Rainfall immediately moves in one of three general directions:

• evaporates back into the atmosphere,
• infiltrates into the soil to become groundwater, or
• becomes "overland flow" (sheet flow) by moving as surface water runoff along the land surface (toward a river or stream)

Overland flow can be particularly intense when the ground is hard and impervious or is covered with an impervious surface like asphalt, cement, or buildings. Since water is unable to infiltrate into the soil in most developed areas, a large volume of water ends up in streams and threaten areas downstream after a storm event.

If the water move just below the surface of land it's called "interflow." Interflow will move in the same direction as overland flow but at a slower rate. If there is a heavy downpour lasting less than one hour there will be a large amount of overland flow. If there is a calm, soaking shower over 4 - 8 hours will produce a large amount of interflow (Cech, 2003).

It is possible to mathematically calculate overland flow using the Rational Formula

Q = KiA

Q = peak rate of runoff in cubic feet per second
K = runoff coefficient (parks & cemeteries = 0.1 - 0.25; downtown business district = 0.7 - 0.95)
i = intensity of rainfall in inches per hour
A = watershed area in acres

Runoff coefficient values can be found in hydrology related textbooks and publications.

To determine the area that contributes water to a particular stream contact your state department of geology (or your local soil-water conservation office) for a topographic map. Using a topographic map you should be able to determine the size of the entire watershed that contributes water to a particular river.

Study of moving water moving across the land surface falls under the discipline of "surface water hydrology."

What's the long-term fate of rain gardens?

• build-up of sediment (like a lake)
• accumulation of minerals and pollutants carried in overland flow

Reference

Cech, T.V. 2003. Principles of Water Resources: History, Development, Management, and Policy. John Wiley & Sons, Inc.

"Runoff" is the flow of water across the land surface after a storm event.