Today circulation of water on earth, amounting to about 500,000 billion cubic meter, is well recognized by experts in the field despite its all complexities. Experts show great interest in examination of water circulation and determination of quantities of different water flows (precipitation, rivers, lakes, aquifers, etc.) which form the totality of the earth’s water. Currently the issues concerning natural water resources are also studied.
Generally, distribution of fresh waters around the world is very irregular, so that they might be completely inaccessible exactly where there is need for great amounts of such waters. For some fundamental reasons, if water resources are renewable, they are considered as among flows and in certain cases follow global scales. But in some countries, there are significant resources of unrenewable water, just as hydrocarbon and mineral resources.
Numerical assessments of volume of water resources have been carried out on different scales and according to different parts of hydrosphere, and have revealed significant exploitation of such resources; these studies included oceans, polar ices, underground waters, soil moisture, lakes, rivers and atmosphere.
M.L. l’Vovich has calculated the average periods of renewal of the above resources as follows:
Oceans: three thousand years; aquifers: four to five thousand years; ice coverage, polar ice and continental glaciers: eight thousand years; lakes: seventeen thousand years; soil moisture: one year; rivers: almost twelve days; atmospheric humidity: one week.
In absolute conditions, underground water is almost in correspondence with natural circulation of water. However, mineral resources take very long periods of time for renewal. Therefore, they can be considered as unrenewable on human life-span scales.