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Water resources of the earth

Seawater: The water stock of the earth

Nearly 97% of all water of the earth is the salt water of the seas. Meaning that the entire water supply of the earth's continents is achieved with only approx. 3% of the global water ammount.

Temporality of water

The natural water cycle begins in the sea. The sun evaporates seawater, turning it into fresh water while warmed humid air rises. On its way to the colder regions of the earth, the air cools down and the water condensates into minute drops, which create the clouds that then rain down upon the continents. Experts calculate the quantity of rain to be around 120,000 cubic kilometers per year. This is a vast amount of water when one considers that a cubic kilometer corresponds to a billion cubic meters and that a cubic meter of water is comprised of 1,000 liters.

The retention times for a drop of water on the mainland (one speaks of renewal rates) are very different. If a drop falls into a creek or river, it takes only a few days or months until it returns to the sea. Should it fall into a lake, however, the process can last a lot longer - depending on the size and depth of the lake, anywhere from a few years to thousands of years. In groundwater the process lasts for hundreds of thousands of years - in the glaciers and at the poles sometimes many millions of years - until the drop returns to the sea. There it can take hundreds of millions of years until the drop commences its journey back to the mainland.

Of course, there are also non-renewable water resources "- the so-called fossil aquifers, such as the groundwater of the Sahara. Its water is millions of years old and comes from a time before the Sahara was a desert. Now, the Sahara is indeed a desert - and the groundwater does not replenish.

We can see that the great natural water cycle consists of many differently paced sub-cycles. This is the temporality of water. It controls the amount of supply. In general, the slower a cycle flows, the more water is moved within it. This makes it easier to understand the fact that most of the water on the earth is in the form of salt water in the seas, and that the greatest amount of fresh water is found at the poles in the form of ice.




Renewable water resources

Water is always in motion. No matter in what form - whether clouds, rain, snow, rivers, seas, groundwater, or the ice of glaciers and poles - in the end it all flows back to the seas. Because of the ongoing evaporation of seawater, this hydrological cycle is never interrupted and rain continues to fall, we can speak of renewable water resources.



Renewal rates

The renewal rates of the continental water resources are defined by the size (amount) of the local resource and the amount of rain falls within the catchment area of the resource. The ranges (mountains) of the continents act like a rain captor: Here the most rivers have their spring.

The sensibility of the resource increases with an increasing renewal rate. By the withdrawal of groundwater the water quantity drops, because rain water percolates into the soil very  slowly. In the long run the groundwater body is also being contaminated by leakages of sewage lines, as  groundwater also drains away  very slowly.

The development of the global water resources is subject to the climatic conditions. One must assume that the global warming doesn't lead to an increased availability of drinking water – more on the contrary.

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