Stormwater runoff describes the water that flows overland during a rain storm. Overland flow is a part of the water cycle. The water cycle describes the path that water takes to create a rain storm.

As a rain storm occurs, the water that is produced from the clouds falls toward the ground. The water that reaches the lower atmosphere is either absorbed by trees and plants, infiltrates into the ground or flows overland. The portion of the water that flows overland is stormwater runoff.

On a property that is in its natural state of meadow or forest the water moves slowly across the land and most of the water is infiltrated into the ground or absorbed by the tree foliage.

After a property is developed with houses or businesses, many of these meadows and forests are replaced with buildings and roadways. The water is no longer able to flow slowly across the land, infiltrating into the ground and being absorbed by the trees. Therefore, more water is moving across the surface of the land at a faster speed.

Stormwater management through the use of best management practices (BMPs) is the method used to slow the water to the conditions prior to development.

After the water from the rain storm reaches the ground the overland flow flows downhill until it reaches the stream system and eventually the Chesapeake Bay. Under the right atmospheric conditions, the water is then evaporated. The water cycle begins again with another rain storm.