In some cases there are hot water tables, water that comes from underground layers of the Earth that are at higher temperatures. Due to their temperature, they have the capacity to dissolve certain minerals contained in the soils in which they are and thus, many of them are rich in different mineral components and can be used for therapeutic uses, such as baths, inhalations, irrigations, (see Hydrotherapy) and also for heating buildings. They are generally found along fault lines since groundwater can enter along the fault plane, which heats up when reaching a certain depth and then rises in the form of steam (which can condense when it reaches the surface, forming a geyser) or hot water springs.

Normally the layers are recharged by rain that runs between the grains of earth that form the soil. Through permeable lands, water filters and reaches the layer. A borehole (a well) allows us to know the upper level of the layer (water table), the level above which the interstices of the earth are not saturated with water. Variations in the water level of the well show the degree of filling of the underground reservoir.

The level may vary due to natural causes (especially rainy or especially dry periods). In dry times, water from unsaturated layers reaches the upper layers of the soil by capillarity and can evaporate. Another way in which a layer loses water is due to plants that collect it through their roots and diffuse it into the atmosphere through evapotranspiration. In rainy seasons, however, the water filtered through the ground is usually easily greater than the water lost through evaporation. In this way, the water content of the layers can vary by natural means and, therefore, the depth of the water table. The layers often maintain a more or less stable water table, except for annual variations due to dry and rainy seasons.

The water content of the layer is often stable when the maximum water table is limited by natural water outlets from springs, such as stream or river sources, so that replenishments equal losses.

In any case, the most important thing is the influence of level variations due to artificial causes. If water is extracted from the aquifer by pumping, for irrigation, for example, at a rate greater than the rate at which water is replaced in the layer, a decrease in the height of the water will occur, and the layer may be exhausted.

There is also the possibility of artificially recharging the layers using surface water that is introduced by many different means into the water table. Among others, purified water can be used (filtration through the ground finishes the purification process) or water infiltrated through the bottom of a canal. Of course, direct pumping through wells is also used in the winter when there is an abundance of surface water.[3]​.

Another system is the use of seasonal rainwater, making a series of small dams along the bed of a stream "Torrente (hydrography)") or rambla "Rambla (geomorphology)") and at the bottom of each one a well, filled with sand, that reaches the aquifer, so that the dam retains the rainwater, which would be lost in the mother river or in the sea, long enough to filter, in whole or in part, to the layer phreatic. In addition, the system serves to moderate, in a certain way, possible floods in times of torrential rain or cold weather.

Another type of layer, also common, is the one formed by river channels when the land on their banks is permeable: they are recharged because the river water extends underground to distances that can become important (alluvial aquifers). Obviously, in times of river dryness the volume of water stored in these layers also decreases. An extreme case in Spain is that of the Guadiana whose channel disappears from the surface for a stretch, between Argamasilla de Alba and Villarrubia de los Ojos (both in the province of Ciudad Real)[4]​ forming a water table.

saturated zone

The saturated zone of the layers distinguishes between two types of water:

• - bound water or trapped water, actually linked to solids through molecular or electrostatic forces, the two processes being limited in space;.

• - free water, capable of moving due to gravity or under the effect of pressure.

The linked water corresponds to:.

• - the layer closest to the grains, in which the thickness is a few dozen molecules (0.1 micrometer) corresponds to an orientation of the water molecules with a H-OH dipolar structure, perpendicular to the surface of the grains. In this zone the viscosity of water can double or triple, its density is of the order of 1.5 and its pressure per unit area can be of the order of 10,000 bar.

• - the transition zone between 0.1 and 0.5 micrometers "Micrometer (unit of length)") has molecules subject to a non-negligible attraction and which are immobile.

Above, the forces of attraction are weaker and it is then free water.

Soil moisture is the amount of water per volume of land in a piece of land.

unsaturated zone

Unlike the saturated zone, which contains a liquid part (water) and a solid part (stone, earth), here there is a third component: air. The part of the holes occupied by water is considered saturated.

In the unsaturated zone, four states are distinguished depending on the water saturation in the ground:

• - funicular water or gravitic: in a terrain almost saturated with water, with some air bubbles, water can circulate under the influence of gravity.

• - saturated soil in equilibrium also called soil capable of capillary retention: the water phase is also continuous, but it does not circulate only due to the effect of gravity. This corresponds to water retained by capillarity.

• - pendular water: in a terrain slightly saturated with water, and occupies discontinuous rings at the points of contact between them. The water phase is always continuous, the pressures are transmitted, but the movements of the water are very slow due to the thinness of the wet film.

• - irreducible saturation: if the water leaves (evapotranspiration), there is nothing left but the bound water.

For most agricultural crops the water table must be in a certain range of depths, because at different depths (both greater and lesser) the yield is reduced.[7]​ For some important crops a classification has been developed:[8]​.

PNF = Depth of Water Table.

In Spain

The Los Arenales aquifer belongs to the Duero River basin in Castilla y León. It covers more than 7000 km² between the south of the Duero River, the Central system and the course of the Tormes River to the west.

The System 23 aquifer or Western La Mancha aquifer is the central unit and one of the key pieces of the hydrological system of the Upper Guadiana Basin. It covers an area of ​​5,500 km² with a depth of 70 m and more than 30 municipalities in the provinces of Ciudad Real, Albacete and Cuenca are located on it. It is, therefore, one of the largest aquifers in Spain.

In France

In France, the largest layer is the Beauce layer "Beauce (France)") whose surface area is about 9000 km², in six departments. Its reserves are estimated at about 20 thousand cubic hectometers.

The largest layer is that of the Rhine plain in Alsace, whose extension is small, but which has reserves estimated at 35,000 cubic hectometers in the Alsatian part alone.[9]​.

In the United States

The largest water table in the United States (the Ogallala Table), with an area comparable to that of France, stretches from South Dakota to Texas.

In South America

• - Guaraní Aquifer, in the Río de la Plata Basin.

Already in 2001, the American expert Lester R. Brown denounced that the level of the water tables was dropping on all continents, due to the availability of electric pumps or explosion engines that allow water to be extracted faster than it is replaced by rain. And he specifically cited three regions: the North China Plain, the Punjab in India/Pakistan, and the southern part of the prairies of the United States, which are irrigated agriculture regions.[10]​.

In 2002, NASA and the German Aerospace Center launched the GRACE (Gravity Recovery and Climate Experiment) mission mainly to evaluate, through satellite observations, the level of the groundwater tables on the planet. Previously it was impossible to make these estimates due to difficulties in accessing them. The mission has found a sharp decline in reserves, not only in China, India and the United States, but in regions such as Argentina, California, the Near East and Australia.[11] It is the case that these countries are large producers of cereals and that their groundwater represents the majority of freshwater reserves available on earth.

Sandra Postel estimates that the annual pumping of aquifers is about 160 billion cubic meters of water throughout the planet, with an approximate equivalent of "a thousand tons of water to produce one ton of cereals", this extraction corresponds to 160 million tons of cereals, that is, half of American production.[12]​.

Given the immense Chinese population of more than 1.4 billion inhabitants, the depletion of China's water tables could influence the world grain market, leading to an increase in food prices around the world.[13]​.

According to three researchers from the University of Utrecht, authors of a study on the subject published on January 25, 2012, "the lack of prudence in the use of groundwater for irrigation is a major problem not only for countries that make intensive use of it, but for the entire world, as a whole, given that international trade induces an intense correlation between food production in one country and consumption in another."[14]​.

In Spain, among others, the so-called 23 or Western Channel Aquifer is in danger, which with its more than 5,000 km² surface is one of the largest on the peninsula. It was declared overexploited in February 1987 by agreement of the Governing Board of the Guadiana Hydrographic Confederation. In fact, the primary source of the Guadiana had stopped gushing.

• - Aquifer.

• - Groundwater.

• - Water cycle.

• - Hydrogeology.

• - Water table.

• - Well.

• - Artesian well.

• - Artificial recharge of aquifers.

• - Overexploitation of aquifers.

In some cases there are hot water tables, water that comes from underground layers of the Earth that are at higher temperatures. Due to their temperature, they have the capacity to dissolve certain minerals contained in the soils in which they are and thus, many of them are rich in different mineral components and can be used for therapeutic uses, such as baths, inhalations, irrigations, (see Hydrotherapy) and also for heating buildings. They are generally found along fault lines since groundwater can enter along the fault plane, which heats up when reaching a certain depth and then rises in the form of steam (which can condense when it reaches the surface, forming a geyser) or hot water springs.

Normally the layers are recharged by rain that runs between the grains of earth that form the soil. Through permeable lands, water filters and reaches the layer. A borehole (a well) allows us to know the upper level of the layer (water table), the level above which the interstices of the earth are not saturated with water. Variations in the water level of the well show the degree of filling of the underground reservoir.

The level may vary due to natural causes (especially rainy or especially dry periods). In dry times, water from unsaturated layers reaches the upper layers of the soil by capillarity and can evaporate. Another way in which a layer loses water is due to plants that collect it through their roots and diffuse it into the atmosphere through evapotranspiration. In rainy seasons, however, the water filtered through the ground is usually easily greater than the water lost through evaporation. In this way, the water content of the layers can vary by natural means and, therefore, the depth of the water table. The layers often maintain a more or less stable water table, except for annual variations due to dry and rainy seasons.

The water content of the layer is often stable when the maximum water table is limited by natural water outlets from springs, such as stream or river sources, so that replenishments equal losses.

In any case, the most important thing is the influence of level variations due to artificial causes. If water is extracted from the aquifer by pumping, for irrigation, for example, at a rate greater than the rate at which water is replaced in the layer, a decrease in the height of the water will occur, and the layer may be exhausted.

There is also the possibility of artificially recharging the layers using surface water that is introduced by many different means into the water table. Among others, purified water can be used (filtration through the ground finishes the purification process) or water infiltrated through the bottom of a canal. Of course, direct pumping through wells is also used in the winter when there is an abundance of surface water.[3]​.

Another system is the use of seasonal rainwater, making a series of small dams along the bed of a stream "Torrente (hydrography)") or rambla "Rambla (geomorphology)") and at the bottom of each one a well, filled with sand, that reaches the aquifer, so that the dam retains the rainwater, which would be lost in the mother river or in the sea, long enough to filter, in whole or in part, to the layer phreatic. In addition, the system serves to moderate, in a certain way, possible floods in times of torrential rain or cold weather.

Another type of layer, also common, is the one formed by river channels when the land on their banks is permeable: they are recharged because the river water extends underground to distances that can become important (alluvial aquifers). Obviously, in times of river dryness the volume of water stored in these layers also decreases. An extreme case in Spain is that of the Guadiana whose channel disappears from the surface for a stretch, between Argamasilla de Alba and Villarrubia de los Ojos (both in the province of Ciudad Real)[4]​ forming a water table.

saturated zone

The saturated zone of the layers distinguishes between two types of water:

• - bound water or trapped water, actually linked to solids through molecular or electrostatic forces, the two processes being limited in space;.

• - free water, capable of moving due to gravity or under the effect of pressure.

The linked water corresponds to:.

• - the layer closest to the grains, in which the thickness is a few dozen molecules (0.1 micrometer) corresponds to an orientation of the water molecules with a H-OH dipolar structure, perpendicular to the surface of the grains. In this zone the viscosity of water can double or triple, its density is of the order of 1.5 and its pressure per unit area can be of the order of 10,000 bar.

• - the transition zone between 0.1 and 0.5 micrometers "Micrometer (unit of length)") has molecules subject to a non-negligible attraction and which are immobile.

Above, the forces of attraction are weaker and it is then free water.

Soil moisture is the amount of water per volume of land in a piece of land.

unsaturated zone

Unlike the saturated zone, which contains a liquid part (water) and a solid part (stone, earth), here there is a third component: air. The part of the holes occupied by water is considered saturated.

In the unsaturated zone, four states are distinguished depending on the water saturation in the ground:

• - funicular water or gravitic: in a terrain almost saturated with water, with some air bubbles, water can circulate under the influence of gravity.

• - saturated soil in equilibrium also called soil capable of capillary retention: the water phase is also continuous, but it does not circulate only due to the effect of gravity. This corresponds to water retained by capillarity.

• - pendular water: in a terrain slightly saturated with water, and occupies discontinuous rings at the points of contact between them. The water phase is always continuous, the pressures are transmitted, but the movements of the water are very slow due to the thinness of the wet film.

• - irreducible saturation: if the water leaves (evapotranspiration), there is nothing left but the bound water.

For most agricultural crops the water table must be in a certain range of depths, because at different depths (both greater and lesser) the yield is reduced.[7]​ For some important crops a classification has been developed:[8]​.

PNF = Depth of Water Table.

In Spain

The Los Arenales aquifer belongs to the Duero River basin in Castilla y León. It covers more than 7000 km² between the south of the Duero River, the Central system and the course of the Tormes River to the west.

The System 23 aquifer or Western La Mancha aquifer is the central unit and one of the key pieces of the hydrological system of the Upper Guadiana Basin. It covers an area of ​​5,500 km² with a depth of 70 m and more than 30 municipalities in the provinces of Ciudad Real, Albacete and Cuenca are located on it. It is, therefore, one of the largest aquifers in Spain.

In France

In France, the largest layer is the Beauce layer "Beauce (France)") whose surface area is about 9000 km², in six departments. Its reserves are estimated at about 20 thousand cubic hectometers.

The largest layer is that of the Rhine plain in Alsace, whose extension is small, but which has reserves estimated at 35,000 cubic hectometers in the Alsatian part alone.[9]​.

In the United States

The largest water table in the United States (the Ogallala Table), with an area comparable to that of France, stretches from South Dakota to Texas.

In South America

• - Guaraní Aquifer, in the Río de la Plata Basin.

Already in 2001, the American expert Lester R. Brown denounced that the level of the water tables was dropping on all continents, due to the availability of electric pumps or explosion engines that allow water to be extracted faster than it is replaced by rain. And he specifically cited three regions: the North China Plain, the Punjab in India/Pakistan, and the southern part of the prairies of the United States, which are irrigated agriculture regions.[10]​.

In 2002, NASA and the German Aerospace Center launched the GRACE (Gravity Recovery and Climate Experiment) mission mainly to evaluate, through satellite observations, the level of the groundwater tables on the planet. Previously it was impossible to make these estimates due to difficulties in accessing them. The mission has found a sharp decline in reserves, not only in China, India and the United States, but in regions such as Argentina, California, the Near East and Australia.[11] It is the case that these countries are large producers of cereals and that their groundwater represents the majority of freshwater reserves available on earth.

Sandra Postel estimates that the annual pumping of aquifers is about 160 billion cubic meters of water throughout the planet, with an approximate equivalent of "a thousand tons of water to produce one ton of cereals", this extraction corresponds to 160 million tons of cereals, that is, half of American production.[12]​.

Given the immense Chinese population of more than 1.4 billion inhabitants, the depletion of China's water tables could influence the world grain market, leading to an increase in food prices around the world.[13]​.

According to three researchers from the University of Utrecht, authors of a study on the subject published on January 25, 2012, "the lack of prudence in the use of groundwater for irrigation is a major problem not only for countries that make intensive use of it, but for the entire world, as a whole, given that international trade induces an intense correlation between food production in one country and consumption in another."[14]​.

In Spain, among others, the so-called 23 or Western Channel Aquifer is in danger, which with its more than 5,000 km² surface is one of the largest on the peninsula. It was declared overexploited in February 1987 by agreement of the Governing Board of the Guadiana Hydrographic Confederation. In fact, the primary source of the Guadiana had stopped gushing.

• - Aquifer.

• - Groundwater.

• - Water cycle.

• - Hydrogeology.

• - Water table.

• - Well.

• - Artesian well.

• - Artificial recharge of aquifers.

• - Overexploitation of aquifers.