The water cycle

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THE NATURAL WATER CYCLE

Water : a limited natural resource

Water is everywhere.It falls as rain, covers the mountains as snow, supplies streams and rivers etc.Yet, less than 1% of all water on the earth can actually be used by man.  This is the freshwater in rivers, lakes and ground-water.

These are the different stages in the natural water cycle:

Evaporation:

Heated by the sun, the water in moist ground and the various water bodies (oceans, rivers and lakes) evaporates and rises into the atmosphere where, in the form of minute water vapour droplets, it gathers to form clouds.

In addition to evaporation there is plant transpiration (an oak tree can release up to 500 litres of water every day).

The sun acts as a huge evaporator:every day more than 1,000 km3 of water vapour is formed this way and spread all around the world.

Condensation:

The evaporated water rises into the sky and condenses, forming clouds made up of billions of water droplets.These clouds are transported by the wind.They continue to accumulate water vapour and grow in size.

Rainfall:

As they cool, the vapour droplets condense forming much bigger drops, which become too heavy to float in the air and then fall in the form of rain. 

If the clouds meet areas of much colder air, the water vapour droplets turn immediately into ice crystals which, in coming together, form snow flakes or hail.

Runoff:

The majority of the water falls directly into the oceans.The rest percolates into the ground (to form ground-water which gives birth to springs) or runs off into rivers, which in turn, feed the oceans.And the cycle starts again etc. 

THE DOMESTIC WATER CYCLE

This cycle is divided into five stages.

1. Catchment – Pumping  60% of water collected in France comes from ground-water, but water can also be pumped from lakes, rivers and springs.The pumping of water is done according to demand and care is always taken to preserve natural reserves.The catchment zones are protected and monitored in order to reduce the risk of pollution.

2. Treatment:  Water is treated in plants using environmentally friendly processes.It is, in turn, cleaned, filtered and disinfected in order to be made fit for human consumption.

3. Storage: After treatment, the water goes into storage areas (reservoirs and water towers).It is transported and distributed by large pipes.Its quality is continuously monitored. 

4. Usage – recovery : After use, the water is recovered by a series of pipes and is transported to the purification plants.

5. Environmental protection : the treatment plants purify the dirty water so that it can be returned to the natural environment.The sewage sludge is then, in part, treated and recycled.

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