Tree-root drying can reduce the water content of a soil in a significant manner and cause foundation settlement in expansive soil. Distortions of lightly loaded structures caused by tree root absorption are often observed in areas of expansive soil. Interest in numerical modeling of the influence of trees on soil shrinkage settlements has intensified recently. Nevertheless, no prescriptive method is given to describe the parameters and conditions in a tree-root-soil-water interaction model. In this paper, a general description of the factors involved in a tree root-absorption model is discussed. The estimation of transpiration, rainfall, evaporation and root distribution is provided. Flow equation and constitutive equation are also explored to describe the soil water interactions.