The mathematical structure of the model consists of the coupling of a model for the transport through soils to a model for transport through plants. The coupled model describes uptake of water and solutes by plants from the soil solution. The rate of uptake is a function of the environmental conditions that determine the transpiration rate. Transport of water, solutes, and heat through the soil is modeled by a one-dimensional approach. The soil is divided into a series of depth increments. Initial root distribution is specified. Water and solutes are taken from each soil layer as determined by water potential. Water transport in the plant is based on water potential and pressure gradients according to the Munch pressure flow hypothesis. Gradients are determined by water availability in the soil and by atmospheric conditions. The model was developed for the specific purpose of making assessments of risks involved in the use of xenobiotic chemicals. It is driven by environmental conditions, specifically atmospheric conditions that determine transpiration rate.