Fresh water or freshwater is any naturally occurring liquid/solid water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include non-salty mineral-rich waters such as chalybeate springs. Fresh water may encompass frozen and meltwater in ice sheets, ice caps, glaciers, snowfields and icebergs, natural precipitations such as rainfall, snowfall, hail/sleet and graupel, and surface runoffs that form inland bodies of water such as wetlands, ponds, lakes, rivers, streams, as well as groundwater contained in aquifers, subterranean rivers and lakes.
Water is critical to the survival of all living organisms. Many organisms can thrive on salt water, but the great majority of higher plants and most insects, amphibians, reptiles, mammals and birds need fresh water to survive.
Fresh water is not always potable water, that is, water safe to drink by humans. Much of the earth's fresh water (on the surface and groundwater) is to a substantial degree unsuitable for human consumption without some treatment. Fresh water can easily become polluted by human activities or due to naturally occurring processes, such as erosion.
Fresh water is a renewable and variable, but finite natural resource. Fresh water can only be replenished through the process of the water cycle, in which water from seas, lakes, forests, land, rivers and reservoirs evaporates, forms clouds, and returns back inland as precipitation. Locally, however, if more fresh water is consumed through human activities than is naturally restored, this may result in reduced fresh water availability (or water scarcity) from surface and underground sources and can cause serious damage to surrounding and associated environments. Water pollution and subsequent eutrophication also reduces the availability of fresh water.
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Fresh water (< 0.05%)
Brackish water (0.05–3%)
Saline water (3–5%)
Brine (> 5% up to 26%-28% max)
|Bodies of water|
Fresh water can be defined as water with less than 500 parts per million (ppm) of dissolved salts.
Fresh water habitats are classified as either lentic systems, which are the stillwaters including ponds, lakes, swamps and mires; lotic which are running-water systems; or groundwaters which flow in rocks and aquifers. There is, in addition, a zone which bridges between groundwater and lotic systems, which is the hyporheic zone, which underlies many larger rivers and can contain substantially more water than is seen in the open channel. It may also be in direct contact with the underlying underground water.
The original source of almost all fresh water is precipitation from the atmosphere, in the form of mist, rain and snow. Fresh water falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled. The precipitation leads eventually to the formation of water bodies that humans can use as sources of freshwater: ponds, lakes, rainfall, rivers, streams, and groundwater contained in underground aquifers.
In coastal areas fresh water may contain significant concentrations of salts derived from the sea if windy conditions have lifted drops of seawater into the rain-bearing clouds. This can give rise to elevated concentrations of sodium, chloride, magnesium and sulfate as well as many other compounds in smaller concentrations.
In desert areas, or areas with impoverished or dusty soils, rain-bearing winds can pick up sand and dust and this can be deposited elsewhere in precipitation and causing the freshwater flow to be measurably contaminated both by insoluble solids but also by the soluble components of those soils. Significant quantities of iron may be transported in this way including the well-documented transfer of iron-rich rainfall falling in Brazil derived from sand-storms in the Sahara in north Africa.
Saline water in oceans, seas and saline groundwater make up about 97% of all the water on Earth. Only 2.5–2.75% is fresh water, including 1.75–2% frozen in glaciers, ice and snow, 0.5–0.75% as fresh groundwater and soil moisture, and less than 0.01% of it as surface water in lakes, swamps and rivers. Freshwater lakes contain about 87% of this fresh surface water, including 29% in the African Great Lakes, 22% in Lake Baikal in Russia, 21% in the North American Great Lakes, and 14% in other lakes. Swamps have most of the balance with only a small amount in rivers, most notably the Amazon River. The atmosphere contains 0.04% water. In areas with no fresh water on the ground surface, fresh water derived from precipitation may, because of its lower density, overlie saline ground water in lenses or layers. Most of the world's fresh water is frozen in ice sheets. Many areas have very little fresh water, such as deserts.
Freshwater ecosystems are a subset of Earth's aquatic ecosystems. They include lakes, ponds, rivers, streams, springs, bogs, and wetlands. They can be contrasted with marine ecosystems, which have a larger salt content. Freshwater habitats can be classified by different factors, including temperature, light penetration, nutrients, and vegetation. Freshwater ecosystems have undergone substantial transformations over time, which has impacted various characteristics of the ecosystems. Freshwater ecosystems can be divided into lentic ecosystems (still water) and lotic ecosystems (flowing water).Original attempts to understand and monitor freshwater ecosystems were spurred on by threats to human health (for example cholera outbreaks due to sewage contamination). Early monitoring focused on chemical indicators, then bacteria, and finally algae, fungi and protozoa. A new type of monitoring involves quantifying differing groups of organisms (macroinvertebrates, macrophytes and fish) and measuring the stream conditions associated with them. Threats to freshwater biodiversity include overexploitation, water pollution, flow modification, destruction or degradation of habitat, and invasion by exotic species.
Threats and challenges
The increase in the world population and the increase in per capita water use puts increasing strains on the finite resources availability of clean fresh water. The response by freshwater ecosystems to a changing climate can be described in terms of three interrelated components: water quality, water quantity or volume, and water timing. A change in one often leads to shifts in the others as well.
Water scarcity (also called water stress or water crisis) is the lack of fresh water resources to meet the standard water demand. Humanity is facing a water crisis, due to unequal distribution (exacerbated by climate change) resulting in some very wet and some very dry geographic locations, plus a sharp rise in global freshwater demand in recent decades driven by industry. Water scarcity can also be caused by droughts, lack of rainfall, or pollution. This was listed in 2019 by the World Economic Forum as one of the largest global risks in terms of potential impact over the next decade. It is manifested by partial or no satisfaction of expressed demand, economic competition for water quantity or quality, disputes between users, irreversible depletion of groundwater, and negative impacts on the environment. Two-thirds of the global population (4 billion people) live under conditions of severe water scarcity at least 1 month of the year. Half a billion people in the world face severe water scarcity all year round. Half of the world's largest cities experience water scarcity.The essence of global water scarcity is the geographic and temporal mismatch between fresh water demand and availability. The increasing world population, improving living standards, changing consumption patterns, and expansion of irrigated agriculture are the main driving forces for the rising global demand for water. Climate change, such as altered weather-patterns (including droughts or floods), deforestation, increased pollution, green house gases, and wasteful use of water can cause insufficient supply. At the global level and on an annual basis, enough freshwater is available to meet such demand, but spatial and temporal variations of water demand and availability are large, leading to (physical) water scarcity in several parts of the world during specific times of the year. Scarcity varies over time as a result of natural hydrological variability, but varies even more so as a function of prevailing economic policy, planning and management approaches. Scarcity can be expected to intensify with most forms of economic development, but, if correctly identified, many of its causes can be predicted, avoided or mitigated.
An important concern for hydrological ecosystems is securing minimum streamflow, especially preserving and restoring instream water allocations. Fresh water is an important natural resource necessary for the survival of all ecosystems. The use of water by humans for activities such as irrigation and industrial applications can have adverse impacts on down-stream ecosystems.
Fresh water withdrawal is the quantity of water removed from available sources for use in any purpose, excluding evaporation losses. Water drawn off is not necessarily entirely consumed and some portion may be returned for further use downstream.
Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities. Water bodies include for example lakes, rivers, oceans, aquifers and groundwater. Water pollution results when contaminants are introduced into the natural environment. For example, releasing inadequately treated wastewater into natural waters can lead to degradation of these aquatic ecosystems. In turn, this can lead to public health problems for people living downstream. They may use the same polluted river water for drinking or bathing or irrigation. Water pollution is the leading worldwide cause of death and disease, e.g. due to water-borne diseases.
Global goals for conservation
The Sustainable Development Goals are a collection of 17 interlinked global goals designed to be a "blueprint to achieve a better and more sustainable future for all". Targets on freshwater conservation are included in SDG 6 (Clean water and sanitation) and SDG 15 (Life on land). For example, Target 6.4 is formulated as "By 2030, substantially increase water-use efficiency across all sectors and ensure sustainable withdrawals and supply of freshwater to address water scarcity and substantially reduce the number of people suffering from water scarcity." Another target, Target 15.1, is: "By 2020, ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services, in particular forests, wetlands, mountains and drylands, in line with obligations under international agreements."
Water is a critical issue for the survival of all living organisms. Some can use salt water but many organisms including the great majority of higher plants and most mammals must have access to fresh water to live. Some terrestrial mammals, especially desert rodents, appear to survive without drinking, but they do generate water through the metabolism of cereal seeds, and they also have mechanisms to conserve water to the maximum degree.
- African Great Lakes – Series of lakes in the Rift Valley
- Aral Sea – Lake between Kazakhstan and Uzbekistan
- Desalination – Removal of salts and minerals from a substance
- Drought – Extended period of deficiency in a region's water supply
- Limnology – The science of inland aquatic ecosystems
- List of countries by freshwater withdrawal
- List of countries by total renewable water resources – list of countries by total renewable water resources mostly based on The World Factbook
- Properties of water – Physical and chemical properties of pure water
- "Nutrients in fresh water"
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