Yang and Nowak demonstrate how vegetation can slow runoff and promote infiltration under a variety of hydrologic conditions.
Yang et al. created a model based on Runoff theory, which asserts that when impervious surface is present water does not infiltrate into the ground but runs off. Urbanization in general reduces the amount of infiltration through soil compaction and reduced roughness of surface. This leads to more water running off the surface which results in water moving at higher velocities (speeds) and higher runoff peaks. This can lead to flooding, which causes damage and risk human life.