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[Images] Five photos of different landscape

Compass Fall 2005
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Compass is a quarterly publication of the USDA Forest Service's Southern Research Station (SRS). As part of the Nation's largest forestry research organization -- USDA Forest Service Research and Development -- SRS serves 13 Southern States and beyond. The Station's 130 scienists work in more than 20 units located across the region at Federal laboratories, universites, and experimental forests.



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Issue 5

The Hydrologic Cycle

Photo of how the Hydrologic Cycle works
Hydrologic cycle: the constant movement of water rising to the atmosphere as water vapor, cooling to condense and precipitate onto the Earth, then evaporating or transpiring back into the atmosphere.

The following seven processes occur simultaneously, and with the exception of precipitation, continuously.
Condensation: water vapor that turns into liquid water as the result of cooling.
Precipitation rain, sleet, or snow that results when water vapor becomes too heavy to remain in air currents.
Interception: precipitation caught on leaves or other vegetative surfaces.
Infiltration: rainfall that seeps into the ground.
Surface runoff: precipitation that reaches the surface of the Earth but does not infiltrate.
Subsurface flow: infiltrated water that moves through subsurface pathways into a stream or river.
Evaporation: water converting from a liquid or solid state to a gaseous state from the plant surfaces, soils, and bodies of water.
Transpiration: process where plants move water from the soil to their aboveground parts, then lose it to the atmosphere.
Evapotranspiration: the combined processes of water evaporating from the ground and transpiring from plants—the total water vapor added back to the atmosphere.
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