Influential Mechanism Of Rainfall And Terrain On Non-Point Source Pollution

With the development of society and economy,point source pollution has been effectively controlled,and non-point source pollution is becoming a major source of pollution due to its characters of extensiveness,uncertainty,lag,latency,difficulty in research and control.Among them,agricultural non-point source pollution is even worse,agriculture is developing in a large scale and intensifying direction.The application of fertilizers and pesticides is increasing in order to increase the yield.The large amount of nutrients rich in it has become the main pollutant of agricultural non-point source pollution.In this study,based on the premise of learning and referring to the existing research results of non-point source pollution,small-scale simulated rainfall experiments were conducted in the study area by selecting the experimental area in the Sichuan Basin.Twenty natural rainfall and topographic scenarios were simulated in the study,including four rainfall intensities(30 mm/h,60 mm/h,90 mm/h and 120 mm/h)and five grades of slopes(5°,10°,15°,20° and 25°)and runoff duration was designed for 40 minutes.In this study,by design of rainfall and slope conditions,soil erosion and non-point source pollution in the purple soil area of Sichuan Basin were simulated.Through the measurement and calculation of the non-point source pollution factors such as runoff intensity,erosion intensity,runoff yield,erosion amount,total nitrogen load,total nitrogen concentration,total phosphorus load and total phosphorus concentration under different rainfall scenarios,Analyze the impact of rainfall and slope conditions on soil erosion and major transport mechanisms of non-point source pollutants,and establish a quantitative relationship.Concluded as follow:(1)The process of runoff has a significant lag relative to the start of rainfall during the rainfall,which is the runoff start time,which varies from 1.31 to 12.43 minutes due to different rainfall and slope conditions.(2)In terms of the impact of precipitation on the loss of non-point source pollution:both the runoff intensity and the erosion intensity increase with the increase of precipitation,and then the increase gradually decrease with the increase of precipitation.Meanwhile,the amplitude of erosion intensity is relatively small during the increase of rainfall.The changes of total nitrogen and total phosphorus were linear with the increase of precipitation while the total nitrogen concentration decreased with the increase of precipitation.Total phosphorus Concentration increased with the increase of precipitation,but the tendency was slowed down with a slightly decline when the slope was greater than 15 °.(3)In terms of the impact of rainfall intensity on the loss of non-point source pollution:the rainfall runoff generally increases with the increase of rainfall intensity.The intensity of erosion increases with the increase of rainfall intensity.The load of total nitrogen loss with the increase of rainfall intensity,the increase increased slowly or even slightly decreased when the rainfall intensity was higher(120 mm/h).As the rainfall intensity increased,the total nitrogen concentration decreased gradually.The total phosphorus load increased with the increase of rainfall intensity.The total phosphorus concentration increased gradually with the increase of rainfall intensity,and the increase was moderate when the rainfall intensity was relatively small but more significant when rainfall intensity is relatively high.(4)In terms of the effect of slope on the loss of non-point source pollution:Under each rainfall intensity,the runoff intensity and erosion intensity all increased with the increase of slope gradient.The change of erosion intensity was slightly in the range of 20° and 25°.Under the different rainfall intensities,the total nitrogen load increased with the increase of slope,and the total nitrogen load changed little or even drop slightly when the slope was greater than 15°.The total nitrogen concentration was increase with slope.The total phosphorus load gradually increases along with the slope,but when the slope is larger,the growth rate is smaller or even declines.With the gradual increase of slope,the total nitrogen concentration increased with the increase of slope more obviously when the slope was less than 15°.However,with the slope greater than 15°,the increase of total nitrogen concentration gradually decreased with the increase of slope,and began to decline.(5)In this study,the critical slope of non-point source pollution loss is about 15 0.