Simulation Study Of Interflow And Nitrogen And Phosphorus Transport In Purple Soil Sloping Farmland In Three Gorges Reservoir Area

The three Gorges Reservoir area is the sensitive area,vulnerable area,easily polluted and easily destroyed area in the ecological environment of China.The water ecological environment of the reservoir area is deteriorating.The eutrophication of water bodies is serious,and disasters such as water blooms are frequent,and the problems of non-point source pollution are prominent,especially non-point source pollution caused by nitrogen and phosphorus.The area of purple soil slope farmland accounts for a large proportion of the total cultivated land area in the Three Gorges reservoir area,and the application amount of fertilizers such as nitrogen and phosphorus fertilizers on sloping farmland is large.Due to the purple soil's own structural characteristics,the interflow occupies a considerable proportion in the total runoff of the purple soil.Therefore,it is of great theoretical and practical significance to study the interflow and movement of nitrogen and phosphorus in purple soil slope farmland to protect sloping farmland in the three Gorges Reservoir area and to prevent the rivers in the three Gorges Reservoir area from the non-point source pollution.In this paper,HYDRUS-3D software is used to build a mathematical model.The experimental results are used to validate the model parameters.This paper uses HYDRUS-3D software to build a mathematical model,and use experimental results to calibrate and validate the model parameters.The research contents mainly includes three parts.(1)Firstly,the infiltration and migration of water in purple soil with three textures of clay,loamy and sandy at a slope of 5 °10 °10 °15 °15 °20 °25 °,respectively were simulated under the established model.Then,the loamy purple soil with good infiltration and water retention properties was selected in the following simulations.Water infiltration and migration processes of loamy purple soil at the tillage layer thicknesses of 15 cm,18 cm,20 cm,22 cm,and 25 cm at a typical slope of 15 °were simulated.(2)The loam purple soil with a 20 cm thick tillage layer and a slope of 15° was selected to study the migration processes of interflow and nitrogen and phosphorus in the slope farmland under different rainfall intensities,rainfall durations and rainfall patterns.(3)The loam purple soil with a 20 cm thick tillage layer and a slope of 15° was selected to study the migration processes of interflow and nitrogen and phosphorus in the slope farmland under initial moisture content of 0.15,0.20,0.25,0.30,0.35 respectively and the upper catchment intensities of 0cm/h,1cm/h,4cm/h,7cm/h,and 10cm/h,respectively.Under the typical rainfall conditions in the reservoir area,the study on infiltration and migration of different texture purple soils on different slopes shows that the size relationship of the infiltration rate and interflow runoff rate between three texture purple soils under the same slope is as follows: sandy soil > loam > mucilage,indicating that the composition of purple soil particles is an important influencing factor for soil infiltration and water retention.The greater the clay content,the poorer the infiltration of purple soil and the better the water retention.The intensity of interflow and accumulated volumes of interflow in the purple soils with three textures were negatively correlated with the gradient.With the increase of slope,the cumulative surface runoff volumes of clay fluctuates,which decreases first and then increases and then decreases;which of the loam increases first and then decreases;which of the sand is always zero.Under the rainfall intensity of 30mm/h and the rainfall duration of 3h,the study on the purple soil with different tillage layer thickness shows that the water infiltration rates,the actual cumulative infiltration volumes,and the runoff rates of the loamy purple soil are positively related to the thickness of the tillage layer.The surface runoff rate is negatively related to the thickness of the tillage layer,and the distribution and loss of water are mainly concentrated in the tillage layer.In simulating the influence of rainfall conditions,(1)Three rainfall intensity studies,hard rain(0.15 cm/h),rainstorm(0.3 cm/h),and downpour(0.45 cm/h),are selected to find out: Migration depth of the wet front,soil moisture content,and migration velocity of nitrate nitrogen and the intensity of inflow are positively correlated with rainfall intensity.The concentrations of nitrogen and phosphorus are negatively correlated with rainfall intensity.The runoff rate of interflow fluctuates with the increase of rainfall intensity.In the rainstorm,the runoff rate of interflow is the smallest,but the loss rate of nitrogen and phosphorus increases with the increase of rainfall intensity,but the increase is decreasing.(2)Selecting 24-hour,48-hour,and 72-hour rainfall durations,the study found that the migration depth of the wet front,the water content in the soil profile,the intensity of inflow are positively correlated with the rainfall duration.While the concentrations of nitrogen and phosphorus are negatively correlated with the rainfall duration.With the increase of rainfall duration,the runoff rate of interflow decreases gradually,while the loss rates of nitrogen and phosphorus gradually increase.(3)Selecting average,increasing,and declining rain patterns: The rate relation between the soil moisture content and the migration depth of the wet front under different rain patterns is: decreasing> average> increasing.The rainfall pattern has no significant effect on the runoff rate of interflow and migration progresses of nitrogen and phosphorus.When considering the influence of different initial moisture content of purple soil,it is found that the migration depth of wetting front,water content in soil profile and the intensity of interflow are all positively related to the initial moisture content,while the concentrations of nitrogen and phosphorus are inversely related to the initial moisture content.The rate relation of water content between different slope positions: mid slope> upslope> downslope.The runoff rate of interflow and the loss rates of nitrogen and phosphorus are positively correlated with the initial moisture content.Considering the upper catchment intensity,the migration speeds of the moisture,nitrogen and phosphorus along the slope was positively related to the intensity of upper water catchment.The greater the strength of the upper catchment,the greater the impetus of the nitrogen and phosphorus along the slope,and the greater the loss rates of nitrogen and phosphorus.In general,the distributions and losses of soil moisture and nitrogen and phosphorus mainly occur in the tillage layer.Compared with the migration of nitrate nitrogen,the adsorption of phosphorus the purple soil is stronger.The influence of the rainfall condition,the initial moisture content and the upper confluence intensity on the migration of dissolved phosphorus is not obvious,and the loss rates of dissolved phosphorus in the interflow are less than 1%.