The Water Cycle
Since Earth’s creation 4 billion years ago, the water we use has always been the same. Water keeps renewing itself through a natural cycle and allows living beings to exist. The water cycle (or hydrologic cycle) is a model representing flows between big reservoirs of liquid, solid and gas water. On Earth, these take the form of oceans, atmosphere, lakes, water courses, ground water and glaciers. The sun is the driving force of the whole cycle. Solar energy makes water change states: the formation and melting of glaciers and evaporation causing exchange processes and the different phases of the cycle.
The water cycle is an essential factor keeping water available for all life forms. This cycle, between the sky and Earth, is made up of four phases: evaporation, condensation, precipitation and collection.
1) Evaporation and transpiration of plants: solar radiation heats streams, rivers, lakes, seas and oceans and transforms liquid water into vapour (gas): this process is called evaporation. Plants and other vegetable species draw water from the soil and release it in the form of vapour. Around 10% of precipitation falling on Earth comes from plant transpiration and 90% is the result of evaporation. Plant transpiration and evaporation resulting from soil humidity, release water vapour that rises into the atmosphere into the form of clouds.
The wind moves the clouds above land. This results in two further phases taking place:
2) Condensation: when water vapour reaches the atmosphere, the vapour cools down and turns into little water drops that form clouds: this phase is called condensation. The water drops will then be moved by the wind and fall on land.
3) Precipitation: carried by the atmosphere, the clouds move and with the help of gravity fall in the form of rain, snow or hail (liquid or solid state), generally called “precipitation”.
4) Collection and infiltration : when water is not absorbed into the soil, it runs off along slopes until it is discharged into streams, rivers and lakes. Then it is carried to seas and oceans. All this forms a “watershed”. When water is in touch with impermeable soil, it runs off and infiltrates permeable soils. Doing so, water can sometimes fill an underground pocket or cave and form a water reservoir. Water in this reservoir (groundwater) sometimes naturally flows up out of the ground. This is called a spring. Human beings can exploit groundwater as drinking water storage.
Note: less than half of precipitation recharges ground water, the remaining part evaporates.