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

分析流域水流动态对预测土壤中农药浓度具有十分重要的意义。因此，本研究分为两部分。第一个问题是如何应用众所周知的非饱和流动方程来分析实际集水区的水运动。二是分析农药在土壤中的扩散和迁移过程。第一部分由藤田（M.Fujita）和松井（y.m atsui）分别进行了研究，认为非饱和流理论最能描述土壤中的物理径流现象。然而，将这一理论应用于实际盆地存在一定的困难。其中之一是如何在土层底部设置渗进深层的边界条件。也就是说，进入深层土层的入渗取决于这些边界条件。藤田先生通过对一个试验流域的径流分析提出了新的边界条件。通过室内土样试验，得到了求解非饱和流动方程的其他方程，如rheta（含水量）- p（渗透性）和更多的hi（吸力）关系。除非在特殊的实验盆地，否则不可能得到rhea -phi关系和非饱和渗透率。这一事实阻碍了非饱和流动方程在实际径流分析中的应用。Fujita指出，基于槽模型和熵法计算的深土入渗与非饱和流动方程的结果相吻合。池模型和熵法是集总参数径流模型中的一种。因此，这些径流模型很容易应用于实际的径流分析。结合两种不同的径流模型来估计一个流域的土壤特性是可能的。农田中使用的农药在降雨时通过土壤进入河水。以下因素对预测饮用水水质起着重要作用。1)什么样的农药容易到达河流？2)哪种土壤容易携带农药？3)什么样的降雨条件促使农药运输？农药在土壤中的迁移现象取决于几个因素。1)农药对土壤的吸收2)农药的分解3)农药在土壤中的分散输送4)农药在土壤中的对流输送本研究的重点是1)、3)和4)。采用二维非饱和流动方程与溶质色散输运模型相结合的计算方法。结果总结如下：亨利定律很好地解释了土壤对农药的吸收。亨利吸收常数是土壤有机和粘土含量的线性函数。2)农药运移的分散系数取决于水的内部流速。3)模拟结果表明，河流中农药浓度随降雨持续时间和降雨强度的增加而增加，农药在田间的扩散量随Henry常数而减小。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