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Glossary

Water terms

Aquatic ecosystems. Any watery environment, such as rivers, lakes, wetlands, and estuaries, in which plants and animals interact with the chemical and physical features of the environment.

Aquifer. An underground stratum of rock or sediment that contains water and transmits water readily.

Catchment area. An area that 'catches' rainfall or snow to supply a river, aquifer, or lake.

Desiccation. Drying up via evaporation, drainage, or outflow of water. This may result from reduced inflow or runoff or increased outflow or abstraction.

Distributaries. Water courses that distribute the flow of flood waters through a delta.

Groundwater. Any water stored underground in aquifers, sometimes supplying springs and wells.

Hydrological cycle (water cycle). The cycle by which water evaporates from soils, vegetation, oceans and other bodies of water; accumulates as water vapor in clouds; returns to the Earth, oceans and other bodies of water as rain and snow; and runs off as river flow, through the soil or an aquifer.

Nonrenewable water. Water in aquifers and other natural reservoirs that is not recharged, or is recharged so slowly that significant withdrawals will cause depletion.

Renewable water. Water continuously renewed within reasonable time spans by the hydrological cycle (see above), such as that in streams, reservoirs, or aquifers that refill from precipitation, runoff, or groundwater recharge.

Runoff. Water originating as rain or snow that runs off the land in streams, eventually reaching oceans, inland seas, or aquifers unless it evaporates first.

Salinization. The appearance of a salt layer on or within the soil due to exposure to salt water, or resulting from the evaporation of soil water that is high in salts.

Salt water intrusion. Movement of saline water into rivers, freshwater lakes, estuaries or coastal freshwater aquifers. This may occur when river flows are low in coastal areas or when there is excessive groundwater pumping in coastal or arid areas.

Water abstraction. Removal of water from a river, lake, or aquifer. This may be associated with construction of dams or barrages (earthen dams).

Water logging. When water is applied to a field that is not adequately drained, it builds up in the root zone, creating conditions unsuitable for plant growth.

Water resources. A general term encompassing the concepts of availability (the location, spatial distribution, or natural fluctuations of water); accessibility (given availability, whether people can access it or afford water in adequate quantities); and quality (whether accessed water is free of contaminants and safe for consumption).

Water scarcity. Condition in which the annual availability of renewable fresh water is 1,000 m3 or less per person.

Water withdrawal. Removal of water from any natural source or reservoir such as a lake, stream, or aquifer for human use. If not consumed, the water may later be returned to the same or another natural reservoir.

Watershed. The boundary of a lake, river, or aquiferís catchment (see above). Sometimes used to refer to the upstream part of a catchment, particularly where hilly or mountainous.

Wetlands. Areas of marsh, fen, peat land, or water that include permanent or temporary areas with static or flowing water that is fresh, brackish, or marine. The areas may be naturally occurring or artificial.

Population terms

Population density. Population per unit of land area; for example, persons per square mile or persons per square kilometer of arable land.

Population momentum. The tendency for population growth to continue beyond the time that replacement fertility has been achieved because of a relatively high concentration of people in the childbearing years.

Population policy. Explicit or implicit measures instituted by a government to influence population size, growth, distribution, or composition.

Population stabilization. The attainment of zero growth, in which the number of births in a population equals the number of deaths. This occurs when fertility attains ìreplacement levelî over a period of time (see below).

Replacement level fertility. The level of fertility at which women, on average, are having only enough children to replace themselves and their partners in the population. In industrialized countries a total fertility rate (see below) of 2.1 is considered to be replacement level.

Total fertility rate (TFR). The average number of children who would be born alive to a woman (or group of women) during her lifetime if she were to pass through her childbearing years conforming to the age-specific fertility rates of a given year. It can thought of as the average number of lifetime births per woman.

Urban population. The population living in urban areas. Countries differ in the way they classify population as ìurbanî or ìrural.î Typically, population living in a community or settlement with a population of 2,000 or more is considered urban.

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