The use of chemical fertilizers and pesticides will not only cause economic problems in agricultural production but also affect the quality and environment of water bodies. The process of migration of agricultural compounds with runoff includes a series of complex physical and chemical processes. It is not only affected by the physical and chemical properties of soils, compounds, and the characteristics of rainfall runoff, but also affected by micro-topography, drainage conditions, surface soil physical properties, and surface conditions. The effect of underlying conditions such as vegetation coverage. Soil bulk density, one of the indicators of soil physical condition, is one of the important indicators of soil fertility. It is also an important parameter that can be reasonably regulated by appropriate tillage or no-tillage measures. Soil bulk density was measured using a soil bulk density analyzer.
The larger the soil bulk density is, the earlier the flow generation time is (Tp value is smaller), and the larger the runoff coefficient is, the more the rainfall is converted into runoff, the greater the erosion intensity of runoff is on the surface soil and the more severe the soil erosion is; In terms of soil, the order of soil bulk density affected by runoff nutrient concentration was determined by soil bulk density meter: NO-3-N>PO3-4-P>K+; there was a “peak point†in the curve of runoff solute mass loss rate. The speed at which the peak point is reached and the size of the peak depend on the bulk density of the soil; the weakly adsorbed compounds reach the peak earlier; the peak size also reflects the amount of solute loss. By fitting the experimental data, it is further confirmed that the power function fits the runoff solute transfer process under unsaturated conditions in the Loess Plateau region is very appropriate.
After the soil bulk density tester determined the soil, under a constant rainfall intensity, the runoff increased drastically within 13 min of the initial runoff, then increased slowly and became stable; as the slope increased, the degree of soil erosion increased and the runoff solute concentration increased. The contribution of soil erosion to runoff solute concentration increases. Runoff concentrations of nutrients (water-soluble phosphorus, potassium, and bromide ions) are high in runoff concentrations within a few minutes of initial production, followed by rapid decay. After comparison, it was found that the use of a mathematical model characterized by a power function model can better simulate the runoff solute concentration change process under unsaturated conditions on the loess slope, and can also use power function to better describe the process of phosphorus and potassium mass change in runoff. However, the initial stage of quality change cannot be fitted. The cumulative loss of runoff potassium and cumulative runoff bromine loss have a good linear relationship with slope.
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