Trees and other vegetation in urban areas considerably reduce urban-water runoff. As rain falls upon vegetation, it clings to the rough surfaces of leaves, branches, and trunks. This is called interception. Some of the water evaporates back to the atmosphere. The rest either drips down to the ground (called throughfall) or runs down along branches and trunks to the ground (called stemflow). By intercepting and slowing precipitation hitting the ground, vegetation substantially reduces the volume and rate of stormwater runoff. This helps protect soil from erosion and reduces flash flooding.
In cities, trees do their best work at preventing runoff in two places. The first is tree canopy over impervious surfaces such as streets, sidewalks, and parking lots. Lots of water runs off of pavement, but trees planted next to impervious surfaces can catch much of this water and slow it before it can erode away sediment and be absorbed in the ground. The other important place is near creeks, streams, and rivers. Trees and vegetation in these places are called riparian buffers because they buffer the waterways from excessive runoff, erosion, and pollution.