The Urban Water Cycle
Our activities and development on land alters how water naturally travels through the landscape or watershed. As we develop the land, we add roads, houses, parking lots, sidewalks, and driveways. These hard surfaces are called impervious surfaces because water can not pass through them as it does through soil so the water is forced to evaporate or run off.
When it rains in a natural, undisturbed environment about 50% of the rainwater infiltrates into the ground, 40% evaporates or is taken up by plants and only about 10% runs off the surface.
At 10 to 20% impervious (such as in rural towns), runoff is doubled, and the amount of water infiltrating is reduced.
At 30 to 50% impervious (such as suburban areas), runoff is tripled.
At 75 to 100% impervious (such as large cities and commercial areas), the majority of rainfall becomes runoff, and infiltration is less than 1/3 of what it was prior to development.