Glossary of Water Cycle Terms | U.S. Geological Survey

A

Agricultural water use

The use of water to cultivate, irrigate and protect crops. Common types of irrigation include flood, sprinkler, and drip irrigation. Agricultural water use is a water cycle flux.

Atmospheric moisture (over land)

Water that is present as water vapor (water in gaseous form) in the atmosphere above land masses. Water vapor in the atmosphere is invisible to the human eye. Atmospheric moisture (over land) is a water cycle pool.

Atmospheric moisture (over ocean)

Water that is present as water vapor (water in gaseous form) in the atmosphere above oceans. Water vapor in the atmosphere is invisible to the human eye. Atmospheric moisture (over ocean) is a water cycle pool.

B

Biological water

Water stored within living things like plants, insects, and animals (including humans). Biological water is a water cycle pool.

C

Condensation

The process that changes gaseous water (water vapor) to liquid water. Condensation is a water phase change.

E

Evaporation

The process that changes liquid water to a gas (water vapor). Evaporation is driven by heat. Evaporation is a water cycle flux.

Evapotranspiration

The sum of all processes by which liquid water is converted to water vapor and is released to the atmosphere. This includes evaporation from water bodies and the land surface as well as transpiration by plants. Evapotranspiration is a water cycle flux.

F

Flux

A flux is a process that moves water between pools, like evaporation, precipitation, discharge, recharge, or human use.

Forestry water use

Water that is used by the forestry industry. In forestry, water is used to irrigate seedlings and grow trees. Water is also used to put out wildfires that burn forests. Water is also used indirectly as trees obtain water from soil moisture, use water during photosynthesis, and store water in their tissues. Forestry activities such as logging, constructing roads, and transporting felled trees can destabilize hillslopes, increasing erosion and sedimentation transport to streams. Forestry water use is a water cycle flux.

G

Grazing water use

Water provided as drinking water for livestock on grazing lands, in feed lots, and in commercial operations like dairies. In these facilities, water is also used for cooling and sanitation. Livestock also indirectly consume water when grazing on vegetation. Grazing water use is a water cycle flux.

Groundwater

Liquid water stored underground, within cracks in rocks of all kinds and in the pore spaces of sediments and sedimentary rocks. The term “aquifer” refers to rocks that can hold substantial amounts of water. Groundwater can be fresh, saline, or brackish. Groundwater is a water cycle pool.

Groundwater discharge to ocean

The flow of fresh or saline water from groundwater aquifers to the ocean in coastal areas. Groundwater discharge to ocean is a water cycle flux.

Groundwater recharge

The process by which water is added to groundwater aquifers. Groundwater recharge occurs when water stored on or moving across the land surface infiltrates into the soil and then percolates through the soil to the saturated zone. Groundwater recharge is a water cycle flux.

I

Ice sheets and glaciers

At high elevations or latitudes where snowpack persists year-round, the weight of the accumulated snow compresses it into ice. When the ice forms in a steep mountain environment, it slowly flows downhill, forming a glacier. Ice sheets are glaciers that have spread outward to cover an area larger than 50,000 square kilometers. Ice sheets and glaciers are a water cycle pool.

Industrial water use

Water that is used in industrial processes, like power generation and material production. Thermoelectric power generation is one of the largest uses of water in the United States. Water taken directly from rivers and streams is used for cooling in industrial processes or may be used to dilute byproducts. Industrial water use can impact water quality through changes in water temperature and chemical contamination. Industrial water use is a water cycle flux.

L

Lakes (fresh)

A lake is formed when water accumulates in a low spot on the landscape as a result of surface-water runoff and groundwater seepage. If the water in the lake is fresh, it is a freshwater lake. Lakes (fresh) are a water cycle pool.

Lakes (saline)

A lake that contains saline water due to the accumulation of salts as water is lost via evaporation. Most saline lakes exist in closed basins and do not have any outlet, meaning they do not release outflow into a river. Lakes (saline) are a water cycle pool.

M

Municipal and domestic water use

Water used by humans in cities, towns, and homes. This includes water we drink and use to prepare food and the water we use to bathe and clean. This includes water used to water lawns and gardens or fill pools. In our communities, we also use water in public parks, to fight fires, and to transport and treat sewage. Municipal and domestic water use is a water cycle flux.

O

Ocean (deep water zone)

The lower portion that is far from the surface of the ocean. It is below the mixed zone. Layers of water form in the deep ocean due to differences in water temperature and salinity (and therefore water density). Ocean (deep water zone) is a water cycle pool.

Ocean (mixed zone)

The upper portion of the ocean that is mixed by wind or because of differences in density that result from cooling/warming, evaporation, or ice formation. This mixing results in near uniform temperature and salinity within this zone. Ocean (mixed zone) is a water cycle pool.

Ocean circulation

The movement and mixing of water horizontally between ocean basins and vertically between mixed and deep zones. Ocean circulation is a water cycle flux.

Ocean evaporation

The conversion of ocean water to water vapor at the interface between the ocean surface and the atmosphere. Ocean evaporation is a water cycle flux.

P

Permafrost

A subsurface layer of soil that contains water and is frozen year-round. Permafrost primarily occurs near the poles of the Earth. Permafrost is a water cycle pool.

Pool

A pool is place where water is stored, like the ocean, ice sheets, groundwater, rivers, lakes, and snowpack.

Precipitation over land

Water released from clouds in the form of rain, freezing rain, sleet, snow, or hail that falls over the land surface. Precipitation over land is a water cycle flux.

Precipitation over ocean

Water released from clouds in the form of rain, freezing rain, sleet, snow, or hail that falls over the ocean. Precipitation over ocean is a water cycle flux.

R

Reservoirs

A reservoir is a human-made lake. Reservoirs form when a dam is built on a river, allowing water to pool behind the dam. Reservoirs are a water cycle pool.

Rivers

A natural body of water that flows from higher to lower elevations as a result of gravity. Also called streams, or creeks, depending on their size, rivers typically flow into lakes or to the ocean but sometimes end due to evaporation, loss of streamflow to groundwater, or due to human intervention. Rivers are a water cycle pool.

S

Snowmelt

Water formed by melting snow. Snowmelt can move across the landscape as runoff or infiltrate into the soil. In the spring, melting seasonal snowpack moves snowmelt into rivers, streams, lakes, and groundwater. Snowmelt is a water cycle flux.

Snowpack

Snow that accumulates and melts seasonally at high elevations or near the poles. Since snowpack accumulates and melts at different times across the globe, scientists quantify the amount of snowpack by estimating the annual maximum snowpack at the peak of winter - February. Snowpack is a water cycle pool.

Soil moisture

Water that is present within soil. The amount of water stored within the soil varies based on precipitation, temperature, and soil type. Soil moisture is a water cycle pool.

Streamflow to closed basins

Closed basins are basins where water flows into, but not out of the basin. Streamflow to closed basins does not flow to the ocean. Instead, that streamflow flows into lakes, into wetlands, is lost to groundwater, or evaporates. The water continues to move through the water cycle by evaporation or via groundwater recharge. Lakes in closed basins may be saline due to the accumulation of transported salts when water evaporates. Streamflow to closed basins is a water cycle flux.

Streamflow to ocean

The movement of fresh water from rivers into oceans. Streamflow to oceans is a water cycle flux.

Sublimation

The process by which snow and ice transform into gaseous water (water vapor) without first melting into water. Sublimation is a water cycle flux.

T

Transpiration

The release of gaseous water (water vapor) from plant leaves. Transpiration is a water cycle flux.

Transport of moisture from ocean to land

When air over the ocean moves over the land, stored water vapor in the air becomes atmospheric moisture over land. Transport of moisture from ocean to land is a water cycle flux.

W

Wetlands (fresh and brackish)

Wetlands are areas where the soil is saturated or flooded year-round or during the growing season. Wetlands support the growth of aquatic (water) and terrestrial (land) plants that are adapted to grow in soils with low oxygen. They include mangroves, marshes (salt, brackish, intermediate, and fresh), swamps, forested wetlands, bogs, wet prairies, prairie potholes, and vernal pools. Wetlands (fresh and brackish) are a water cycle pool.