As the Council of Europe Water Charter says, life is not possible without water. This resource is of such importance that any human activity (industrial, livestock, agricultural or service) is conditioned by the necessary availability of the precious liquid.
About 97% of the Earth's water is found in the seas and oceans; the rest, 2%, corresponds to the ice caps. Consequently, human life depends on the minuscule amount of remaining water (1%) that exists in the subsoil, rivers and continental lakes.
These volumes are constantly and perpetually renewed by the hydrological cycle, a process governed by solar radiation, which acts as a motor and is supported by the atmosphere. Water is constantly evaporating into the atmosphere to return, only a part, to Earth in the form of rain, snow, hail, dew or frost.
It is estimated that the amount of water that evaporates into the atmosphere each year is about 500,000 km3, an amount that falls back as precipitation.
A large part (almost two thirds) evaporates or is absorbed by plants, which use it in their biological processes. Therefore, only 10% of the total precipitation falls on the continental surface.
The rain is not homogeneous on the planet, there are areas where it almost never rains (the great deserts) and, in others, the rivers are so mighty that a large part of their contribution ends up in the sea. On the other hand, in addition to this spatial irregularity, temporal irregularity is superimposed; Spain is an example: there are regions where there are prolonged annual droughts that end with heavy rains, producing dangerous and devastating floods.
Of the total available runoff, at this time, only 6% is used, of which almost 70% is used for agriculture. The other 30% is used to supply water to the population and industry.
The endowment of consumption in supply is variable, since it depends on the standard of living of the population supplied. In the US, the average supply is around 450 l/inhab./day and, in Spain, it is around 300 l/inhab./day.
Of this endowment, the amount of water necessary to maintain the basic functions of the human body is surprisingly small, since it only reaches about 2 liters / day. The rest corresponds to industrial consumption (70%) and, the remaining volume, to domestic consumption, in which a large part (40%) is consumed by the toilet, and another, in the sink and bathroom (35%).
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