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

分析流域内水流的动态特性对预测土壤中农药浓度具有重要意义。因此，本研究由两部分组成。第一个问题是如何应用非饱和流方程来分析真实的小流域的水流运动。第二个是农药在土壤中的扩散和迁移过程的分析。第一部分由藤田先生研究，第二部分由松井先生研究，认为非饱和流理论最能描述土壤中的物理径流现象。然而，在真实的盆地中应用这一理论还存在一些困难。其中之一是如何设置土壤层底部的边界条件，使土壤层进入深层。也就是说，向深层土壤层的入渗取决于这些边界条件。藤田先生通过实验流域的径流分析提出了新的边界条件。其他方程，如rheta（水含量）- p ...更多信息 通过室内土样试验，得到了非饱和渗流方程的吸力关系式和非饱和渗透系数。只有在特殊的试验池中才能获得rheta-phi关系和非饱和渗透率。这一事实妨碍了非饱和水流方程在实际径流分析中的应用。Fujita指出，基于水箱模型和熵方法计算的土壤深层入渗与非饱和流方程的结果一致。水箱模型和熵值法是集总参数径流模型的一种。因此，这些径流模型易于应用于实际径流分析。通过联合收割机两种不同的径流模型可以估算流域内的土壤性质。以下因素在预测饮用水水质方面起着重要作用。1）哪种农药容易到达河流？2)什么样的土壤容易运输农药？3)什么样的降雨条件会促使农药的运输？农药在土壤中的迁移现象取决于以下几个因素：1）农药在土壤中的吸附; 2）农药在土壤中的分解; 3）农药在土壤中的分散迁移; 4）农药在土壤中的对流迁移。所采用的计算方法是将二维非饱和渗流方程与溶质弥散运移模型相结合。结果表明：1）农药在土壤中的吸附符合亨利定律。土壤吸附亨利常数与土壤有机质和粘粒含量呈线性关系; 2）农药在河流中的扩散系数取决于水流的内流速; 3）模拟结果表明，农药在河流中的浓度随降雨历时、降雨强度和农药在田间的扩散量的增加而增加，随亨利常数的增加而减小。少

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