Water Cycle Glossary

Condensation– the process by which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds.

Evaporation– the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. Studies have shown that the oceans, seas, lakes, and rivers provide nearly 90 percent of the moisture in the atmosphere via evaporation, with the remaining 10 percent being contributed by plant transpiration.

Evapotranspiration– the sum of evaporation from the land surface plus transpiration from plants.

Freshwater storage– freshwater existing on the Earth’s surface, which includes streams, ponds, lakes, reservoirs, and freshwater wetlands. The definition of freshwater is water containing less than 1,000 milligrams per liter of dissolved solids, most often salt.

Groundwater discharge– groundwater is discharged when it emerges back to the surface of the Earth and flows into stream beds.

Groundwater storage– water existing for long periods below the Earth’s surface. The water is still moving, possibly very slowly, and it is still part of the water cycle. Most of the water in the ground comes from precipitation that infiltrates downward from the land surface.

Infiltration– the downward movement of water from the land surface into soil or porous rock.

Precipitation– water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.

Saturated water content– the water content when water fills every available pore space in a soil. When soils are saturated, new rain has nowhere to go and flooding is likely.

Snowmelt runoff to streams– the movement of water as surface runoff from snow and ice to surface water. It is a major component of the global movement of water.

Soil water content-  soil is made of many small mineral and organic particles, with pore space between them that can be filled with air or water. Soil water content is the percentage of a soil volume that is water. In other words, if soil water content is 25%, that means that a 1 cubic meter of soil contains 0.25 cubic meters of water.

Solar radiation – A measurement of global solar radiation—the combination of direct and diffuse solar radiation—in the 400 to 1100 nm range. Measurement units are in watts per square meter (W m-2)

Springs– a water resource formed when the side of a hill, a valley bottom or other excavation intersects a flowing body of groundwater at or below the local water table. A spring is the result of an aquifer being filled to the point that the water overflows onto the land surface. They range in size from intermittent seeps, which flow only after much rain, to huge pools with a flow of hundreds of millions of gallons per day.

Streamflow– the movement and amount of water flowing in a natural channel, such as a river.

Sublimation– the process by which water changes from a solid (ice or snow) to a gas, bypassing the liquid phase. When dry air hits snow, it changes the snow directly into water vapor, bypassing the liquid phase. Sublimation is a common way for snow to disappear quickly in arid climates.

Surface runoff– the water in rivers which comes directly from runoff from the land surface; this accounts for most of the water in rivers (other inputs are direct precipitation and ground water).

Transpiration– is essentially evaporation of water from plant leaves. Studies have revealed that transpiration accounts for about 10 percent of the moisture in the atmosphere, with oceans, seas, and other bodies of water (lakes, rivers, streams) providing nearly 90 percent.

Water in the atmosphere– water stored in the atmosphere as vapor, such as clouds and humidity. Only about 0.001 percent of the total Earth’s water volume is stored in the atmosphere at any given time.

Water storage in ice and snow– freshwater stored in frozen form, generally in glaciers, icecaps and fields, and snowfields. Water may be locked up in this state for a relatively long period of time.

Water storage in oceans– saline water existing in oceans and inland seas. Much more water is “in storage” for long periods of time than is actually moving through the cycle; 96.5 % of the world’s water supply is stored in the oceans. It is also estimated that the oceans supply about 90 percent of the evaporated water that goes into the water cycle.

Water vapor– Vapor Pressure Deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. Once air becomes saturated water will condense out to form clouds, dew or films of water over leaves or surfaces.

Most definitions from the USGS Water Science School