Prediction of Two-Dimensional Pesticide Concentration Based on Unsaturated Flow Model and Dispersive Transport Model

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It is very important to analyze the behavior water flow in a basin in order to predict the pesticide concentration in a soil. Consequently, this research consists of two parts. The first one is how to apply the well known unsaturated flow equations to analyze the water movement in a real catchment. The second is an analysis of a diffusion and transport process of pesticide in a soil. The first part is studied by M.Fujita and the last one by Y.Matsui.It is belived that the unsaturated flow theory best describes physical runoff phenomena in a soil. However, there are some difficulties to apply this theory in a real basin. One of them is how to set the boundary conditions at the bottom of soil layr at which the infiltration into the deep soil layr occurs. That is, the infiltration into the deep soil layr depends on the these boundary conditions. M.Fujita proposes new boundary conditions through the runoff analysis of an experimental basin. Other equations such as rheta (water content) - p … More hi(suction) relation and unsaturated permeability to solve the unsaturated flow equations are obtained from a laboratory test using the sampled soil. It is impossible to obtain rheta-phi relation and unsaturated permeability except a special experimental basin. This fact hampers the application of the unsaturated flow equation to a practical runoff analysis. Fujita points out that the calculated infiltration into the deep soli based on the tank model and entropy method coincides with the results from the unsaturated flow equation. The tank model and entropy method are one of the lumped parameter runoff model. So, it is easy to apply these runoff model to a practical runoff analysis. It is possible to estimate a soil properties in a basin to combine two different runoff models.Pesticide used in a farm land reaches river water through the soil during rainfall. The following factors play an important role to predict the water quality for drinking water.1) What kind of pesticide does easily reach river flow ?2) What kind of soil does easily transport pesticide ?3) What kind of rainfall condition does prompt the transportation of pesticide ?The transport phanomena of pesticide in a soil depends on several factors.1) absorption of pesticide to soil2) decomposition of pesticide3) Dispersive transport of pesticide in a soil4) convective transport of pesticide in a soilThis research focuses on 1), 3) and 4). The adopted calculation method is carried out by combining 2-dimensional unsaturated flow equation with the dispersive transport model of solute. The results are summarized as follows.1) The absorption of pesticide on soil is well explained by Henry's law. Henry constant of absorption is a linear function of organic and clay content of a soil.2) The dispersion coefficient of pesticide transportation depends on internal velocity of water.3) The simulation results show that the concentration of pesticide in a river flow increases with duration time of rainfall and rainfall intensity and the amount of pesticide spread over the field and decrease with Henry constant. Less