Discrete modeling of water and pesticide movement in soil

Simulations of water and pesticide movement are presented for the unsaturated zone. Water movement is studied with a two-dimensional Monte Carlo technique, where particles hop between the sites of a square lattice according to probabilistic rules. We developed this lattice gas method for two-dimensional simulations of water flow in inhomogeneous media. Generalization to three dimensions is straightforward. The dynamics of the pesticide is simulated by particle tracking. After presenting the underlying discrete algorithms we emphasize calibration of the system. A soil is divided into homogeneous horizons, and our model is compared with the conventional one-dimensional model PESTLA. We observe good agreement for both water content profile and pesticide distribution. The random elements in our algorithms cause moderate fluctuations in the results. As an example of two-dimensional simulation, spatiotemporal pesticide patterns under a ridge are compared with the distribution under a flat soil surface.