basin, orchards andvegetable plots of Xinzheng village in Shibao Town, located in the hinterland of ThreeGorges Reservoir area, were chosen as typical case study area in this paper to simulatemigration and transformation, and spatial and temporal distribution of TN, TP, andpesticide non-point source pollutants with the hydrological processes. The criticalsource areas of pollution load were identified and the environmental impacts of nitrogen, phosphorus and pesticide non-point source pollution were analyzed and assessed,respectively. Therefore, the non-point source pollution coupled model was built toachieve the comprehensive simulation of nitrogen, phosphorus and pesticide non-pointsource pollution. The main findings are as follows:①Taking into account nitrogen, phosphorus non-point source pollution of thesubsurface flow, the SLURP hydrological model with the physical mechanisms wasintroduced and assisted to build distributed-dissolved non-point source pollution modelbased on surface runoff and subsurface flow. Through calibration and optimization ofthe parameters, the simulation accuracy of the surface runoff and subsurface flow, andthe estimation accuracy of the dissolved non-point source nitrogen and phosphorusloading were both improved.②In view of temporal and spatial variations of the rainfall erosion factor andsediment delivery ratio factor, the dynamic model which can reflect the annualvariations of the watershed sediment load was established. According to the uncertaintycharacteristics for each factor in the universal soil loss equation, each factor in theequation was defined as the blind factor, the distributed-adsorbed nitrogen andphosphorus pollution gray model was built via nitrogen and phosphorus content, andcontaminants enrichment efficiency. Overall, the dynamic coupling among the GIStechnology, uncertainty theory and adsorbed non-point source pollution model wasachieved successfully.③In order to simulate watershed sedimenttransport accurately, the sedimenttransport capacity, which is closely related to sediment delivery ratio, was chosen as thestarting point, and the cumulative runoff amount from a pixel and the upper reaches ofthis pixel was selected as a key breakthrough. According to the mathematicalrelationship between sediment transport capacity and the cumulative runoff, thetopographic index and the flow accumulation were coupled with the cumulative runoff,and the sediment delivery ratio of the pixel with maximum sediment transport capacitywas assumed as1, a new calculation method of watershed slope and river channelsediment delivery ratio was proposed by means of the GIS spatial analysis techniques.This could provide scientific theory basis for the accurate estimation of the basin-scalesoil loss.④The migration mechanism of pollutants in the plant canopy is essentiallydifferent from soil sub-environment system, the former is pesticides volatile from plantcanopy, the driving force is evaporation, and the corresponding equation is the first order differential equation; the latter is mainly the degradation of pesticides in the air,water and soil environment, the main driving force is the water potential gradient andconcentration gradient, and the corresponding equation is the second order partialdifferential equations. A comprehensive migration and transformation model ofpesticide non-point source pollution was built on the basis of dynamic mechanism ofpollutants migration and transformation in different media, and used to describe thespatial and temporal distribution and output process of pesticide pollutants in the air,water and soil environment.⑤An integrated dynamic basin model, with physical and distributed mechanism,was built by coupling nitrogen, phosphorus and pesticide non-point source pollutionmodel to simulate pollution source strength of nitrogen, phosphorus and pesticidenon-point source pollution, and thus to quantitatively evaluate and analysis the temporaland spatial distribution of non-point source pollution load. This will be more accuratelyand comprehensively conducive to quantitatively analysis and evaluate the dynamicvariations of non-point source pollution load in a typical area of the Three Gorgesreservoir area, and will achieve a comprehensive simulation of the agricultural nitrogen,phosphorus and pesticide non-point source pollution.⑥The Xiaojiang River watershed and the agricultural land in Xinzheng village,Shibao Town, Zhongxian in the Three Gorges Reservoir Area were selected as the casestudy of non-point source pollution, according to the established comprehensive model,the fate, migration and transformation of non-point source pollution of nitrogen,phosphorus and pesticides were simulated dynamically in support of GIS technology.The verifcation results show that the construction method of the model is reasonable.Based on this, the spatial and temporal distribution, the source composition, thecontribution rate, the critical source areas, runoff and erosion output of non-point sourcepollution load were analyzed and evaluated, respectively. |
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