Naturally occurring water with low amounts of dissolved salts
This article is about a naturally occurring type of water. For all types of waters that are of potential use to humans, seeWater resources. For other uses, seeFreshwater (disambiguation).
Fresh water is thewater resource that is of the most and immediate use to humans. Fresh water is not alwayspotable water, that is, water safe to drink byhumans. Much of theearth's fresh water (on the surface and groundwater) is to a substantial degree unsuitable for human consumption withouttreatment. Fresh water can easily becomepolluted by human activities or due to naturally occurring processes, such as erosion.
Fresh water makes up less than 3% of the world's water resources, and just 1% of that is readily available. About 70% of the world's freshwater reserves are frozen inAntarctica. Just 3% of it is extracted for human consumption. Agriculture uses roughly two thirds of all fresh water extracted from the environment.[1][2][3]
Fresh water is a renewable and variable, but finitenatural resource. Fresh water is replenished through the process of the naturalwater cycle, in which water from seas, lakes, forests, land, rivers andreservoirs evaporates, formsclouds, and returns inland as precipitation.[4] Locally, however, if more fresh water is consumed through human activities than is naturally restored, this may result in reduced fresh water availability (orwater scarcity) from surface and underground sources and can cause serious damage to surrounding and associated environments.[5]Water pollution also reduces the availability of fresh water. Where available water resources are scarce, humans have developed technologies likedesalination andwastewater recycling to stretch the available supply further. However, given the high cost (both capital and running costs) and - especially for desalination - energy requirements, those remain mostly niche applications.
A non-sustainable alternative is using so-called "fossil water" from undergroundaquifers. As some of those aquifers formed hundreds of thousands or even millions of years ago when local climates were wetter (e.g. from one of theGreen Sahara periods) and are not appreciably replenished under current climatic conditions - at least compared to drawdown, these aquifers form essentially non-renewable resources comparable to peat or lignite, which are also continuously formed in the current era but orders of magnitude slower than they are mined.
Fresh water habitats are classified as eitherlentic systems, which are the stillwaters includingponds, lakes,swamps andmires;lotic which are running-water systems; orgroundwaters which flow in rocks andaquifers. There is, in addition, a zone which bridges between groundwater and lotic systems, which is thehyporheic 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 isprecipitation from theatmosphere, in the form ofmist,rain andsnow. Fresh water falling as mist, rain or snow contains materials dissolved from theatmosphere and material from the sea and land over which the rain bearing clouds have traveled. The precipitation leads eventually to the formation ofwater bodies that humans can use as sources of freshwater:ponds,lakes,rainfall,rivers,streams, andgroundwater contained in undergroundaquifers.
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 ofsodium,chloride,magnesium andsulfate as well as many other compounds in smaller concentrations.
Indesert areas, or areas with impoverished or dusty soils, rain-bearing winds can pick upsand anddust 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 ofiron may be transported in this way including the well-documented transfer of iron-rich rainfall falling in Brazil derived from sand-storms in theSahara innorth Africa.[9]
In Africa, it was revealed that groundwater controls are complex and do not correspond directly to a single factor. Groundwater showed greater resilience to climate change than expected, and areas with an increasing threshold between 0.34 and 0.39 aridity index exhibited significant sensitivity to climate change. Land-use could affect infiltration and runoff processes. The years of most recharge coincided with the most precipitation anomalies, such as duringEl Niño andLa Niña events. Three precipitation-recharge sensitivities were distinguished: in super arid areas with more than 0.67 aridity index, there was constant recharge with little variation with precipitation; in most sites (arid, semi-arid, humid), annual recharge increased as annual precipitation remained above a certain threshold; and in complex areas down to 0.1 aridity index (focused recharge), there was very inconsistent recharge (low precipitation but high recharge). Understanding these relationships can lead to the development of sustainable strategies for water collection. This understanding is particularly crucial in Africa, where water resources are often scarce and climate change poses significant challenges.[10]
Saline water inoceans,seas and salinegroundwater make up about 97% of all the water onEarth. Only 2.5–2.75% is fresh water, including 1.75–2% frozen inglaciers,ice and snow, 0.5–0.75% as fresh groundwater. The water table is the level below which all spaces are filled with water, while the area above this level, where spaces in the rock and soil contain both air and water, is known as the unsaturated zone. The water in this unsaturated zone is referred to as soil moisture.
Below the water table, the entire region is known as the saturated zone, and the water in this zone is called groundwater.[12] Groundwater plays a crucial role as the primary source of water for various purposes including drinking, washing, farming, and manufacturing, and even when not directly used as a drinking water supply it remains vital to protect due to its ability to carry contaminants and pollutants from the land into lakes and rivers, which constitute a significant percentage of other people's freshwater supply. It is almost ubiquitous underground, residing in the spaces between particles of rock and soil or within crevices and cracks in rock, typically within 100 m (330 ft) of the surface,[12] andsoil moisture, and less than 0.01% of it assurface water inlakes,swamps andrivers.[13][14]
Freshwater lakes contain about 87% of this fresh surface water, including 29% in theAfrican Great Lakes, 22% inLake Baikal in Russia, 21% in theNorth American Great Lakes, and 14% in other lakes. Swamps have most of the balance with only a small amount in rivers, most notably theAmazon River. The atmosphere contains 0.04% water.[15] In areas with no fresh water on the ground surface, fresh water derived fromprecipitation may, because of its lower density, overlie saline ground water in lenses or layers. Most of the world's fresh water is frozen inice sheets. Many areas have very little fresh water, such asdeserts.
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 mostmammals must have access to fresh water to live. Some terrestrial mammals, especially desertrodents, appear to survive without drinking, but they do generate water through themetabolism ofcereal seeds, and they also have mechanisms to conserve water to the maximum degree.
There are three basic types of freshwater ecosystems:lentic (slow moving water, includingpools,ponds, andlakes),lotic (faster movingstreams, for examplecreeks andrivers) andwetlands (semi-aquatic areas where the soil is saturated or inundated for at least part of the time).[17][16] Freshwater ecosystems contain 41% of the world's knownfish species.[18]
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 byfreshwater ecosystems to achanging 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.[19]
Water scarcity (closely related to water stress orwater crisis) is the lack of fresh waterresources to meet the standard water demand. There are two types of water scarcity. One isphysical. The other iseconomic water scarcity.[20]: 560 Physical water scarcity is where there is not enough water to meet all demands. This includes water needed forecosystems to function. Regions with adesert climate often face physical water scarcity.[21]Central Asia,West Asia, andNorth Africa are examples of arid areas. Economic water scarcity results from a lack of investment in infrastructure or technology to draw water from rivers,aquifers, or other water sources. It also results from weak human capacity to meet water demand.[20]: 560 Many people insub-Saharan Africa are living with economic water scarcity.[22]: 11
There is enough freshwater available globally and averaged over the year to meet demand. As such, water scarcity is caused by a mismatch between when and where people need water, and when and where it is available.[23] This can happen due to anincrease in the number of people in a region, changing living conditions and diets, and expansion ofirrigated agriculture.[24][25][26]Climate change (includingdroughts orfloods),deforestation,water pollution and wasteful use of water can also mean there is not enough water.[27] These variations in scarcity may also be a function of prevailingeconomic policy and planning approaches.
An important concern for hydrological ecosystems is securing minimumstreamflow, especially preserving and restoringinstream water allocations.[28] Fresh water is an important natural resource necessary for the survival of allecosystems.
TheSustainable Development Goals are a collection of 17 interlinked global goals designed to be a "blueprint to achieve a better and moresustainable future for all".[34] Targets on freshwater conservation are included inSDG 6 (Clean water and sanitation) andSDG 15 (Life on land). For example, Target 6.4 is formulated as "By 2030, substantially increasewater-use efficiency across all sectors and ensure sustainable withdrawals andsupply of freshwater to address water scarcity and substantially reduce the number of people suffering fromwater scarcity."[34] Another target, Target 15.1, is: "By 2020, ensure the conservation, restoration and sustainable use of terrestrial and inlandfreshwater ecosystems and their services, in particular forests,wetlands,mountains anddrylands, in line with obligations under international agreements."[34]
^Each tiny cube[i] (such as the one representing biological water) corresponds to approximately 1400 cubic km of water, with a mass of approximately 1.4 trillion tonnes (235000 times that of theGreat Pyramid of Giza or 8 times that ofLake Kariba, arguably the heaviest human-made object).[11]
^Only 3% of the Earth's water is fresh water. Most of it is in icecaps and glaciers (69%) and groundwater (30%), while all lakes, rivers and swamps combined only account for a small fraction (0.3%) of the Earth's total freshwater reserves.[citation needed]
^Schaffner, Monika; Bader, Hans-Peter; Scheidegger, Ruth (15 August 2009). "Modeling the contribution of point sources and non-point sources to Thachin River water pollution".Science of the Total Environment.407 (17):4902–4915.doi:10.1016/j.scitotenv.2009.05.007.ISSN0048-9697.
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