The Study Of Agricultural Non-point Source Pollution Based On SWAT Model In Typical Basin Of Southern China

With rapid development of social economy, the increasingly shortage of water resource and water environmental pollution are becoming global problems. The agricultural non-point source pollution also became an important factor that effect water quality because of its randomness, hysteresis, uncertainty and concealment. The Siheshui watershed of Tanjiang Valley, located in Guangdong Province, was taken as the research region to study the non-point source pollution problems, get the main results as follows:(1) Summarized the research status of mechanism and model of agricultural non-point source pollution in China and abroad, introduced Chinese current situation of non-point pollution load calculation methods; Chose SWAT model and the average concentration method as research tools, and determined the ultimate technical route of paper.(2) Analysis of loss characteristics of non-point pollutants for each rainfall eventSimultaneous monitoring of quality and quantity of water induced by several rainfall events was conducted in order to study the loss characteristics of non-point pollutants. The results indicates that the concentration of pollutants like BOD5, TP and TN were high at the preliminary stage of rainfall runoff generation, the preliminary stage of rainfall was the fastigium for the losses of non-point pollutants. The concentation of ammonia shows the first flush affect during the first half procedure, while becomes negatively correlated with the volume of runoff in the rest procedure. Concentration of CODMn shows wavy and jagged, and in the later period, the concentration is influenced by water withdrawal process dominant by the interflow.The pollutant flux and the flow had an approximately identical change trend, and remarkable correlation between them could be detected through statistical analysis.(3) Establish of SWAT model and analysis of model applicabilitySpatial database and attribute database of Siheshui watershed were established by use of tools such as ArcGIS and SPAW, and then the SWAT model used for non-point pollution simulation can be completed. Within limited quantity data, the non-point pollution load at outlets in the watershed was estimated by the mean concentration method, and then applied for model parameter calibration and verification. The results show that this model is applicable in the humid area of south China. The efficiency coefficients of monthly runoff in the calibration and verification period were 0.83 and 0.92, respectively, the relative error of yearly pollution load was restricted to less than 25%, while the monthly pollution load to less than 60%, the daily to 100%.(4) Analysis of model running results and management practices scence simulationAfter SWAT model was calibrated, it can be used to simulate the Non-point source of Siheshui watershed. For the relationship between TSS、TP、TN and precipitation, apparent consistency can be observed with regard to the changes in a single year. The pollution load in the flood period occupied more than 80% of those throughout the year, and the pollution content in the middle and lower reaches was significantly affected by the population density and the human activities. The load quantity of pollution content, including sediment, phosphorus, and organic nitrogen, in unit area of various land usage were estimated and the descending order is farmland, grassland, construction land, and forest land. If the farmland was returned to forests, the pollution load of runoff, sediment, nitrogen and phosphorus would reduce 3.2%,35.8%,46.1%,35.8%, respectively. Moreover, the total nitrogen load or phosphorus load would respectively decrease 18.1% or 8.9% if the amount of nitrogen or phosphorus fertilizer was drop down to 50%.