ABSTRACT
Precipitation is any form of solid or liquid water that falls from the atmosphere to the earth’s surface. Rain, hail, sleet, and snow are examples. Precipitation is the single strongest variable driving hydrologic processes and events, but it must be considered in the context of other variables. For example, areas with more than 20 in. of annual precipitation are often considered humid, while those areas with less are considered arid. However, 20 in. of precipitation in South Dakota is usually adequate to grow corn, soybeans, and wheat, while in Texas it would be quite inadequate. The difference lies in differences of evapotranspiration (ET) losses between the two regions (see Chapter 3). Likewise, a 50-year return period storm on dry soil might produce a 10-year runoff event because much of the water would be absorbed, while a 10-year storm falling on an already-saturated soil might produce a 50-year runoff event.
