Recall that most of the precipitation that falls on land either enters the soil(infiltration) or remains at the surface, moving downslope as runoff The amount of water that runs off rather than soaking into the ground depends on several factors:()intensity and duration of rainfall, (2) amount of water already in the soil, (3) nature of the surface material, (4) slope of the land, and(S) the extent and type of vegetation, When the surface material is highly impermeable, or when it becomes saturated, runoff is the dominant process Runoff is also high in urban areas because large areas are covered by impermeable buildings, roads, and parking lots