Precipitation

Precipitation plays a big role in runoff. Runoff is the total amount of water flowing into a stream. This consists of direct runoff and groundwater recharge (or baseflow). The intensity and duration of a rain or snow storm along with the properties of the subsurface materials determine the infiltration rates and the distribution of surface runoff ¹¹.

When precipitation falls to the Earth, some part of it may be intercepted by trees, vegetation, and buildings. During brief or light storms, this intercepted water usually evaporates rapidly and is known as interception loss. (Scientists have not really been able to measure the amount of water lost to interception, but the actual volume is small ⁹.)

During longer and heavier storms, water does reach the ground and can follow many different paths as the tree diagram shows.

Water can pool on the surface and evaporate, infiltrate the ground, or become surface runoff. If the water infiltrates the ground it can become interflow, be taken up by plant roots, or recharge the groundwater and become baseflow. Interflow is water moving laterally or horizontally in the zone of aeration during and immediately after storms. This water discharges directly into a stream or lake. ^2,9,10

Vegetation that is actively growing during the summer absorbs the water as it infiltrates the ground, so not much water reaches the zone of saturation. However, when vegetation is growing at a slower rate, more water will recharge the groundwater. The major periods for recharge of the aquifer occur in the spring and fall when precipitation is usually greater and plant growth is slower ^2,12.

Precipitation that falls at a fast rate may not be absorbed by the soil, so it becomes surface runoff. Surface runoff can take many forms such as overland flow, or saturation excess overland flow ^2,9.

(Italicized words defined in the glossary.)

To see a diagram with the different types of runoff and flow, click here or the tree diagram.