ESS terms in Water

The removal of water from a river, lake, reservoir or aquifer by humans, for drinking water, irrigation, industry or energy.

The attraction of water molecules to the molecules of other substances, such as the walls of plant xylem or soil particles.

The horizontal transfer of water vapour, or of the droplets and ice crystals in clouds, by the wind from one place to another.

Water flowing over or through farmland into streams and lakes, carrying fertiliser nutrients, manure, pesticides and soil particles with it.

A rapid increase in the population of phytoplankton, such as algae or cyanobacteria, in a water body, usually after an increase in a limiting nutrient, that discolours the water.

The complete absence of dissolved oxygen in water or sediment, in which only anaerobic microorganisms can survive.

A chemical coating, often containing copper or other biocides, applied to nets, cages and boat hulls to stop algae, barnacles and other organisms growing on them.

The farming of aquatic organisms, including fish, molluscs, crustaceans and aquatic plants, with intervention in the rearing process such as stocking, feeding and protection from predators.

A food production system that combines raising fish with growing plants in water, in which fish waste supplies nutrients to the plants and the plants clean the water returned to the fish.

A body of permeable rock or sediment that holds groundwater and lets it flow, so that water can be stored in it and abstracted from wells and boreholes.

The injection of surplus water, such as treated river water or wastewater, into an aquifer through wells for storage, and its pumping back out through the same wells when it is needed.

The deliberate addition of water to an aquifer at a rate faster than natural recharge, for example from infiltration basins or injection wells, to replenish groundwater.

The Atlantic part of the thermohaline circulation: warm, salty surface water flows north, cools and sinks in the North Atlantic, and returns south as cold deep water.

A low dam or barrier with gates built across a river mouth or estuary, used to keep out seawater, store freshwater behind it or control flooding.

The amount of dissolved oxygen consumed by microorganisms as they decompose the organic matter in a water sample, usually measured in mg per litre over five days at 20 °C.

A scale that indirectly measures the quality of an environment, such as a river, from the pollution tolerance, diversity and relative abundance of the organisms living there.

The use of explosives to stun or kill schools of fish so they can be collected, a destructive and widely illegal method that shatters coral reefs.

A fishing method that drags a large cone-shaped net, weighted to stay in contact with the seabed, to catch fish and shellfish living on or near the bottom.

A highly concentrated salt solution, such as the waste water discharged by desalination plants, which is much saltier and denser than seawater.

The unwanted animals caught unintentionally while fishing for a target species, such as juvenile fish, turtles, dolphins and seabirds, which are often discarded dead.

The catch obtained for a standard amount of fishing effort, such as kilograms per hour of trawling, used as an indirect index of the abundance of a fish stock.

A period during which fishing for a species is banned, usually timed to protect it while it spawns or while its young grow.

A weather modification technique in which particles such as silver iodide or salt are released into clouds to encourage water droplets or ice crystals to form and fall as precipitation.

The attraction of water molecules to each other through hydrogen bonds, which gives water its high surface tension and lets it be pulled up through plants in columns.

The transformation of water vapour into liquid water droplets when air cools, which releases latent heat and forms clouds, fog and dew.

The whitening of corals when heat or other stress causes them to expel the symbiotic algae living in their tissues, leaving them without their main food source and at risk of death.

The use of sodium cyanide sprayed onto coral reefs to stun fish for capture, mostly for the live reef-fish food and aquarium trades, which kills coral and non-target animals.

Photosynthetic bacteria, often called blue-green algae, that can form dense blooms in nutrient-rich water; some species produce toxins.

A toxin produced by cyanobacteria, such as microcystin, which damages the liver, that can poison people and animals that drink or swim in affected water.

A barrier built across a river to hold back water, creating a reservoir used for water supply, irrigation, flood control or hydroelectric power.

An area of a sea or lake where the water, usually near the bottom, is hypoxic or anoxic, so that it cannot support most aquatic animal life.

The removal of salt and other dissolved minerals from seawater or brackish water to produce freshwater, mainly by reverse osmosis or by distillation.

A single-celled protist with two flagella, common in marine plankton; some species form red tides and produce neurotoxins that accumulate in shellfish.

The area of land drained by a river and its tributaries, separated from neighbouring basins by high ground called a watershed.

A method of irrigation that delivers small amounts of water slowly through pipes and emitters directly to the roots of each plant, reducing losses by evaporation and run-off.

A prolonged period of below-average precipitation that leads to a shortage of water for ecosystems, agriculture or human supply.

A crop variety that can grow and produce a yield with less water than usual, through traits such as deep roots or reduced water loss, whether bred conventionally or genetically modified.

A label on a product showing that it meets an independent standard for environmental sustainability, such as seafood from a fishery certified as well managed.

A shortage of water caused by a lack of investment in storage, treatment and distribution, or by a lack of human capacity, even though enough freshwater exists naturally.

Liquid waste discharged into the environment from a pipe or outfall, such as treated or untreated sewage or industrial wastewater.

A chemical that interferes with the production or action of hormones in an organism, disrupting growth, development or reproduction, often at very low concentrations.

A lake in a basin with no outlet to the sea, which loses water only by evaporation and seepage, so dissolved salts build up in it.

The quantity, timing and quality of water flow that must be left in a river, wetland or estuary to sustain its ecosystems and the people who depend on them.

A farmed aquatic animal that has escaped from its cage or pond into the wild, where it may compete with, breed with or spread disease to wild populations.

The enrichment of lakes, estuaries and coastal waters with mineral nutrients, especially nitrates and phosphates, often causing excessive phytoplankton growth and oxygen depletion.

The transformation of liquid water into water vapour, using heat energy mainly from solar radiation; the main flow from the oceans to the atmosphere.

The combined flow of water vapour to the atmosphere by evaporation from soil and water surfaces and by transpiration from plants.

A zone of sea extending up to 200 nautical miles (370 km) from a country's coast, in which that country has sovereign rights over fishing and other natural resources under UNCLOS.

A measure of water availability based on renewable freshwater per person per year: below 1,700 m³ indicates water stress, below 1,000 m³ scarcity and below 500 m³ absolute scarcity.

A population of one fish species in a defined area that breeds largely within itself and is assessed and managed as a single unit.

An activity or industry that catches or farms a particular aquatic species in a particular area, together with the people, vessels and methods involved.

A dramatic and lasting decrease in a fish stock to a level at which it can no longer support a commercial fishery and cannot recover quickly, even when fishing stops.

The amount of fishing carried out over a period, measured for example as the number of vessels, days at sea, hours of trawling or hooks set.

A legal limit on the amount of a species, usually in tonnes, that a country, fleet or vessel may catch in a period, often set from a total allowable catch.

A sudden flood that rises within minutes to a few hours of intense rainfall, usually because surface run-off reaches rivers or drains faster than they can carry it.

Water with a low concentration of dissolved salts, usually below about 1 g per litre, found in rivers, lakes, groundwater, ice and the atmosphere, and usable for drinking and irrigation.

An area at the centre of an oceanic gyre where floating debris, mostly plastic fragments and lost fishing gear, accumulates at unusually high concentrations.

The continued catching and killing of marine animals by fishing gear, such as nets, lines and traps, that has been lost or abandoned at sea.

Lightly used wastewater from showers, baths, washbasins and washing machines, which can be filtered and reused for flushing toilets or watering gardens.

Water held underground in the pores and cracks of soil and rock below the water table, where the ground is fully saturated.

The slow transfer of water through saturated rock or sediment, which feeds springs, rivers, lakes and the sea.

A layer in a water body in which salinity changes rapidly with depth, separating fresher surface water from saltier, denser water below.

A bloom of algae, cyanobacteria or other phytoplankton that harms people, animals or ecosystems, especially by producing toxins or by clogging the gills of fish.

The parts of the ocean beyond any country's exclusive economic zone, open to all states and governed only by international agreements and regional fisheries bodies.

The UN agreement on marine biodiversity of areas beyond national jurisdiction, adopted in 2023 and in force from January 2026, which allows marine protected areas and impact assessments on the high seas.

A weak attraction between a slightly positive hydrogen atom of one molecule and a slightly negative atom, such as oxygen, of another molecule.

The continuous circulation of water between the oceans, atmosphere, land and living things, driven by solar radiation and gravity, through stores linked by transfers and transformations.

A condition in which dissolved oxygen in water is too low to support most animal life, commonly defined as 2 mg per litre or less.

A surface such as concrete, asphalt or a roof that water cannot pass through, so rain becomes surface run-off instead of infiltrating.

The development of male sex organs in female marine snails, such as dog whelks, caused by exposure to endocrine disruptors such as tributyltin.

A species whose presence, absence or abundance shows a particular environmental condition, such as a level of pollution, because it is sensitive to or tolerant of that condition.

The transfer of water from the ground surface into the soil.

A form of aquaculture that farms species from different trophic levels together, so that wastes from fed species such as fish become food or nutrients for shellfish, deposit feeders and seaweeds.

The temporary storage of precipitation on leaves, stems and other surfaces before it evaporates or drips and flows to the ground.

The international body set up under the 1946 International Convention for the Regulation of Whaling that sets catch limits and has kept a moratorium on commercial whaling since 1986.

The artificial supply of water to farmland from rivers, reservoirs or groundwater to grow crops where or when rainfall alone is not enough.

The seasonal mixing of a whole lake from top to bottom when surface and deep water reach the same temperature and density, usually in spring and autumn.

Energy absorbed or released when a substance changes state without a change in temperature, such as the heat taken in by evaporation and given out by condensation.

An aquatic plant large enough to be seen with the naked eye, growing emergent, submerged or floating in fresh water or shallow coastal water.

A clearly defined area of sea or coast managed by law or other effective means to conserve nature, with restrictions on activities such as fishing, which range from partial limits to full protection.

The size of the holes in a fishing net, which can be regulated so that young, small fish escape and reach breeding age.

A negatively charged ion of nitrogen and oxygen (NO₃⁻); a highly soluble plant nutrient that is easily washed from soil into water and contributes to eutrophication.

An area of sea or fresh water in which all fishing and other removal of living or non-living resources is banned.

Pollution that enters the environment from many dispersed sources over a wide area, such as run-off from farmland or roads, so that no single source can easily be identified.

The total amount of nutrients, mainly nitrates and phosphates, entering a body of water over a period from natural sources and human activities such as farming and sewage.

A large, roughly circular system of ocean surface currents driven by prevailing winds and the Earth’s rotation, which can concentrate floating debris at its centre.

The release of crude oil or refined fuel into the environment, especially the sea, from tankers, offshore rigs, pipelines or storage facilities.

The removal of fish from a stock faster than the stock can replace them by reproduction and growth, so that the population declines over time.

A graph of dissolved oxygen against distance downstream of an input of organic pollution, showing a fall to a minimum and a gradual recovery as the organic matter is decomposed.

The movement of water down through the soil and into the rock below, under the influence of gravity.

A negatively charged ion of phosphorus and oxygen (PO₄³⁻); a plant nutrient, often limiting in fresh water, that enters water in sewage, detergents, manure and eroded soil.

A shortage of water caused by the actual amount of freshwater available being too small to meet the demands of a population, typical of arid regions.

Microscopic photosynthetic organisms, such as diatoms and cyanobacteria, that drift in the surface waters of oceans, seas and fresh water and form the base of most aquatic food webs.

Organisms that live in water and drift with currents because they cannot swim strongly against them, including photosynthetic phytoplankton and consuming zooplankton.

The accumulation of plastic objects and particles in the environment in amounts that harm wildlife, habitats or people.

Pollution released into the environment from a single, clearly identifiable location, such as a discharge pipe, sewage outfall or chimney, which makes it easier to monitor and regulate.

An uneven distribution of electric charge across a molecule; in water the oxygen atom is slightly negative and the hydrogen atoms slightly positive.

A substance or form of energy, such as heat, that harms living things or the environment when it is added faster than it can be dispersed, broken down or stored.

The addition of a substance or form of energy to the environment at a rate faster than it can be made harmless by dispersal, breakdown, recycling or storage, causing harm to living things.

A model matching management strategies to three stages of pollution: changing the human activity that produces a pollutant, controlling its release, and cleaning up and restoring ecosystems.

The transfer of water from the atmosphere to the Earth's surface as rain, snow, sleet, hail or drizzle.

The first, physical stage of sewage treatment, in which screens and settling tanks remove large objects, grit and settleable solids.

The collection and storage of rain that falls on roofs or other surfaces, so that it can be used for gardens, toilets, livestock, crops or, after treatment, drinking.

The addition of water to an aquifer or other store, mainly by percolation of rain and snowmelt; the natural input that limits sustainable abstraction.

A coastal bloom of phytoplankton, usually dinoflagellates, dense enough to discolour the water red or brown; many red tides produce toxins harmful to marine life and people.

A natural or artificial lake used to store water, usually behind a dam, so that it can be supplied to people, farms and industry when needed.

A desalination process in which seawater or brackish water is pushed at high pressure through a semi-permeable membrane that lets water molecules through but holds back dissolved salts.

Drinking water from an improved source that is located on the premises, available when needed and free from faecal and priority chemical contamination; the standard used to monitor SDG 6.

The movement of seawater into a coastal freshwater aquifer, usually because groundwater is pumped out faster than it is recharged, making the water too salty to use.

The provision of facilities and services for the safe collection, removal, treatment and disposal of human urine and faeces, which protects health and the environment.

Any aquatic animal or plant from the sea or from fresh water that people eat, including fish, molluscs, crustaceans and seaweeds.

A flat disc with black and white quarters lowered into water on a marked line; the depth at which it disappears from view measures water clarity.

The biological stage of sewage treatment, in which aerobic microorganisms break down dissolved and suspended organic matter, greatly reducing biochemical oxygen demand.

Wastewater from homes and other buildings, containing human waste, food residues, detergents and other dissolved and suspended material, carried away by drains and sewers.

The process of removing solids, organic matter, nutrients and pathogens from wastewater in primary, secondary and tertiary stages so that the effluent can be released or reused safely.

A method of producing freshwater in which the sun's energy evaporates salty or dirty water and the vapour is condensed on a cool surface, leaving salts and impurities behind.

A liquid in which other substances dissolve; water dissolves more substances than any other common liquid because its molecules are polar.

The amount of energy needed to raise the temperature of 1 kg of a substance by 1 °C; for water it is about 4,180 J kg⁻¹ °C⁻¹, which is high.

The movement of adult fish, and of eggs and larvae, from a protected area into neighbouring fished waters, which raises catches outside the protected area.

The scientific estimation of the size, age structure and productivity of a fish stock, and of how fishing affects it, used to advise on safe catch limits.

A graph of a river's discharge over time after a period of rainfall; the delay between peak rainfall and peak discharge is the lag time.

The formation of distinct layers in a water body, usually warm, less dense water above cold, denser water, which restricts mixing between them.

The transfer of water along a river or stream channel, downhill towards a lake or the sea.

The transformation of ice or snow directly into water vapour, without first melting into liquid water.

The upper layer of a lake or ocean in which wind and waves keep the water well mixed, so that its temperature, oxygen and salinity are nearly uniform.

The transfer of water over the land surface when rain or meltwater cannot infiltrate the ground fast enough, or the ground is already saturated or impermeable.

Fresh water held on the land surface in lakes, rivers, reservoirs, swamps and other wetlands.

The advanced stage of sewage treatment, using chemical and biological processes to remove nutrients such as nitrate and phosphate and to disinfect the effluent.

A change in the temperature of a water body, usually an increase caused by the discharge of cooling water, that lowers dissolved oxygen and harms aquatic organisms.

The layer in a stratified water body in which temperature falls rapidly with depth, separating the warm surface mixed layer from the cold deep water.

The global ocean circulation driven by differences in water density, which are caused by differences in temperature and salinity; it carries heat around the planet.

The dry mass of particles in a known volume of water that can be trapped by a filter, usually given in milligrams per litre.

A conflict between countries or regions over the use, allocation or control of a river, lake or aquifer that crosses the boundaries between them.

The loss of water vapour from plants, mainly through the stomata of leaves, after water taken up by the roots has moved through the plant.

A biotic index for rivers, published in 1964, that scores water quality from 10 (clean) to 0 (grossly polluted) using the most sensitive invertebrate group present and the number of groups.

A toxic tin compound formerly used in anti-fouling ship paints; an endocrine disruptor that causes imposex in marine snails, banned on ships worldwide from 2008.

The cloudiness of water caused by suspended particles such as clay, silt, organic matter and plankton, which reduces the depth to which light penetrates.

The United Nations Convention on the Law of the Sea (1982), the international treaty defining the rights and responsibilities of states in the ocean, including territorial seas and exclusive economic zones.

The rising of cold, dense, nutrient-rich water from depth to the surface, replacing surface water that winds have moved away.

A satellite system that automatically reports the position, course and speed of fishing vessels to the authorities, used to enforce closed areas and other rules.

The volume of freshwater used to produce a good or service, which is effectively traded between countries when that good is exported or imported.

An account of all the inputs and outputs of water for a store such as a lake, aquifer or drainage basin over a period, showing the change in storage.

The use of practices, technologies and regulations that reduce the amount of water used or wasted, so that existing supplies last longer and meet more needs.

The measurement of the volume of water each household or business uses with a meter, so that it pays for what it uses and has an incentive to use less.

The chemical, physical and biological characteristics of water, measured against the standards needed for a particular use or for a healthy aquatic ecosystem.

A single number, usually on a scale of 0 to 100, calculated as a weighted average of the quality values of several water tests, summarising the overall degree of contamination.

A limit set by an authority on how much water each person, household or sector may use in a given period, usually during a drought or supply crisis.

The limited availability of freshwater to human societies, either because there is too little water (physical scarcity) or because storage and transport systems are inadequate (economic scarcity).

The condition in which a population has reliable access to sufficient quantities of safe drinking water for health, livelihoods and well-being; a key component of a sustainable society.

A condition in which the demand for clean, accessible water exceeds supply, taking account of water quality, environmental flows and access; defined as less than 1,700 m³ per person per year.

The movement of large volumes of water by canals, tunnels or pipelines from a river basin with a surplus to one with a deficit.

The removal of freshwater from a river, lake or aquifer for human use, whether or not it is later returned to the environment.

An area of land that is saturated or covered with shallow water for all or part of the year, such as a marsh, swamp or floodplain, which stores and filters water.

The hunting of whales for meat, oil, blubber and other products, whether commercially, for scientific permits or for the subsistence of Indigenous communities.

A model graph showing how the sustainable catch from a fish stock changes as fishing effort increases, rising to a peak at maximum sustainable yield and falling at higher effort.

Small, mostly microscopic animals and animal-like organisms that drift in water, many of which feed on phytoplankton and are in turn eaten by fish and other consumers.