Rain Garden Project
The Water Cycle
To understand the effects that urbanization has on water runoff, consider the water cycle, represented graphically in the figure below.
Water that is in lakes and oceans evaporates into the air. The water vapor forms into clouds and air currents move the cloud over land. Rain falls on the land; some of it infiltrates into the soil into underground aquifers, which feed wells and underground streams. Some of the water that infiltrates the ground is absorbed by the root systems of trees and plants, where it is transported into the leaves and evaporated back to the air by the process of transpiration. The rest of the rain runs downhill across the surface of the land, collecting into creeks, streams, and rivers. Finally, the rivers and streams return to a lake or ocean and the cycle begins again.
As portions of the ground is covered with impervious surfaces, such as asphalt, concrete, or buildings, less of that storm water is able to infiltrate into the ground and more of the rain becomes runoff. This is what happens in urbanized areas.
On undeveloped land with natural ground cover, trees grasses and other plants, roughly 10% of the storm water becomes surface runoff and 50% of the rainfall infiltrates back into the ground (the remaining 40% evaporates or is transpired by the plants). On fully urbanized land, with 75-100% of the ground surface transformed into impervious surface (such as a city made of buildings, sidewalks and parking lots), approximately 55% of the storm water turns into surface runoff and only 15% is infiltrated into the ground.
The addition of asphalt and other impervious surfaces in urbanized areas inverts the amount of storm water between the categories of runoff and infiltration, and this significantly reduces the amount of water that is allowed to return to underground sources and significantly increases the amount of water that is passed to the downstream watershed. If storm water runoff is directed into the sewage water system (as is the case in for the city of Evansville), this also means the water treatment plants must be designed for a higher capacity before releasing the runoff into waterways. Increasing the volume flow of a river can significantly change the downstream flow patterns such as river deltas or even river beds. Directing the runoff from impervious surfaces into a rain garden allows the rain water to infiltrate back into the ground, imitating the natural ground cover.