Water in the Universe

Water is a fairly common compound in our solar system,[43]​ and in the universe,[43][44]​ where it is found mainly in the form of ice and vapor "Vapor (state)"). It constitutes a large part of the material that makes up comets and in 2016 "magmatic water" was found from the interior of the Moon in small mineral grains on the lunar surface.[45] Some satellites of Jupiter and Saturn, such as Europa "Europa (satellite)") and Enceladus "Enceladus (satellite)"), possibly have liquid water under their thick layer of ice.[43]​.

Most of the water that exists in the universe may have arisen as a byproduct of the formation of stars that subsequently produced water vapor when they exploded. The birth of stars usually causes a strong flow of gases and cosmic dust. When this material collides with the gas in the outer areas, the shock waves produced compress and heat the gas. It is thought that water is produced in this warm, dense gas.[46]​.

Water has been detected in interstellar clouds within our Milky Way galaxy. These interstellar clouds can sometimes condense into the form of a solar nebula. In addition, it is thought that water may be abundant in other galaxies, given that its components (hydrogen and oxygen) are among the most common in the universe.[47]​ In the first decade of the century, water was found on exoplanets, such as HD 189733 b[48]​[49]​ and HD 209458 b.[50]​.

In July 2011, the journal Astrophysical Journal Letters published the discovery by a group of astronomers from NASA's Jet Propulsion Laboratory (JPL) and the California Institute of Technology (CALTECH) of a cloud of water vapor surrounding the quasar APM 08279+5255, which represents the largest reserve of water in the Universe discovered to date, about 140 billion times more than in the land.[51]​.

Water in the solar system

Water vapor has been detected in the atmosphere of several planets, satellites and other bodies in the solar system, as well as in the Sun itself. Several examples are listed below:

• - Mercury "Mercury (planet)"): It has been detected in high proportions in the exosphere.[52]​.

• - Venus "Venus (planet)"): 0.002% in the atmosphere.[53][54]​.

• - Earth: reduced quantities in the atmosphere, subject to climatic variations.

• - Mars "Mars (planet)"): Variable quantities depending on the place and season of the year.[55]​.

• - Enceladus "Enceladus (satellite)") (Saturn's moon): 91% of its atmosphere.[56]​.

Water in its liquid state is abundant on Earth, where it covers 71% of the surface. In 2015, NASA confirmed the presence of liquid water on the surface of Mars.[57]​.

There are indications that Saturn's moon Enceladus has a 10 km deep liquid ocean about 30-40 km below the satellite's south pole; interior.[61]​[62]​ On Ganymede "Ganymede (satellite)"), another moon of Jupiter, there could also be liquid water between layers of high-pressure ice and rock.[63]​ In 2015, the New Horizons space probe found signs of water inside Pluto "Pluto (dwarf planet)").[64]​.

Regarding ice, it exists on Earth, especially in polar areas and glaciers; In the polar caps of Mars "Mars (planet)"), solid water is also found, although they are mainly composed of dry ice. Ice is likely to be part of the internal structure of planets such as Uranus "Uranus (planet)"), Saturn "Saturn (planet)"), and Neptune "Neptune (planet)"). Ice forms a thick layer on the surface of some satellites, such as Europa "Europa (satellite)") and Titan "Titan (satellite)"), where it can reach 50 km thick.[65]​.

Ice also exists in the material that forms Saturn's rings,[66]​ in comets[67]​ and objects of meteoric origin, arriving for example from the Kuiper Belt or the Oort Cloud. Ice has been found on the Moon, and on dwarf planets such as Ceres "Ceres (dwarf planet)") and Pluto.[68]​[64]​.

Water and the habitable zone

The existence of water in a liquid state is necessary for terrestrial living beings and its presence is considered an important factor in the origin and evolution of life on the planet.[69]​[70]​ The Earth is located in an area of ​​the solar system that meets very specific conditions, but if it were 5% – eight million kilometers – closer or further from the Sun, it could not house water in a liquid state, only water vapor or ice.[69]​[71]​.

The mass of the Earth also plays an important role in the state of water on the surface: the force of gravity prevents gases in the atmosphere from dispersing. Water vapor and carbon dioxide combine, causing what is known as the greenhouse effect, which maintains the stability of temperatures, acting as a protective layer for life on the planet. If the Earth were smaller, the lower gravity exerted on the atmosphere would make it less thick, which would result in extreme temperatures and prevent the accumulation of water except in the polar caps, as occurs on Mars. On the other hand, if the mass of the Earth were much greater, water would remain in a solid state even at high temperatures, given the high pressure caused by gravity.[72] Therefore, both the size of a planet and the distance from the star are factors in the extent of the habitable zone.

La Tierra se caracteriza por contener un alto porcentaje de su superficie cubierta por agua líquida, y el volumen total ocupa 1 400 000 000 km³. Este líquido se mantiene constante gracias al ciclo hídrico. Se piensa que el agua formaba parte de la composición de la tierra primigenia[73]​ y apareció en la superficie a partir de procesos de desgasificación del magma del interior de la tierra y de condensación del vapor de agua al enfriarse el planeta, aunque no se descartan aportes de agua por impactos con otros cuerpos solares.[74]​.

Distribution of water in the Earth's mantle

The Earth's mantle contains an undetermined amount of water, which according to sources is between 35% and 85% of the total.[75] This substance can be found practically anywhere in the biosphere and in the three states of aggregation of matter: solid, liquid and gaseous. Water present in any state above or below the surface of the planet, including underground, forms the hydrosphere, which is subject to a complex dynamics of transport and change of state that defines a water cycle.

Saltwater oceans and seas cover 71% of the Earth's surface. Only 3% of Earth's water is fresh, and of this volume, only 1% is in a liquid state. The remaining 2% is found in a solid state in layers, fields and ice shelves or ice packs in the latitudes close to the poles. Outside the polar regions, fresh water is found mainly in wetlands and, underground, in aquifers. According to a study published in the journal Nature Geoscience, it is estimated that the total groundwater on the planet represents a volume of 23 million cubic kilometers.[76]​.

In total, the Earth contains about 1,386,000,000 km³ of water[n. 3]​ which are distributed as follows:[4]​.

Water plays a very important role in geological processes. On the surface, water acts as a very active agent on chemical and physical erosion processes. Water in its liquid state and, to a lesser extent, in the form of ice, is also an essential factor in sediment transport. The deposit of these remains is a tool used by geology to study the formative phenomena that occurred on Earth.[77]​.

The water cycle

The water cycle—scientifically known as the hydrological cycle—is the continuous exchange of water within the hydrosphere, between the atmosphere, surface and groundwater, and living organisms.

Water constantly changes its position from one part of the water cycle to another and numerous components can be distinguished[78]​ that basically involve the following physical processes:

• - evaporation from the oceans and other masses of water and transpiration of living beings (animals and plants) into the atmosphere.

• - precipitation "Precipitation (meteorology)"), caused by the condensation of water vapor, and which can take multiple forms.

• - transport of water by surface runoff or by underground flows after infiltration into the subsoil.

The sun's energy heats the water, generating the energy necessary to break the bonds between the liquid water molecules, which thus pass into the gaseous state. The evaporated water rises to the upper layers of the atmosphere where it cools until it condenses and forms clouds composed of tiny droplets. Under certain conditions, these small water particles join together to form larger droplets that cannot be kept suspended by rising air currents and fall as rain or hail or snow depending on the temperature. 90% of the water vapor present in the atmosphere comes from evaporation from the oceans, where most of it returns directly; However, the wind moves 10% towards the mainland, in which the volume of precipitation thus exceeds the volume of evaporation, coming mainly from aquatic bodies and the transpiration of living beings, predominantly from plants.[78]​.

Some of the water that falls on the land as rain or from snowmelt seeps into the ground or evaporates, but about a third moves along the surface along the slope.[78] Runoff water often forms basins (watersheds), where smaller watercourses often join together to form rivers. The constant movement of water masses over different geological terrains is a very important factor in the formation of the relief. In the parts of the course with a high slope, the rivers drag minerals during their movement, which they deposit in the lower parts of the course. Therefore, rivers play a very important role in enriching the soil. Part of the waters of these rivers are diverted for agricultural use. Rivers flow into the sea forming estuaries or deltas.[79]​ Groundwater, for its part, can rise to the surface as springs or descend into deep aquifers, where it can remain for millennia.[78]​.

Water can occupy land with disastrous consequences: Floods occur when a mass of water exceeds its usual margins or when it communicates with a larger mass—such as the sea—in an irregular manner. On the other hand, and although the lack of precipitation is a major obstacle to life, it is natural that some regions periodically suffer droughts. When dryness is not temporary, vegetation disappears, while soil erosion accelerates. This process is called desertification[80]​ and many countries adopt policies[81]​ to stop its advance. In 2007, the UN declared June 17 as the World Day to Combat Desertification and Drought.[82]​.

the ocean

The ocean encompasses the part of the Earth's surface occupied by sea water. There are several theories about its formation. There are indications that it comes from the water present in the interior of the planet, transported to the surface in the form of water vapor by volcanic processes,[83]​ but it is not ruled out that its origin is in collisions with bodies rich in water during the formation of the solar system.[84]​ During the different geological eras the distribution of ocean waters has constantly varied. During the Cenozoic, the Antarctic, Arctic, Atlantic, Indian and Pacific oceans, as well as the seas, smaller bodies of salt water, reached their current configuration.[n. 4]​.

It covers 71% of the Earth's surface and the average depth is about 4 km. In the Mariana Trench, it reaches a depth of 11,033 m.[85] In the oceans there is a surface layer of water at about 17 °C on average, although the temperature varies significantly between the equatorial and tropical areas, where it can reach 36 °C, and the polar areas, where it drops to around −2 °C, a temperature at which it freezes. The surface water layer, whose thickness is normally about four or five hundred meters, is maintained at an almost constant temperature, until reaching an area, called the thermocline, where a rapid drop in temperature occurs. Below this limit zone, the temperature drops to 3 and 0 °C.[86]​.

The oceans contain many elements in solution, although most are found in minute concentrations. The most abundant are sodium and chlorine, which, in their solid form, combine to form sodium chloride or common salt, which represents 80% of the salts dissolved in seawater. These elements are followed in order of abundance by magnesium - 4% -, sulfur, mainly in the form of sulfates, calcium, potassium, bromine, strontium, boron and fluorine.[87]​.

Tides are cyclical movements of large masses of water caused by the gravitational force of the moon and the sun. Tides are due to current movements of large masses of water, which oscillate within a constant range of hours. The tide is perceptibly reflected in a notable variation in the height of sea level - among other things - caused by the relative positions of the Sun and the Moon in combination with the effect of the Earth's rotation and local bathymetry.[88] The strip of sea subject to these changes - exposed at low tide and covered at high tide - is called the intertidal zone and represents an ecological niche of great value.[89]​.

Fresh water in nature

Fresh water in nature is renewed thanks to the atmosphere, which has 13,900 km³ of water vapor, 10% of the planet's fresh water, excluding groundwater, ice in the polar caps and permafrost. It is a dynamic volume that is constantly increasing in the form of evaporation and decreasing in the form of precipitation, with the annual volume in the form of precipitation estimated to be between 113,500 and 120,000 km³ in the world. In countries with a temperate and cold climate, precipitation in the form of snow represents an important part of the total.[90]​.

68.7% of the world's fresh water is in glaciers and ice sheets. Those present in Antarctica, the Arctic and Greenland, despite their size, are not considered water resources due to their inaccessibility. On the other hand, continental glaciers are an important part of the water resources of many countries.[90]​.

Surface waters encompass lakes, reservoirs, rivers and wetlands, accounting for only 0.3% of the planet's fresh water; however, they represent 80% of annually renewable freshwater, hence its importance.[90]​.

Stored fresh groundwater, which represents 96% of the Earth's unfrozen fresh water, is also an important resource. According to Morris, the groundwater systems used to supply populations account for between 25 and 40% of the total drinking water supplied. Thus, half of the world's large megalopolises depend on them for their consumption. In areas where there is no other source of supply, it represents a form of quality supply at low cost.[90]​.

The largest source of fresh water in the world suitable for consumption is Lake Baikal in Siberia, which has a very low salt and calcium index and is not yet contaminated.[91]​.

El agua es la molécula más común en todos los seres vivos en la Tierra; la masa de la mayoría de los organismos contiene entre un setenta y noventa por ciento de agua, aunque el porcentaje varía considerablemente según la especie, la etapa de desarrollo del individuo y, en organismos multicelulares complejos, el tipo de tejido.[92]​ Las algas llegan al 98 % de agua en peso, mientras que los pinos contienen un 47 %. El cuerpo humano incluye entre un 65 % a un 75 % de agua en peso, y el porcentaje es menor a medida que la persona crece. El contenido en los tejidos varía entre el 99 % del líquido cefalorraquídeo y el 3 % de la dentina.[93]​[94]​.

El agua desempeña un papel biológico importante y todas las formas de vida conocidas dependen del agua a nivel molecular. Sus propiedades como disolvente posibilitan las diversas reacciones químicas de los compuestos orgánicos cruciales para todas las funciones vitales, el transporte de moléculas a través de las membranas y para disolver los productos de excreción.[95]​ También es un agente activo esencial en muchos de los procesos metabólicos de los seres vivos. La extracción de agua de moléculas —mediante reacciones químicas enzimáticas que consumen energía— permite la síntesis de macromoléculas complejas, como los triglicéridos o las proteínas; el agua actúa asimismo como agente catabólico sobre los enlaces entre átomos, reduciendo el tamaño de moléculas como glucosas, ácidos grasos y aminoácidos, y produciendo energía en el proceso. Es un compuesto esencial para la fotosíntesis. En este proceso, las células fotosintéticas utilizan la energía del sol para separar el oxígeno y el hidrógeno presentes en la molécula de agua; el hidrógeno se combina con CO —absorbido del aire o del agua— para formar glucosa, liberando oxígeno en el proceso.[96]​ El agua, por su carácter anfiprótico es también el eje de las funciones enzimáticas y la neutralidad respecto a ácidos y bases. La bioquímica en muchos medios intracelulares funciona de manera ideal alrededor de un valor pH de alrededor de 7,0 hasta 7,2.[95]​.

Aquatic life

The various theories on the origin of life agree that it had its origin in the oceans, either in surface waters thanks to the energy supplied by solar radiation, cosmic rays and even electrical discharges from the atmosphere or in the deep sea, next to the hydrothermal vents of oceanic trenches,[97]​[98]​ It is estimated that 25% of all species are aquatic.[99]​ Bacteria are particularly abundant in water and a 2006 study counted about 20,000 species per liter of seawater.[100]​ These microorganisms, together with phytoplankton, are the basis of the underwater food chain, so water provides not only the environment, but also the sustenance of all marine fauna.[101]​.

Aquatic animals must obtain oxygen to breathe, extracting it from the water in various ways. Pulmonary breathing vertebrates, such as mammals, birds, reptiles and amphibians in their adult phase, take in air out of water and hold their breath when submerged. Most multicellular aquatic fauna, however, use gills to extract oxygen from the water. Some species such as dipnoos have both respiratory systems. Some invertebrate organisms lack a respiratory system and absorb oxygen directly from water by diffusion.[102]​.

La historia muestra que las primeras civilizaciones florecieron en zonas favorables a la agricultura, como las cuencas de los ríos. Es el caso de Mesopotamia, considerada la cuna de la civilización humana, surgida en el fértil valle del Éufrates y el Tigris; y también el de Egipto, que dependía por completo del Nilo y sus periódicas crecidas. Muchas otras grandes ciudades, como Róterdam, Londres, Montreal, París, Nueva York, Buenos Aires, Shanghái, Tokio, Chicago y Hong Kong deben su riqueza a la conexión con alguna gran vía de agua que favoreció su crecimiento y su prosperidad. Las islas que contaban con un puerto natural seguro —como Singapur— florecieron por la misma razón. Del mismo modo, las áreas en las que el agua es muy escasa tienen dificultades de desarrollo, a no ser que posean otros recursos en grandes cantidades.[103]​.

Water as a human right

The General Assembly of the United Nations approved on July 28, 2010, in its sixty-fourth session, a resolution that recognizes drinking water and basic sanitation as a human right essential for the full enjoyment of life and all human rights.[104]​[n. 5] This resolution was preceded, in November 2002, by "General Comment No. 15 on the right to water", which establishes the right to affordable access to water as an indispensable condition for "a dignified human life." Article I.1 establishes that "The human right to water is indispensable for a dignified human life."[104]​.

In the 2010 General Assembly resolution, the number of people without access to drinking water was estimated at 884 million, and the number of people without basic sanitation at more than 2,600,000,000. Likewise, it estimated that some 1.5 million children under 5 years of age died annually as a result of lack of water.

Drinking water: need of the human body

The human body is composed of between 55% and 78% water, depending on its measurements and complexion.[105]​ Metabolic activity, such as the oxidation of fats or carbohydrates, generates a certain amount of water; However, metabolic water is insufficient to compensate for losses through urine, feces, sweat, or exhalation of breath, so to maintain the body's water balance it is necessary to consume water. Water can be absorbed from both liquid drinks and foods, among which fresh fruits and vegetables contain the highest percentage, up to 85%, similar to that of many beverages, while cereals or nuts usually consist of only 5% water.[106]​.

Water is also useful for lubricating joints, facilitating the digestion process, and keeping organs functioning and in good condition.[107]​.

To avoid problems associated with dehydration, a document from the Food and Nutrition Platform of the National Research Council of the United States recommended in 1945 consuming one milliliter of water for each calorie of food.[108]​ The last reference offered by this same organization speaks of 2.7 liters of water per day for a woman and 3.7 liters for a man, including the consumption of water through food.[109]​ Naturally, during the During pregnancy and breastfeeding, women must consume more water to stay hydrated. According to the Institute of Medicine - which recommends an average of 2.2 liters/day for a woman, and 3.0 liters/day for a man - a pregnant woman should consume 2.4 liters, and up to 3 liters during breastfeeding, considering the large amount of fluid lost during this period. Smaller minimum amount, typically one liter of water daily for an adult male individual.[113]​ In any case, exact amount will vary depending on activity level, temperature, humidity, diet, and other factors.

Excessive water intake—for example, during physical exercise—can cause hyperhydration, or water intoxication, a condition that can be dangerous. There are several unproven myths about water consumption and health, such as the alleged relationship between water consumption, weight loss and constipation.[114]​.

Unlike water losses through the skin or lungs, the volume excreted with urine is subject to strict control, carried out in the kidneys. The percentage of water present in urine can vary greatly, depending on the amount of waste substances, such as minerals and urea, to be excreted. The maximum concentration or osmolarity of these solutes in the urine is 1200 mOsm/L, which defines the minimum volume of liquid necessary for their elimination, regardless of the hydration status of the organism.[106]​.

Drinking water is one of the main transmitters of disease-causing microorganisms, mainly bacteria, viruses and intestinal protozoa. The great epidemics of humanity have prospered due to water pollution. From references, it is known that boiling water was recommended since five hundred years before our era.[115]​.

Currently, in developed countries, the problems posed by contaminated water are practically controlled. Water filtration and disinfection processes prior to human consumption were imposed in the 20th century and are estimated to be responsible for a 50% increase in life expectancy in developed countries in the last century. Life magazine considered water chlorination and filtration as probably the most important public health progress of the millennium. There are several agents that can be used to disinfect water, including peroxide, chlorine compounds and other halogens, silver-copper, ozone and ultraviolet radiation.[116]​.

Chlorine, either in gas form or as hypochlorite, is the material most used as a water disinfectant, due to its oxidizing properties. Once it has crossed the membrane of the microorganisms, the chlorinated compounds are eliminated by oxidation by their respiratory enzymes.[117]​.

Chlorine can be irritating to the mucous membranes and skin, so its use is strictly monitored. The proportion used varies between 1ppm when it comes to purifying water for consumption, and between 1-2 ppm for the preparation of bathing water. Improper application of chemical components in water can be dangerous. The application of chlorine as a disinfectant began in 1912 in the United States. The following year Wallace and Tiernan designed equipment that could measure chlorine gas and form a concentrated solution that was added to the water to be treated. Since then the chlorination technique has continued to progress. In addition to its germ-destroying capacity, its action is also very beneficial in the elimination of iron, manganese, hydrogen sulfides, sulfides and other reducing substances from water. Many countries in their regulations establish disinfection using chlorine and require the maintenance of a certain residual concentration of disinfectant in their water distribution pipe networks. Chloramines are sometimes used as a secondary disinfectant to maintain a certain concentration of chlorine in the drinking water supply system for a longer period of time.[118]​.

The world population has increased from 2,630,000,000 in 1950 to 6,671,000,000 in 2008. In this period, the urban population has increased from 733,000,000 to 3,505,000,000. It is in human settlements where non-agricultural water use is concentrated and where most water-related diseases are contracted. water.[119]​.

Given the difficulty of having drinking water for human consumption in many places on the planet, an intermediate concept has been consolidated, safe water as water that does not contain dangerous bacteria, dissolved toxic metals, or chemicals harmful to health, and is therefore considered safe to drink, therefore it is used when the supply of drinking water is compromised. It is a water that is not harmful to humans, although it does not meet the ideal conditions for consumption.

For various reasons, the availability of water is problematic in much of the world, and for this reason it has become one of the main concerns of governments around the world. Currently, it is estimated that around one billion[120]​ people have poor access to drinking water. This situation is aggravated by the consumption of water in poor conditions, which favors the proliferation of diseases and epidemic outbreaks. Many of the countries meeting in Evian at the XXIX G8 conference set 2015 as the deadline to achieve universal access to better water around the world.[121] Even if this difficult goal were achieved, it is estimated that there would still be around 500 million without access to drinking water, and more than a billion would lack an adequate sanitation system. Poor water quality and irregular sanitation seriously affect the health status of the population: the consumption of contaminated water alone causes 5,000,000 deaths a year, according to several reports[122]​ from the United Nations, which declared 2005-2015 the Decade of Action. The WHO estimates that the adoption of safe water policies could prevent the deaths of 1,400,000 children a year from diarrhea.[123]​[124]​ 50 countries, which account for almost a third of the world's population, lack an adequate water supply,[125]​ and 17 of them annually extract more water from their aquifers than can be renewed naturally.[126]​ Pollution, on the other hand, does not only pollute. the water of rivers and seas, but the underground water resources that serve as a supply for human consumption.[127]​.

Domestic water use

In addition to human beings needing water for their existence, they need water for their own hygiene and cleaning. It has been estimated that humans directly or indirectly consume about 54% of the world's available surface freshwater. This percentage is broken down into:

• - 20%, used to maintain fauna and flora, for the transportation of goods (boats) and for fishing, and.

• - the remaining 34%, used as follows: 70% in irrigation, 20% in industry and 10% in cities and homes.[128]​[129]​.

Direct human consumption represents a small percentage of the volume of water consumed daily in the world. It is estimated that an inhabitant of a developed country consumes around five liters a day in the form of food and drinks.[130] These figures rise dramatically when total domestic consumption is considered. An approximate calculation[131]​ of water consumption per person/day in a developed country, considering domestic industrial consumption, yields the following data:

These consumption habits and the increase in population in the last century have simultaneously caused an increase in water use. This has caused the authorities to carry out campaigns for the good use of water. Currently, raising awareness is a task of great importance to guarantee the future of water on the planet, and as such it is the subject of constant activities at both the national and municipal levels.[132]​ On the other hand, the enormous differences in daily consumption per person between developed and developing countries[133]​ indicate that the current water model is not only ecologically unviable: it is also unviable from a humanitarian point of view,[134]​ which is why numerous NGOs are strive[135]​ to include the right to water among human rights.[136]​ During the V World Water Forum, convened on March 16, 2009 in Istanbul (Turkey), Loic Fauchon (president of the World Water Council) stressed the importance of regulating consumption in these terms:.

1. • Avoid throwing any type of oil down the drains.

2. • Keep all water taps closed while scrubbing/washing/soaping.

3. • Address leaks. Some are not visible, but can be seen on the consumption receipt.

4. • Reduce the use of the bathtub.

5. • Take 5-8 minute showers.

6. • Collect the water that comes out of the shower (which is usually wasted before starting to bathe, and use the water to wash the car, water the garden, etc.

Water in agriculture

According to the FAO, agriculture accounts for 69% of the total water extracted in the world, a percentage that in some arid areas can exceed 90%. The need for water resources for food production must be reconciled with the demand from other sectors, such as use in urban areas and the preservation of ecosystems.[139]​ In many places, agriculture places significant pressure on natural bodies of water, and the water required for irrigation represents a decrease in the natural flows of rivers and a decrease in groundwater levels that cause a negative effect on ecosystems. aquatic.[140]​.

According to UNESCO data, less than 20% of irrigation water reaches the plant; The rest is wasted and also transports waste with toxic substances that inevitably end up in rivers.[141]​ The use of nitrates and pesticides in agricultural work represents the main diffuse contamination of both surface and underground water bodies. The most significant is nitrate pollution, which causes water eutrophication. In Spain, the annual consumption of fertilizers is estimated at 1,076,000 tons of nitrogen, 576,000 tons of phosphorus and 444,000 tons of potassium. Although most of the fertilizers are absorbed by crops, the rest is a potential water contaminant.[140]​.

Because agriculture is an ancient production system, it has been adapted to the different water regimes of each region: Thus, in areas where there is abundant rainfall "Precipitation (meteorology)") irrigated crops are usually grown, while in drier areas rainfed crops are common. Given that irrigated lands are approximately three times more productive than rainfed lands, investments in the development of irrigation infrastructure and water resource management are important for the sustainable development of agriculture.[139]​ This development occurs very unevenly in different parts of the world. For example, in Africa, only 7% of the arable area is irrigated, while in Asia, it is 38%.[139]​.

More recently, new forms of cultivation and irrigation have been experimented with aimed at minimizing water use. Localized irrigation techniques - drip or sprinkler -, greenhouse agriculture under controlled environmental conditions and the selection of varieties genetically adapted to dry climates, are part of these practices.[142] Currently one of the most active aspects of genetic research attempts to optimize the water consumption of the species that man uses as food.[143]​ In space agriculture experiments"), as the cultivation of plants in the conditions of space stations is known, have also been developed. technologies that limit water consumption between 25 and 45%.[144]​ Agroforestry and mortars are solutions to build microclimates and allow the circulation of water to the interior of the land thanks to the evapotranspiration phenomena of the plants. For example, a hectare of beech forest, which consumes between 2,000 and 5,000 tons of water per year, returns 2,000 tons per year. evaporation.[145]​.

The use of water in industry

The industry requires water for multiple applications, such as for heating and cooling in heat exchangers, to produce water vapor in steam turbines or as a solvent, as raw material or for cleaning. Pressurized water is used in hydrodemolition equipment, in water jet cutting machines, and is also used in high-pressure water guns to efficiently and precisely cut various materials such as steel, concrete, reinforced concrete, ceramics, etc. and as a cooling liquid to prevent overheating of machinery such as electric saws "Saw (tool)") or between elements subjected to intense friction. After use, most of it is disposed of, returning it back to nature. Sometimes discharges are treated, but other times industrial wastewater contaminated with heavy metals, chemicals or organic matter returns to the water cycle without adequate treatment, which has a negative impact on water quality and the aquatic environment.[146] Indirect pollution can also occur: through solid waste containing contaminated water or other liquids, leachate, which ends up leaking into the ground and contaminating aquifers if they are not isolated. properly.[147]​ Thermal pollution also occurs due to the discharge of water used as coolant.

The largest consumers of water for industry in 2000 were: the United States (220.7 km³); China (162 km³); Russian Federation (48.7 km³); India (35.2 km³); Germany (32 km³); Canada (31.6 km³) and France (29.8 km³). In Spanish-speaking countries, the highest consumption occurred in Spain (6.6 km³); Mexico (4.3 km³); Chile (3.2 km³) and Argentina (2.8 km³).[148]​ Global industrial water consumption exceeds domestic water consumption by more than double.[149]​.

Water is used for the generation of electrical energy. Hydroelectricity is that which is obtained through hydraulic energy. Hydroelectric energy is produced when water previously impounded in a dam falls by gravity into a hydroelectric power plant, rotating a turbine geared to an electric power alternator in this process. This type of energy is low cost, does not produce pollution, and is renewable, although the construction of reservoirs has an environmental impact.[150]​[151]​.

Water and steam are used as heat transmitters in various heat exchange systems, due to their abundance and high heat capacity, which allows them to absorb large amounts of heat energy without excessively changing their temperature.[152]​ Condensed steam is an efficient heater due to its high latent heat.[153]​ The disadvantage of water and steam is that, without treatment, they are corrosive to many metals, such as steel and copper. In most power plants, water is used as a coolant, either by heat exchange or evaporation.

In the nuclear industry, water can be used as a nuclear moderator. In a pressurized water reactor, water acts as a coolant and moderator. This increases the effectiveness of the nuclear power plant's passive safety system, since the water slows down the nuclear reaction, maintaining the chain reaction.[154]​.

The water used as a solvent

Water is often described as the universal solvent, because it dissolves many of the known compounds. However, it does not dissolve all the compounds.

In chemical terms, water is an effective solvent because it allows ions and polar molecules to dissolve "Polarity (chemistry)"). In the dissolution process, water molecules group around the ions or molecules of the substance to keep them away or dispersed. Anions or negatively charged portions of the molecule attract hydrogens present in the water molecule, while oxygens have an affinity for cations or positively charged surfaces.[162] Solvation or suspension "Suspension (chemistry)") of substances in water is used daily for washing clothing, floors, food, pets, automobiles, and the human body. The use of water as a cleaning solvent is very high in industrialized countries.

Water facilitates the biological and chemical processing of wastewater. The aqueous environment helps to break down contaminants, due to its ability to become a homogeneous solution, which can be treated flexibly. Microorganisms that live in water can access dissolved waste and feed on it, breaking it down into less polluting substances. For this, aerobic treatments are widely used by adding oxygen or air to the solution, increasing the speed of decomposition and reducing the reactivity of the harmful substances that compose it. Other examples of biological systems for wastewater treatment are sugarcane fields and anaerobic biodigesters. Generally, in chemical and biological waste treatments, solid residues remain from the treatment process. Depending on its composition, the remaining residue can be dried and used as fertilizer if its properties are beneficial, or it can be disposed of in a landfill "Landfill (garbage)") or incinerated.

Other uses

Water's high latent heat of vaporization and relatively low chemical reactivity make it an effective firefighting fluid. Water extinguishes fire by cooling, by absorbing the heat from combustion. Water also decreases the oxygen concentration as it evaporates, thus helping to smother the fire. However, the use of water to extinguish flames on electrical equipment is not recommended, due to its electrically conductive properties, which can cause electrocution. Likewise, it should not be used to extinguish liquid fuels or organic solvents, since they float in water and the explosive boiling of water tends to spread the fire.[163]​.

When using water to put out fires, the risk of a steam explosion must be considered, as it can occur when used in confined spaces and on overheated fires.[n. 6]​

The danger of an explosion must also be taken into account when certain substances, such as alkali metals or hot graphite, decompose in water producing hydrogen.

Humans use water for various recreational purposes, including exercise and sports. Some of these sports include swimming, water skiing, sailing, surfing and jumping. There are also other sports that are practiced on an ice surface such as ice hockey and ice skating.

Lakeshores, beaches, and water parks are popular places for relaxation and fun. The sound of flowing water has a calming effect, due to its white noise nature.[164] Other people keep aquariums or ponds with fish and marine life for fun, company, or for display. Humans also practice snow sports such as skiing or snowboarding. It is also used for fighting games by throwing snowballs, water balloons), and even with the use of water guns.

The fountains "Fountain (architecture)") and canals, originally built to facilitate the use of water for human consumption, irrigation and transportation, have become ornamental elements to decorate public or private places.[165]​.

On April 7, 1795, the gram was defined in France as "the absolute weight of a volume of pure water equal to a cube of one hundredth of a meter, at the melting temperature of ice."[166] For practical reasons, a thousand-fold larger reference measurement for metals and other solids became popular. The task assigned was therefore to accurately calculate the mass of a liter of water. Despite the fact that the definition of the gram itself specified 0 °C, a very stable temperature point, scientists preferred to redefine the standard and make their measurements based on the maximum density of water, that is, around 4 °C.[167]​.

The SI Kelvin temperature scale is based on the triple point of water or 273.16 K (0.01 °C).[168] The Kelvin scale is based on the same increment as the Celsius scale, defined by the boiling point (100 °C) and melting point (0 °C) of water at atmospheric pressure.

Natural water is composed mainly of the isotopes hydrogen-1 and oxygen-16, but also contains a small amount of heavier isotopes such as hydrogen-2 or deuterium. The amount of deuterium oxides in heavy water is also very small, but it greatly affects the properties of the water. River and lake water usually has less deuterium than sea water. For this reason, a water standard was defined according to its deuterium content, known as the Vienna Standard Mean Ocean Water or VSMOW ().[169]​.

Pollution and water purification

The deposition of untreated waste in the atmosphere, on land and in water causes the pollution of precipitation, surface waters, groundwater and the degradation of natural ecosystems.[170]​ Population growth and the expansion of economic activities have a negative effect on the ecosystems of coastal waters, rivers, lakes, wetlands and aquifers. Examples are the construction of new ports and urban areas along the coast, the alteration of river systems for navigation and for water storage reservoirs, the drainage of wetlands to increase agricultural area, the overexploitation of fishing grounds, the multiple sources of pollution from agriculture, industry, tourism and household wastewater. According to UNESCO data, between 1990 and 2006, water extraction has doubled the population growth rate. The quality of natural bodies of water is being reduced due to all these factors.[171]​.

In 2000, the UN General Assembly established eight goals for the future (Millennium Development Goals). Among them was reversing the trend of loss of environmental resources, since the need to preserve ecosystems was recognized, essential for maintaining biodiversity and human well-being, since obtaining drinking water and food depends on them.[172] To achieve this, in addition to sustainable development policies, purification systems are required that improve the quality of the waste generated by human activity. Water purification is the set of physical, chemical or biological treatments that improve water quality or eliminate or reduce pollution. There are two types of treatments: those that are applied to obtain quality water suitable for human consumption and those that reduce water pollution in discharges into nature after use. It is imperative to generate research projects to find viable and economical alternatives for the non-conventional treatment of this valuable resource.[173]​.

Water intended for human consumption is that used for drinking, cooking, preparing food or other domestic uses. Each country regulates by law the quality of water intended for human consumption. European law protects "people's health from the adverse effects derived from any type of contamination of water intended for human consumption, guaranteeing its health and cleanliness" and therefore it cannot contain any type of microorganism, parasite or substance, in a quantity or concentration that may pose a danger to human health. Thus, it must be totally free of Escherichia coli and Enterococcus bacteria, and its composition must meet certain restrictions, such as containing less than 50 milligrams of nitrates per liter of water or less than 2 milligrams of copper and other chemical substances.[174]​.

Drinking water is usually collected in reservoirs, springs or extracted from the ground through artificial tunnels or wells from an aquifer. Other sources of water are rainwater, rivers and lakes. However, water must be treated for human consumption, and the extraction of dissolved substances, undissolved substances and microorganisms harmful to health may be necessary. There are different technologies to make water drinkable. They usually include various processes where all the water that is treated can go through filtration, coagulation, flocculation or decantation treatments. One of the methods used is water filtration with sand, in which only undissolved substances are removed. On the other hand, through chlorination, dangerous microbes are eliminated. There are more advanced water purification techniques such as reverse osmosis. There is also the desalination method, a process by which salt is removed from seawater, through physical and chemical processes; However, it is expensive,[175]​ due to the high cost of electrical energy it entails and is usually used more frequently in coastal areas with arid climates.

There are policies designed to allocate, distribute and manage water resources and water.[178]​ The availability of drinking water per capita has been decreasing due to several factors such as pollution, overpopulation, excessive irrigation, misuse[179]​ and the increasing rate of consumption.[180]​ For this reason, water is a strategic resource for the world and an important factor in many contemporary conflicts.[181]​[182]​ Undoubtedly, the Water scarcity has an impact on health[183]​ and biodiversity.[184]​.

Between 1990 and 2015, 2.6 billion people have gained access to a source of drinking water.[9] It has been estimated that the proportion of people in developed countries with access to safe water has increased from 30% in 1970[8] to 71% in 1990, and from 79% in 2000 to 84% in 2004.[185] 2015, it reached 91%.[9]​ In 2017, the UN predicted that the expenditure necessary for universal access to water is about 114 billion dollars per year.[186]​.

According to a 2006 United Nations report, "there is enough water for everyone globally," but access has been hampered by corruption and mismanagement.[187]

In UNESCO's World Water Development Report (WWDR, ​​2003) of its World Water Assessment Programme, WWAP predicts that in the next twenty years the amount of water available to everyone will decrease to 30%; Indeed, 40% of the world's population has insufficient drinking water for basic hygiene. More than 2.2 million people died in 2000 as a result of waterborne diseases (related to drinking contaminated water) or drought. In 2004, the non-profit organization WaterAid reported that every 15 seconds a child dies from preventable water-related illnesses[188] and is usually due to the lack of a wastewater treatment system.

There are several international conventions related to water, such as the United Nations Convention to Combat Desertification (UNCCD), the International Convention to Prevent Pollution from Ships, the United Nations Convention on the Law of the Sea, the Ramsar Convention, and the Water Convention. World Water Day is celebrated on March 22[189]​ and World Oceans Day is celebrated on June 8.

Water is considered a purifying element in most religions. Many religious doctrines incorporate a ritual of washing or ablutions: Christianity, Hinduism, the Rastafari movement, Islam, Shintoism, Taoism and Judaism. One of the central sacraments of Christianity is baptism, which is performed by immersion, sprinkling or pouring a person into water. This practice is also carried out in other religions such as Judaism where it is called mikveh and in Sikhism where it takes the name Amrit Sanskar. Likewise, in many religions including Judaism and Islam, ritual purification baths are performed on the dead in water. According to Islam, the five prayers a day (or salat) must be carried out after washing certain parts of the body using clean water or abdesto; however, in case there is no clean water they are performed ablutions with dust or sand which are called tayammum. In Shintoism, water is used in almost all rituals to purify a person or a place, as is the case of the misogi ritual. Ethnologists such as Frazer have highlighted the purifying role of water in the culture.[190]

Many religions also consider some sources or bodies of water to be sacred or at least favorable; and some examples include: the city of Lourdes "Lourdes (High Pyrenees)") according to Catholicism, the Jordan River (at least symbolically) in some Christian churches, the well of Zamzam in Islam, and the Ganges River in Hinduism and other cults of the region. Several cults use specially prepared water for religious purposes, such as holy water in some Christian denominations or amrita in Sikhism and Hinduism. Ancient mythologies and religions also attributed spiritual powers to water; In Celtic mythology, Sulis "Sulis (goddess)") is the goddess of hot springs; In Hinduism, the Ganges is personified by a goddess, and according to the Vedic texts the Hindu goddess Sárasuati represents the river of the same name. Water is also in Vishnuism one of the five basic elements or mahābhūta, which include: fire, earth, space and air. Alternatively, the gods may be considered patrons of fountains, rivers or lakes. In fact, in Greek and Roman mythology, Peneus "Peneus (mythology)") was the river god, one of the three thousand rivers or sometimes included among the three thousand Oceanids. In Islam, water is not only the source of life, but each life is considered to be composed of water: "And what do we get every living being out of water?"[191]​[192]​.

Regarding philosophy, Thales of Miletus, one of the seven Greek wise men, who stated that water was the ultimate substance, the Arjé, of the cosmos, where everything is made up of water. Empedocles, a philosopher of ancient Greece, held the hypothesis that water is one of the four classical elements along with fire, earth "Earth (element)") and air "Air (element)"), and was considered the basic substance of the universe or ylem. According to the theory of the four humors, water is related to phlegm. In traditional Chinese philosophy, water is one of the five elements along with earth, fire, wood and metal.

Water also plays an important role in literature as a symbol of purification. Some examples include a river as the central axis where the main actions take place, as is the case in the novel As I Lay Dying by William Faulkner and the drowning of Ophelia in Hamlet.

In Jungian psychology, water is the symbol of the unconscious par excellence, it represents the depth of the shadow in the most primitive part of our psyche. To reach the heights it is necessary to descend to the depths and face them, only then will it be possible to be reborn and rise. Immersing yourself in the waters of the psyche means entering into the darkness and the unknown, there is our unconscious, and in its waters we must navigate to reach what Carl Jung called Individuation.

• - Wikimedia Commons hosts a multimedia category on Water.

• - The Dictionary of the Royal Spanish Academy has a definition for water.

• - Unesco web portal on water.

• - Observatory of public policies in Mercosur.

• - European Community Legislation for water protection.

Water in the Universe

Water is a fairly common compound in our solar system,[43]​ and in the universe,[43][44]​ where it is found mainly in the form of ice and vapor "Vapor (state)"). It constitutes a large part of the material that makes up comets and in 2016 "magmatic water" was found from the interior of the Moon in small mineral grains on the lunar surface.[45] Some satellites of Jupiter and Saturn, such as Europa "Europa (satellite)") and Enceladus "Enceladus (satellite)"), possibly have liquid water under their thick layer of ice.[43]​.

Most of the water that exists in the universe may have arisen as a byproduct of the formation of stars that subsequently produced water vapor when they exploded. The birth of stars usually causes a strong flow of gases and cosmic dust. When this material collides with the gas in the outer areas, the shock waves produced compress and heat the gas. It is thought that water is produced in this warm, dense gas.[46]​.

Water has been detected in interstellar clouds within our Milky Way galaxy. These interstellar clouds can sometimes condense into the form of a solar nebula. In addition, it is thought that water may be abundant in other galaxies, given that its components (hydrogen and oxygen) are among the most common in the universe.[47]​ In the first decade of the century, water was found on exoplanets, such as HD 189733 b[48]​[49]​ and HD 209458 b.[50]​.

In July 2011, the journal Astrophysical Journal Letters published the discovery by a group of astronomers from NASA's Jet Propulsion Laboratory (JPL) and the California Institute of Technology (CALTECH) of a cloud of water vapor surrounding the quasar APM 08279+5255, which represents the largest reserve of water in the Universe discovered to date, about 140 billion times more than in the land.[51]​.

Water in the solar system

Water vapor has been detected in the atmosphere of several planets, satellites and other bodies in the solar system, as well as in the Sun itself. Several examples are listed below:

• - Mercury "Mercury (planet)"): It has been detected in high proportions in the exosphere.[52]​.

• - Venus "Venus (planet)"): 0.002% in the atmosphere.[53][54]​.

• - Earth: reduced quantities in the atmosphere, subject to climatic variations.

• - Mars "Mars (planet)"): Variable quantities depending on the place and season of the year.[55]​.

• - Enceladus "Enceladus (satellite)") (Saturn's moon): 91% of its atmosphere.[56]​.

Water in its liquid state is abundant on Earth, where it covers 71% of the surface. In 2015, NASA confirmed the presence of liquid water on the surface of Mars.[57]​.

There are indications that Saturn's moon Enceladus has a 10 km deep liquid ocean about 30-40 km below the satellite's south pole; interior.[61]​[62]​ On Ganymede "Ganymede (satellite)"), another moon of Jupiter, there could also be liquid water between layers of high-pressure ice and rock.[63]​ In 2015, the New Horizons space probe found signs of water inside Pluto "Pluto (dwarf planet)").[64]​.

Regarding ice, it exists on Earth, especially in polar areas and glaciers; In the polar caps of Mars "Mars (planet)"), solid water is also found, although they are mainly composed of dry ice. Ice is likely to be part of the internal structure of planets such as Uranus "Uranus (planet)"), Saturn "Saturn (planet)"), and Neptune "Neptune (planet)"). Ice forms a thick layer on the surface of some satellites, such as Europa "Europa (satellite)") and Titan "Titan (satellite)"), where it can reach 50 km thick.[65]​.

Ice also exists in the material that forms Saturn's rings,[66]​ in comets[67]​ and objects of meteoric origin, arriving for example from the Kuiper Belt or the Oort Cloud. Ice has been found on the Moon, and on dwarf planets such as Ceres "Ceres (dwarf planet)") and Pluto.[68]​[64]​.

Water and the habitable zone

The existence of water in a liquid state is necessary for terrestrial living beings and its presence is considered an important factor in the origin and evolution of life on the planet.[69]​[70]​ The Earth is located in an area of ​​the solar system that meets very specific conditions, but if it were 5% – eight million kilometers – closer or further from the Sun, it could not house water in a liquid state, only water vapor or ice.[69]​[71]​.

The mass of the Earth also plays an important role in the state of water on the surface: the force of gravity prevents gases in the atmosphere from dispersing. Water vapor and carbon dioxide combine, causing what is known as the greenhouse effect, which maintains the stability of temperatures, acting as a protective layer for life on the planet. If the Earth were smaller, the lower gravity exerted on the atmosphere would make it less thick, which would result in extreme temperatures and prevent the accumulation of water except in the polar caps, as occurs on Mars. On the other hand, if the mass of the Earth were much greater, water would remain in a solid state even at high temperatures, given the high pressure caused by gravity.[72] Therefore, both the size of a planet and the distance from the star are factors in the extent of the habitable zone.

La Tierra se caracteriza por contener un alto porcentaje de su superficie cubierta por agua líquida, y el volumen total ocupa 1 400 000 000 km³. Este líquido se mantiene constante gracias al ciclo hídrico. Se piensa que el agua formaba parte de la composición de la tierra primigenia[73]​ y apareció en la superficie a partir de procesos de desgasificación del magma del interior de la tierra y de condensación del vapor de agua al enfriarse el planeta, aunque no se descartan aportes de agua por impactos con otros cuerpos solares.[74]​.

Distribution of water in the Earth's mantle

The Earth's mantle contains an undetermined amount of water, which according to sources is between 35% and 85% of the total.[75] This substance can be found practically anywhere in the biosphere and in the three states of aggregation of matter: solid, liquid and gaseous. Water present in any state above or below the surface of the planet, including underground, forms the hydrosphere, which is subject to a complex dynamics of transport and change of state that defines a water cycle.

Saltwater oceans and seas cover 71% of the Earth's surface. Only 3% of Earth's water is fresh, and of this volume, only 1% is in a liquid state. The remaining 2% is found in a solid state in layers, fields and ice shelves or ice packs in the latitudes close to the poles. Outside the polar regions, fresh water is found mainly in wetlands and, underground, in aquifers. According to a study published in the journal Nature Geoscience, it is estimated that the total groundwater on the planet represents a volume of 23 million cubic kilometers.[76]​.

In total, the Earth contains about 1,386,000,000 km³ of water[n. 3]​ which are distributed as follows:[4]​.

Water plays a very important role in geological processes. On the surface, water acts as a very active agent on chemical and physical erosion processes. Water in its liquid state and, to a lesser extent, in the form of ice, is also an essential factor in sediment transport. The deposit of these remains is a tool used by geology to study the formative phenomena that occurred on Earth.[77]​.

The water cycle

The water cycle—scientifically known as the hydrological cycle—is the continuous exchange of water within the hydrosphere, between the atmosphere, surface and groundwater, and living organisms.

Water constantly changes its position from one part of the water cycle to another and numerous components can be distinguished[78]​ that basically involve the following physical processes:

• - evaporation from the oceans and other masses of water and transpiration of living beings (animals and plants) into the atmosphere.

• - precipitation "Precipitation (meteorology)"), caused by the condensation of water vapor, and which can take multiple forms.

• - transport of water by surface runoff or by underground flows after infiltration into the subsoil.

The sun's energy heats the water, generating the energy necessary to break the bonds between the liquid water molecules, which thus pass into the gaseous state. The evaporated water rises to the upper layers of the atmosphere where it cools until it condenses and forms clouds composed of tiny droplets. Under certain conditions, these small water particles join together to form larger droplets that cannot be kept suspended by rising air currents and fall as rain or hail or snow depending on the temperature. 90% of the water vapor present in the atmosphere comes from evaporation from the oceans, where most of it returns directly; However, the wind moves 10% towards the mainland, in which the volume of precipitation thus exceeds the volume of evaporation, coming mainly from aquatic bodies and the transpiration of living beings, predominantly from plants.[78]​.

Some of the water that falls on the land as rain or from snowmelt seeps into the ground or evaporates, but about a third moves along the surface along the slope.[78] Runoff water often forms basins (watersheds), where smaller watercourses often join together to form rivers. The constant movement of water masses over different geological terrains is a very important factor in the formation of the relief. In the parts of the course with a high slope, the rivers drag minerals during their movement, which they deposit in the lower parts of the course. Therefore, rivers play a very important role in enriching the soil. Part of the waters of these rivers are diverted for agricultural use. Rivers flow into the sea forming estuaries or deltas.[79]​ Groundwater, for its part, can rise to the surface as springs or descend into deep aquifers, where it can remain for millennia.[78]​.

Water can occupy land with disastrous consequences: Floods occur when a mass of water exceeds its usual margins or when it communicates with a larger mass—such as the sea—in an irregular manner. On the other hand, and although the lack of precipitation is a major obstacle to life, it is natural that some regions periodically suffer droughts. When dryness is not temporary, vegetation disappears, while soil erosion accelerates. This process is called desertification[80]​ and many countries adopt policies[81]​ to stop its advance. In 2007, the UN declared June 17 as the World Day to Combat Desertification and Drought.[82]​.

the ocean

The ocean encompasses the part of the Earth's surface occupied by sea water. There are several theories about its formation. There are indications that it comes from the water present in the interior of the planet, transported to the surface in the form of water vapor by volcanic processes,[83]​ but it is not ruled out that its origin is in collisions with bodies rich in water during the formation of the solar system.[84]​ During the different geological eras the distribution of ocean waters has constantly varied. During the Cenozoic, the Antarctic, Arctic, Atlantic, Indian and Pacific oceans, as well as the seas, smaller bodies of salt water, reached their current configuration.[n. 4]​.

It covers 71% of the Earth's surface and the average depth is about 4 km. In the Mariana Trench, it reaches a depth of 11,033 m.[85] In the oceans there is a surface layer of water at about 17 °C on average, although the temperature varies significantly between the equatorial and tropical areas, where it can reach 36 °C, and the polar areas, where it drops to around −2 °C, a temperature at which it freezes. The surface water layer, whose thickness is normally about four or five hundred meters, is maintained at an almost constant temperature, until reaching an area, called the thermocline, where a rapid drop in temperature occurs. Below this limit zone, the temperature drops to 3 and 0 °C.[86]​.

The oceans contain many elements in solution, although most are found in minute concentrations. The most abundant are sodium and chlorine, which, in their solid form, combine to form sodium chloride or common salt, which represents 80% of the salts dissolved in seawater. These elements are followed in order of abundance by magnesium - 4% -, sulfur, mainly in the form of sulfates, calcium, potassium, bromine, strontium, boron and fluorine.[87]​.

Tides are cyclical movements of large masses of water caused by the gravitational force of the moon and the sun. Tides are due to current movements of large masses of water, which oscillate within a constant range of hours. The tide is perceptibly reflected in a notable variation in the height of sea level - among other things - caused by the relative positions of the Sun and the Moon in combination with the effect of the Earth's rotation and local bathymetry.[88] The strip of sea subject to these changes - exposed at low tide and covered at high tide - is called the intertidal zone and represents an ecological niche of great value.[89]​.

Fresh water in nature

Fresh water in nature is renewed thanks to the atmosphere, which has 13,900 km³ of water vapor, 10% of the planet's fresh water, excluding groundwater, ice in the polar caps and permafrost. It is a dynamic volume that is constantly increasing in the form of evaporation and decreasing in the form of precipitation, with the annual volume in the form of precipitation estimated to be between 113,500 and 120,000 km³ in the world. In countries with a temperate and cold climate, precipitation in the form of snow represents an important part of the total.[90]​.

68.7% of the world's fresh water is in glaciers and ice sheets. Those present in Antarctica, the Arctic and Greenland, despite their size, are not considered water resources due to their inaccessibility. On the other hand, continental glaciers are an important part of the water resources of many countries.[90]​.

Surface waters encompass lakes, reservoirs, rivers and wetlands, accounting for only 0.3% of the planet's fresh water; however, they represent 80% of annually renewable freshwater, hence its importance.[90]​.

Stored fresh groundwater, which represents 96% of the Earth's unfrozen fresh water, is also an important resource. According to Morris, the groundwater systems used to supply populations account for between 25 and 40% of the total drinking water supplied. Thus, half of the world's large megalopolises depend on them for their consumption. In areas where there is no other source of supply, it represents a form of quality supply at low cost.[90]​.

The largest source of fresh water in the world suitable for consumption is Lake Baikal in Siberia, which has a very low salt and calcium index and is not yet contaminated.[91]​.

El agua es la molécula más común en todos los seres vivos en la Tierra; la masa de la mayoría de los organismos contiene entre un setenta y noventa por ciento de agua, aunque el porcentaje varía considerablemente según la especie, la etapa de desarrollo del individuo y, en organismos multicelulares complejos, el tipo de tejido.[92]​ Las algas llegan al 98 % de agua en peso, mientras que los pinos contienen un 47 %. El cuerpo humano incluye entre un 65 % a un 75 % de agua en peso, y el porcentaje es menor a medida que la persona crece. El contenido en los tejidos varía entre el 99 % del líquido cefalorraquídeo y el 3 % de la dentina.[93]​[94]​.

El agua desempeña un papel biológico importante y todas las formas de vida conocidas dependen del agua a nivel molecular. Sus propiedades como disolvente posibilitan las diversas reacciones químicas de los compuestos orgánicos cruciales para todas las funciones vitales, el transporte de moléculas a través de las membranas y para disolver los productos de excreción.[95]​ También es un agente activo esencial en muchos de los procesos metabólicos de los seres vivos. La extracción de agua de moléculas —mediante reacciones químicas enzimáticas que consumen energía— permite la síntesis de macromoléculas complejas, como los triglicéridos o las proteínas; el agua actúa asimismo como agente catabólico sobre los enlaces entre átomos, reduciendo el tamaño de moléculas como glucosas, ácidos grasos y aminoácidos, y produciendo energía en el proceso. Es un compuesto esencial para la fotosíntesis. En este proceso, las células fotosintéticas utilizan la energía del sol para separar el oxígeno y el hidrógeno presentes en la molécula de agua; el hidrógeno se combina con CO —absorbido del aire o del agua— para formar glucosa, liberando oxígeno en el proceso.[96]​ El agua, por su carácter anfiprótico es también el eje de las funciones enzimáticas y la neutralidad respecto a ácidos y bases. La bioquímica en muchos medios intracelulares funciona de manera ideal alrededor de un valor pH de alrededor de 7,0 hasta 7,2.[95]​.

Aquatic life

The various theories on the origin of life agree that it had its origin in the oceans, either in surface waters thanks to the energy supplied by solar radiation, cosmic rays and even electrical discharges from the atmosphere or in the deep sea, next to the hydrothermal vents of oceanic trenches,[97]​[98]​ It is estimated that 25% of all species are aquatic.[99]​ Bacteria are particularly abundant in water and a 2006 study counted about 20,000 species per liter of seawater.[100]​ These microorganisms, together with phytoplankton, are the basis of the underwater food chain, so water provides not only the environment, but also the sustenance of all marine fauna.[101]​.

Aquatic animals must obtain oxygen to breathe, extracting it from the water in various ways. Pulmonary breathing vertebrates, such as mammals, birds, reptiles and amphibians in their adult phase, take in air out of water and hold their breath when submerged. Most multicellular aquatic fauna, however, use gills to extract oxygen from the water. Some species such as dipnoos have both respiratory systems. Some invertebrate organisms lack a respiratory system and absorb oxygen directly from water by diffusion.[102]​.

La historia muestra que las primeras civilizaciones florecieron en zonas favorables a la agricultura, como las cuencas de los ríos. Es el caso de Mesopotamia, considerada la cuna de la civilización humana, surgida en el fértil valle del Éufrates y el Tigris; y también el de Egipto, que dependía por completo del Nilo y sus periódicas crecidas. Muchas otras grandes ciudades, como Róterdam, Londres, Montreal, París, Nueva York, Buenos Aires, Shanghái, Tokio, Chicago y Hong Kong deben su riqueza a la conexión con alguna gran vía de agua que favoreció su crecimiento y su prosperidad. Las islas que contaban con un puerto natural seguro —como Singapur— florecieron por la misma razón. Del mismo modo, las áreas en las que el agua es muy escasa tienen dificultades de desarrollo, a no ser que posean otros recursos en grandes cantidades.[103]​.

Water as a human right

The General Assembly of the United Nations approved on July 28, 2010, in its sixty-fourth session, a resolution that recognizes drinking water and basic sanitation as a human right essential for the full enjoyment of life and all human rights.[104]​[n. 5] This resolution was preceded, in November 2002, by "General Comment No. 15 on the right to water", which establishes the right to affordable access to water as an indispensable condition for "a dignified human life." Article I.1 establishes that "The human right to water is indispensable for a dignified human life."[104]​.

In the 2010 General Assembly resolution, the number of people without access to drinking water was estimated at 884 million, and the number of people without basic sanitation at more than 2,600,000,000. Likewise, it estimated that some 1.5 million children under 5 years of age died annually as a result of lack of water.

Drinking water: need of the human body

The human body is composed of between 55% and 78% water, depending on its measurements and complexion.[105]​ Metabolic activity, such as the oxidation of fats or carbohydrates, generates a certain amount of water; However, metabolic water is insufficient to compensate for losses through urine, feces, sweat, or exhalation of breath, so to maintain the body's water balance it is necessary to consume water. Water can be absorbed from both liquid drinks and foods, among which fresh fruits and vegetables contain the highest percentage, up to 85%, similar to that of many beverages, while cereals or nuts usually consist of only 5% water.[106]​.

Water is also useful for lubricating joints, facilitating the digestion process, and keeping organs functioning and in good condition.[107]​.

To avoid problems associated with dehydration, a document from the Food and Nutrition Platform of the National Research Council of the United States recommended in 1945 consuming one milliliter of water for each calorie of food.[108]​ The last reference offered by this same organization speaks of 2.7 liters of water per day for a woman and 3.7 liters for a man, including the consumption of water through food.[109]​ Naturally, during the During pregnancy and breastfeeding, women must consume more water to stay hydrated. According to the Institute of Medicine - which recommends an average of 2.2 liters/day for a woman, and 3.0 liters/day for a man - a pregnant woman should consume 2.4 liters, and up to 3 liters during breastfeeding, considering the large amount of fluid lost during this period. Smaller minimum amount, typically one liter of water daily for an adult male individual.[113]​ In any case, exact amount will vary depending on activity level, temperature, humidity, diet, and other factors.

Excessive water intake—for example, during physical exercise—can cause hyperhydration, or water intoxication, a condition that can be dangerous. There are several unproven myths about water consumption and health, such as the alleged relationship between water consumption, weight loss and constipation.[114]​.

Unlike water losses through the skin or lungs, the volume excreted with urine is subject to strict control, carried out in the kidneys. The percentage of water present in urine can vary greatly, depending on the amount of waste substances, such as minerals and urea, to be excreted. The maximum concentration or osmolarity of these solutes in the urine is 1200 mOsm/L, which defines the minimum volume of liquid necessary for their elimination, regardless of the hydration status of the organism.[106]​.

Drinking water is one of the main transmitters of disease-causing microorganisms, mainly bacteria, viruses and intestinal protozoa. The great epidemics of humanity have prospered due to water pollution. From references, it is known that boiling water was recommended since five hundred years before our era.[115]​.

Currently, in developed countries, the problems posed by contaminated water are practically controlled. Water filtration and disinfection processes prior to human consumption were imposed in the 20th century and are estimated to be responsible for a 50% increase in life expectancy in developed countries in the last century. Life magazine considered water chlorination and filtration as probably the most important public health progress of the millennium. There are several agents that can be used to disinfect water, including peroxide, chlorine compounds and other halogens, silver-copper, ozone and ultraviolet radiation.[116]​.

Chlorine, either in gas form or as hypochlorite, is the material most used as a water disinfectant, due to its oxidizing properties. Once it has crossed the membrane of the microorganisms, the chlorinated compounds are eliminated by oxidation by their respiratory enzymes.[117]​.

Chlorine can be irritating to the mucous membranes and skin, so its use is strictly monitored. The proportion used varies between 1ppm when it comes to purifying water for consumption, and between 1-2 ppm for the preparation of bathing water. Improper application of chemical components in water can be dangerous. The application of chlorine as a disinfectant began in 1912 in the United States. The following year Wallace and Tiernan designed equipment that could measure chlorine gas and form a concentrated solution that was added to the water to be treated. Since then the chlorination technique has continued to progress. In addition to its germ-destroying capacity, its action is also very beneficial in the elimination of iron, manganese, hydrogen sulfides, sulfides and other reducing substances from water. Many countries in their regulations establish disinfection using chlorine and require the maintenance of a certain residual concentration of disinfectant in their water distribution pipe networks. Chloramines are sometimes used as a secondary disinfectant to maintain a certain concentration of chlorine in the drinking water supply system for a longer period of time.[118]​.

The world population has increased from 2,630,000,000 in 1950 to 6,671,000,000 in 2008. In this period, the urban population has increased from 733,000,000 to 3,505,000,000. It is in human settlements where non-agricultural water use is concentrated and where most water-related diseases are contracted. water.[119]​.

Given the difficulty of having drinking water for human consumption in many places on the planet, an intermediate concept has been consolidated, safe water as water that does not contain dangerous bacteria, dissolved toxic metals, or chemicals harmful to health, and is therefore considered safe to drink, therefore it is used when the supply of drinking water is compromised. It is a water that is not harmful to humans, although it does not meet the ideal conditions for consumption.

For various reasons, the availability of water is problematic in much of the world, and for this reason it has become one of the main concerns of governments around the world. Currently, it is estimated that around one billion[120]​ people have poor access to drinking water. This situation is aggravated by the consumption of water in poor conditions, which favors the proliferation of diseases and epidemic outbreaks. Many of the countries meeting in Evian at the XXIX G8 conference set 2015 as the deadline to achieve universal access to better water around the world.[121] Even if this difficult goal were achieved, it is estimated that there would still be around 500 million without access to drinking water, and more than a billion would lack an adequate sanitation system. Poor water quality and irregular sanitation seriously affect the health status of the population: the consumption of contaminated water alone causes 5,000,000 deaths a year, according to several reports[122]​ from the United Nations, which declared 2005-2015 the Decade of Action. The WHO estimates that the adoption of safe water policies could prevent the deaths of 1,400,000 children a year from diarrhea.[123]​[124]​ 50 countries, which account for almost a third of the world's population, lack an adequate water supply,[125]​ and 17 of them annually extract more water from their aquifers than can be renewed naturally.[126]​ Pollution, on the other hand, does not only pollute. the water of rivers and seas, but the underground water resources that serve as a supply for human consumption.[127]​.

Domestic water use

In addition to human beings needing water for their existence, they need water for their own hygiene and cleaning. It has been estimated that humans directly or indirectly consume about 54% of the world's available surface freshwater. This percentage is broken down into:

• - 20%, used to maintain fauna and flora, for the transportation of goods (boats) and for fishing, and.

• - the remaining 34%, used as follows: 70% in irrigation, 20% in industry and 10% in cities and homes.[128]​[129]​.

Direct human consumption represents a small percentage of the volume of water consumed daily in the world. It is estimated that an inhabitant of a developed country consumes around five liters a day in the form of food and drinks.[130] These figures rise dramatically when total domestic consumption is considered. An approximate calculation[131]​ of water consumption per person/day in a developed country, considering domestic industrial consumption, yields the following data:

These consumption habits and the increase in population in the last century have simultaneously caused an increase in water use. This has caused the authorities to carry out campaigns for the good use of water. Currently, raising awareness is a task of great importance to guarantee the future of water on the planet, and as such it is the subject of constant activities at both the national and municipal levels.[132]​ On the other hand, the enormous differences in daily consumption per person between developed and developing countries[133]​ indicate that the current water model is not only ecologically unviable: it is also unviable from a humanitarian point of view,[134]​ which is why numerous NGOs are strive[135]​ to include the right to water among human rights.[136]​ During the V World Water Forum, convened on March 16, 2009 in Istanbul (Turkey), Loic Fauchon (president of the World Water Council) stressed the importance of regulating consumption in these terms:.

1. • Avoid throwing any type of oil down the drains.

2. • Keep all water taps closed while scrubbing/washing/soaping.

3. • Address leaks. Some are not visible, but can be seen on the consumption receipt.

4. • Reduce the use of the bathtub.

5. • Take 5-8 minute showers.

6. • Collect the water that comes out of the shower (which is usually wasted before starting to bathe, and use the water to wash the car, water the garden, etc.

Water in agriculture

According to the FAO, agriculture accounts for 69% of the total water extracted in the world, a percentage that in some arid areas can exceed 90%. The need for water resources for food production must be reconciled with the demand from other sectors, such as use in urban areas and the preservation of ecosystems.[139]​ In many places, agriculture places significant pressure on natural bodies of water, and the water required for irrigation represents a decrease in the natural flows of rivers and a decrease in groundwater levels that cause a negative effect on ecosystems. aquatic.[140]​.

According to UNESCO data, less than 20% of irrigation water reaches the plant; The rest is wasted and also transports waste with toxic substances that inevitably end up in rivers.[141]​ The use of nitrates and pesticides in agricultural work represents the main diffuse contamination of both surface and underground water bodies. The most significant is nitrate pollution, which causes water eutrophication. In Spain, the annual consumption of fertilizers is estimated at 1,076,000 tons of nitrogen, 576,000 tons of phosphorus and 444,000 tons of potassium. Although most of the fertilizers are absorbed by crops, the rest is a potential water contaminant.[140]​.

Because agriculture is an ancient production system, it has been adapted to the different water regimes of each region: Thus, in areas where there is abundant rainfall "Precipitation (meteorology)") irrigated crops are usually grown, while in drier areas rainfed crops are common. Given that irrigated lands are approximately three times more productive than rainfed lands, investments in the development of irrigation infrastructure and water resource management are important for the sustainable development of agriculture.[139]​ This development occurs very unevenly in different parts of the world. For example, in Africa, only 7% of the arable area is irrigated, while in Asia, it is 38%.[139]​.

More recently, new forms of cultivation and irrigation have been experimented with aimed at minimizing water use. Localized irrigation techniques - drip or sprinkler -, greenhouse agriculture under controlled environmental conditions and the selection of varieties genetically adapted to dry climates, are part of these practices.[142] Currently one of the most active aspects of genetic research attempts to optimize the water consumption of the species that man uses as food.[143]​ In space agriculture experiments"), as the cultivation of plants in the conditions of space stations is known, have also been developed. technologies that limit water consumption between 25 and 45%.[144]​ Agroforestry and mortars are solutions to build microclimates and allow the circulation of water to the interior of the land thanks to the evapotranspiration phenomena of the plants. For example, a hectare of beech forest, which consumes between 2,000 and 5,000 tons of water per year, returns 2,000 tons per year. evaporation.[145]​.

The use of water in industry

The industry requires water for multiple applications, such as for heating and cooling in heat exchangers, to produce water vapor in steam turbines or as a solvent, as raw material or for cleaning. Pressurized water is used in hydrodemolition equipment, in water jet cutting machines, and is also used in high-pressure water guns to efficiently and precisely cut various materials such as steel, concrete, reinforced concrete, ceramics, etc. and as a cooling liquid to prevent overheating of machinery such as electric saws "Saw (tool)") or between elements subjected to intense friction. After use, most of it is disposed of, returning it back to nature. Sometimes discharges are treated, but other times industrial wastewater contaminated with heavy metals, chemicals or organic matter returns to the water cycle without adequate treatment, which has a negative impact on water quality and the aquatic environment.[146] Indirect pollution can also occur: through solid waste containing contaminated water or other liquids, leachate, which ends up leaking into the ground and contaminating aquifers if they are not isolated. properly.[147]​ Thermal pollution also occurs due to the discharge of water used as coolant.

The largest consumers of water for industry in 2000 were: the United States (220.7 km³); China (162 km³); Russian Federation (48.7 km³); India (35.2 km³); Germany (32 km³); Canada (31.6 km³) and France (29.8 km³). In Spanish-speaking countries, the highest consumption occurred in Spain (6.6 km³); Mexico (4.3 km³); Chile (3.2 km³) and Argentina (2.8 km³).[148]​ Global industrial water consumption exceeds domestic water consumption by more than double.[149]​.

Water is used for the generation of electrical energy. Hydroelectricity is that which is obtained through hydraulic energy. Hydroelectric energy is produced when water previously impounded in a dam falls by gravity into a hydroelectric power plant, rotating a turbine geared to an electric power alternator in this process. This type of energy is low cost, does not produce pollution, and is renewable, although the construction of reservoirs has an environmental impact.[150]​[151]​.

Water and steam are used as heat transmitters in various heat exchange systems, due to their abundance and high heat capacity, which allows them to absorb large amounts of heat energy without excessively changing their temperature.[152]​ Condensed steam is an efficient heater due to its high latent heat.[153]​ The disadvantage of water and steam is that, without treatment, they are corrosive to many metals, such as steel and copper. In most power plants, water is used as a coolant, either by heat exchange or evaporation.

In the nuclear industry, water can be used as a nuclear moderator. In a pressurized water reactor, water acts as a coolant and moderator. This increases the effectiveness of the nuclear power plant's passive safety system, since the water slows down the nuclear reaction, maintaining the chain reaction.[154]​.

The water used as a solvent

Water is often described as the universal solvent, because it dissolves many of the known compounds. However, it does not dissolve all the compounds.

In chemical terms, water is an effective solvent because it allows ions and polar molecules to dissolve "Polarity (chemistry)"). In the dissolution process, water molecules group around the ions or molecules of the substance to keep them away or dispersed. Anions or negatively charged portions of the molecule attract hydrogens present in the water molecule, while oxygens have an affinity for cations or positively charged surfaces.[162] Solvation or suspension "Suspension (chemistry)") of substances in water is used daily for washing clothing, floors, food, pets, automobiles, and the human body. The use of water as a cleaning solvent is very high in industrialized countries.

Water facilitates the biological and chemical processing of wastewater. The aqueous environment helps to break down contaminants, due to its ability to become a homogeneous solution, which can be treated flexibly. Microorganisms that live in water can access dissolved waste and feed on it, breaking it down into less polluting substances. For this, aerobic treatments are widely used by adding oxygen or air to the solution, increasing the speed of decomposition and reducing the reactivity of the harmful substances that compose it. Other examples of biological systems for wastewater treatment are sugarcane fields and anaerobic biodigesters. Generally, in chemical and biological waste treatments, solid residues remain from the treatment process. Depending on its composition, the remaining residue can be dried and used as fertilizer if its properties are beneficial, or it can be disposed of in a landfill "Landfill (garbage)") or incinerated.

Other uses

Water's high latent heat of vaporization and relatively low chemical reactivity make it an effective firefighting fluid. Water extinguishes fire by cooling, by absorbing the heat from combustion. Water also decreases the oxygen concentration as it evaporates, thus helping to smother the fire. However, the use of water to extinguish flames on electrical equipment is not recommended, due to its electrically conductive properties, which can cause electrocution. Likewise, it should not be used to extinguish liquid fuels or organic solvents, since they float in water and the explosive boiling of water tends to spread the fire.[163]​.

When using water to put out fires, the risk of a steam explosion must be considered, as it can occur when used in confined spaces and on overheated fires.[n. 6]​

The danger of an explosion must also be taken into account when certain substances, such as alkali metals or hot graphite, decompose in water producing hydrogen.

Humans use water for various recreational purposes, including exercise and sports. Some of these sports include swimming, water skiing, sailing, surfing and jumping. There are also other sports that are practiced on an ice surface such as ice hockey and ice skating.

Lakeshores, beaches, and water parks are popular places for relaxation and fun. The sound of flowing water has a calming effect, due to its white noise nature.[164] Other people keep aquariums or ponds with fish and marine life for fun, company, or for display. Humans also practice snow sports such as skiing or snowboarding. It is also used for fighting games by throwing snowballs, water balloons), and even with the use of water guns.

The fountains "Fountain (architecture)") and canals, originally built to facilitate the use of water for human consumption, irrigation and transportation, have become ornamental elements to decorate public or private places.[165]​.

On April 7, 1795, the gram was defined in France as "the absolute weight of a volume of pure water equal to a cube of one hundredth of a meter, at the melting temperature of ice."[166] For practical reasons, a thousand-fold larger reference measurement for metals and other solids became popular. The task assigned was therefore to accurately calculate the mass of a liter of water. Despite the fact that the definition of the gram itself specified 0 °C, a very stable temperature point, scientists preferred to redefine the standard and make their measurements based on the maximum density of water, that is, around 4 °C.[167]​.

The SI Kelvin temperature scale is based on the triple point of water or 273.16 K (0.01 °C).[168] The Kelvin scale is based on the same increment as the Celsius scale, defined by the boiling point (100 °C) and melting point (0 °C) of water at atmospheric pressure.

Natural water is composed mainly of the isotopes hydrogen-1 and oxygen-16, but also contains a small amount of heavier isotopes such as hydrogen-2 or deuterium. The amount of deuterium oxides in heavy water is also very small, but it greatly affects the properties of the water. River and lake water usually has less deuterium than sea water. For this reason, a water standard was defined according to its deuterium content, known as the Vienna Standard Mean Ocean Water or VSMOW ().[169]​.

Pollution and water purification

The deposition of untreated waste in the atmosphere, on land and in water causes the pollution of precipitation, surface waters, groundwater and the degradation of natural ecosystems.[170]​ Population growth and the expansion of economic activities have a negative effect on the ecosystems of coastal waters, rivers, lakes, wetlands and aquifers. Examples are the construction of new ports and urban areas along the coast, the alteration of river systems for navigation and for water storage reservoirs, the drainage of wetlands to increase agricultural area, the overexploitation of fishing grounds, the multiple sources of pollution from agriculture, industry, tourism and household wastewater. According to UNESCO data, between 1990 and 2006, water extraction has doubled the population growth rate. The quality of natural bodies of water is being reduced due to all these factors.[171]​.

In 2000, the UN General Assembly established eight goals for the future (Millennium Development Goals). Among them was reversing the trend of loss of environmental resources, since the need to preserve ecosystems was recognized, essential for maintaining biodiversity and human well-being, since obtaining drinking water and food depends on them.[172] To achieve this, in addition to sustainable development policies, purification systems are required that improve the quality of the waste generated by human activity. Water purification is the set of physical, chemical or biological treatments that improve water quality or eliminate or reduce pollution. There are two types of treatments: those that are applied to obtain quality water suitable for human consumption and those that reduce water pollution in discharges into nature after use. It is imperative to generate research projects to find viable and economical alternatives for the non-conventional treatment of this valuable resource.[173]​.

Water intended for human consumption is that used for drinking, cooking, preparing food or other domestic uses. Each country regulates by law the quality of water intended for human consumption. European law protects "people's health from the adverse effects derived from any type of contamination of water intended for human consumption, guaranteeing its health and cleanliness" and therefore it cannot contain any type of microorganism, parasite or substance, in a quantity or concentration that may pose a danger to human health. Thus, it must be totally free of Escherichia coli and Enterococcus bacteria, and its composition must meet certain restrictions, such as containing less than 50 milligrams of nitrates per liter of water or less than 2 milligrams of copper and other chemical substances.[174]​.

Drinking water is usually collected in reservoirs, springs or extracted from the ground through artificial tunnels or wells from an aquifer. Other sources of water are rainwater, rivers and lakes. However, water must be treated for human consumption, and the extraction of dissolved substances, undissolved substances and microorganisms harmful to health may be necessary. There are different technologies to make water drinkable. They usually include various processes where all the water that is treated can go through filtration, coagulation, flocculation or decantation treatments. One of the methods used is water filtration with sand, in which only undissolved substances are removed. On the other hand, through chlorination, dangerous microbes are eliminated. There are more advanced water purification techniques such as reverse osmosis. There is also the desalination method, a process by which salt is removed from seawater, through physical and chemical processes; However, it is expensive,[175]​ due to the high cost of electrical energy it entails and is usually used more frequently in coastal areas with arid climates.

There are policies designed to allocate, distribute and manage water resources and water.[178]​ The availability of drinking water per capita has been decreasing due to several factors such as pollution, overpopulation, excessive irrigation, misuse[179]​ and the increasing rate of consumption.[180]​ For this reason, water is a strategic resource for the world and an important factor in many contemporary conflicts.[181]​[182]​ Undoubtedly, the Water scarcity has an impact on health[183]​ and biodiversity.[184]​.

Between 1990 and 2015, 2.6 billion people have gained access to a source of drinking water.[9] It has been estimated that the proportion of people in developed countries with access to safe water has increased from 30% in 1970[8] to 71% in 1990, and from 79% in 2000 to 84% in 2004.[185] 2015, it reached 91%.[9]​ In 2017, the UN predicted that the expenditure necessary for universal access to water is about 114 billion dollars per year.[186]​.

According to a 2006 United Nations report, "there is enough water for everyone globally," but access has been hampered by corruption and mismanagement.[187]

In UNESCO's World Water Development Report (WWDR, ​​2003) of its World Water Assessment Programme, WWAP predicts that in the next twenty years the amount of water available to everyone will decrease to 30%; Indeed, 40% of the world's population has insufficient drinking water for basic hygiene. More than 2.2 million people died in 2000 as a result of waterborne diseases (related to drinking contaminated water) or drought. In 2004, the non-profit organization WaterAid reported that every 15 seconds a child dies from preventable water-related illnesses[188] and is usually due to the lack of a wastewater treatment system.

There are several international conventions related to water, such as the United Nations Convention to Combat Desertification (UNCCD), the International Convention to Prevent Pollution from Ships, the United Nations Convention on the Law of the Sea, the Ramsar Convention, and the Water Convention. World Water Day is celebrated on March 22[189]​ and World Oceans Day is celebrated on June 8.

Water is considered a purifying element in most religions. Many religious doctrines incorporate a ritual of washing or ablutions: Christianity, Hinduism, the Rastafari movement, Islam, Shintoism, Taoism and Judaism. One of the central sacraments of Christianity is baptism, which is performed by immersion, sprinkling or pouring a person into water. This practice is also carried out in other religions such as Judaism where it is called mikveh and in Sikhism where it takes the name Amrit Sanskar. Likewise, in many religions including Judaism and Islam, ritual purification baths are performed on the dead in water. According to Islam, the five prayers a day (or salat) must be carried out after washing certain parts of the body using clean water or abdesto; however, in case there is no clean water they are performed ablutions with dust or sand which are called tayammum. In Shintoism, water is used in almost all rituals to purify a person or a place, as is the case of the misogi ritual. Ethnologists such as Frazer have highlighted the purifying role of water in the culture.[190]

Many religions also consider some sources or bodies of water to be sacred or at least favorable; and some examples include: the city of Lourdes "Lourdes (High Pyrenees)") according to Catholicism, the Jordan River (at least symbolically) in some Christian churches, the well of Zamzam in Islam, and the Ganges River in Hinduism and other cults of the region. Several cults use specially prepared water for religious purposes, such as holy water in some Christian denominations or amrita in Sikhism and Hinduism. Ancient mythologies and religions also attributed spiritual powers to water; In Celtic mythology, Sulis "Sulis (goddess)") is the goddess of hot springs; In Hinduism, the Ganges is personified by a goddess, and according to the Vedic texts the Hindu goddess Sárasuati represents the river of the same name. Water is also in Vishnuism one of the five basic elements or mahābhūta, which include: fire, earth, space and air. Alternatively, the gods may be considered patrons of fountains, rivers or lakes. In fact, in Greek and Roman mythology, Peneus "Peneus (mythology)") was the river god, one of the three thousand rivers or sometimes included among the three thousand Oceanids. In Islam, water is not only the source of life, but each life is considered to be composed of water: "And what do we get every living being out of water?"[191]​[192]​.

Regarding philosophy, Thales of Miletus, one of the seven Greek wise men, who stated that water was the ultimate substance, the Arjé, of the cosmos, where everything is made up of water. Empedocles, a philosopher of ancient Greece, held the hypothesis that water is one of the four classical elements along with fire, earth "Earth (element)") and air "Air (element)"), and was considered the basic substance of the universe or ylem. According to the theory of the four humors, water is related to phlegm. In traditional Chinese philosophy, water is one of the five elements along with earth, fire, wood and metal.

Water also plays an important role in literature as a symbol of purification. Some examples include a river as the central axis where the main actions take place, as is the case in the novel As I Lay Dying by William Faulkner and the drowning of Ophelia in Hamlet.

In Jungian psychology, water is the symbol of the unconscious par excellence, it represents the depth of the shadow in the most primitive part of our psyche. To reach the heights it is necessary to descend to the depths and face them, only then will it be possible to be reborn and rise. Immersing yourself in the waters of the psyche means entering into the darkness and the unknown, there is our unconscious, and in its waters we must navigate to reach what Carl Jung called Individuation.

• - Wikimedia Commons hosts a multimedia category on Water.

• - The Dictionary of the Royal Spanish Academy has a definition for water.

• - Unesco web portal on water.

• - Observatory of public policies in Mercosur.

• - European Community Legislation for water protection.