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Competency Area 2: Soil hydrology AEM

PO 8. Know the components of the hydrology cycle.

The hydrologic cycle refers to the fate of water in and on planet Earth, from the time precipitation falls on the Earth's surface until the water is returned to Earth's atmosphere. The general principle is simple, and the driving force behind it comes primarily from the Sun's solar energy.

The components and processes of the hydrologic cycle include:

Precipitation – the condensed liquid or crystalline water falling from the atmosphere, in the form of rain, hail, sleet, or snow.

Interception – the process where precipitation is caught and temporarily held in a vegetative canopy before it reaches the land surface. Some of this precipitation may be evaporated directly or evaporated directly or adsorbed by the plant, or it occurs as throughfall (i.e., water dripping off the leaves) or as stemflow (i.e., the portion that flows down the stem to the ground).

Evaporation - the process where water passes directly from its liquid or solid state to a vapor state. Evaporation can occur from vegetation, soil, or water (and ice) surfaces.

Transpiration – the process where water is extracted from soil by plants, passing up through the plant to the plant leaves and then discharged to the atmosphere through the stomata.

Evapotranspiration – the combined processes of evaporation and transpiration.

Condensation – the process where water passes from its vapor state to a liquid or solid state, the opposite of evaporation.

Runoff - the portion of precipitation on a land area that is discharged from the area enters through streams. The portion lost without entering the soil is called surface runoff, and the portion which the soil before reaching a stream is called groundwater runoff or seepage flow from groundwater.  In Soil Science terminology "runoff" usually refers to any water lost from an area by surface flow; whereas in Geology and Hydrology, "runoff" usually includes both surface and subsurface flow that eventually reaches a stream.

Infiltration – the downward entry of water through the soil surface and into the upper soil layers.

Illustration by Tom Schultz
Courtesy of Iowa State University Department of Natural Resource Ecology and Management