The a small part of it forms

The water stocks of our planet are considerable: nearly 1.4 billion km³ or 1.4 billion billion cubic meters.
However, the majority (97.5%) of this water is present in the form of salt water in the seas and oceans, difficult to exploit for human activities.
Of the remaining 2.5%, which still represents 35 million km³, more than two thirds are glaciers, very inaccessible. The remaining third mainly comprises groundwater (less than 1% of the total water of the globe) and a small part of it forms the surface water contained in freshwater lakes and rivers (less than 0.01% of the water of the globe).
Groundwater reserves may appear to be important, but only a very small proportion (about 200 000 km 3 or less than 1%) is mobilizable and potentially usable by humans. In addition, these groundwater reserves are unevenly distributed on the planet.
Feeding groundwater
The free groundwater is mainly fed by the infiltration of a part of the rainwater which, considering the evapotranspiration, is especially effective between the months of October and April when the infiltration of the waters is easy. However, in highly cracked aquifers, the rainy episodes of the spring and summer months can momentarily recharge the aquifer reservoirs.

The recharge by the rains causes the rise of the aquifer levels (A) and the seasonal increase of the flow of the sources (B) (J.-J. Collin © 2004, modified 2015)
Flow of water to the water table
The vertical flow of water to the aquifer is a function of the porosity and permeability of the soil, and the nature and thickness of the unsaturated zone .
The speed of infiltration can be of the order of the meter per year (Chalk of Champagne), of one meter per month ( Limestone of Beauce), of one meter per day ( alluvium ), and of several tens of meters to the hour in very fissured karstic terrain.
The ability of soils to infiltrate can be mapped from the study of the density of river systems. This IDPR (network development and persistence index) map is available in the map area.

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Circulation of water in the groundwater
Between the infiltration zone and the outlet ( source ), the groundwater moves in the aquifer by gravity from the highest zones to the lowest points. The surface of the sheet is sloping.
This piezometric surface is revealed by the level of water measured in the wells and the level of the location of the sources.
It makes it possible to determine the direction of flow of the sheet.


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