Rainfall and Runoff

Water that falls to the earth's surface is precipitation, and includes rain, snow, sleet, or hail. Some precipitation seeps into the ground; some falls directly into oceans, lakes, or rivers.

Water that stays on the surface is called runoff. It moves from higher to lower elevations. As it moves, it picks up and carries material, like soil or pollutants, along with it. It eventually flows into waterways that drain into the ocean.

Runoff amounts vary depending on features of the precipitation. The amount, intensity, type and distribution of the precipitation all affect runoff. Physical characteristics of the land affect runoff, too. In investigating runoff, scientists look at land use, vegetation, soil type, elevation and slope.