Water cycle

Water cycle

The water cycle is the cyclical movement and transportation of water both above and below the Earth’s surface. The cycle is continuous and has no end or origin point. Within the cycle water can change its state at any point, whether changing to or from liquid, ice and vapour. Despite the fact that individual water molecules can come and go the Earth’s general water balance stays pretty well fixed over time.

In terms of the way in which the cycle operates despite there being no definite origin point in the closed system of the planet we can start at the sun, for description’s sake. The sun, acting as the engine of the cycle, heats all of the water whether it is in seas, oceans lakes, rivers or any other standing pools upon the surface. Any water lying as snow or ice can be transformed directly into water vapour, and water can also be transpired from leaves and from plants.

Air currents travelling upwards generated by the sun’s heat transport the water vapour into the earth’s atmosphere where it is subsequently condensed to form clouds. At this stage air currents transport water vapour across all of the world’s skies where the many cloud particles crash into each other, gradually growing and finally falling from the sky as rain. Subsequently, this rain falls in the form of water or, in cooler temperatures, can fall as hail or snow. In this state it can fall and settle as ice caps and glaciers, where it can be stored as frozen water – potentially over the course of millennia. Other than this fallen snow will generally thaw and eventually melt, subsequently flowing over land and then back into the rivers, lakes and oceans or stored as groundwater.

Some of the water that is absorbed deep into the Earth is stored as infiltration, where it refills aquifers. Some groundwater can also resurface as groundwater springs where, if they are hot enough, can also release steam vapour. Eventually, however, all water will return to the ocean to be once again driven across the cycle by the engine of the sun.

In terms of water being stored within the Earth in reservoirs, for instance, this is known as residence time and can be used to measure the average age of the water stored. As far as groundwater is concerned it can be stored beneath the surface of the Earth for more than 10,000 years before being transported onwards. Extremely old groundwater is referred to as fossil water, and is not found in many places due to the constant cyclical nature water exists in. In general, once water is in the soil it stays there for just a short time and is easily lost due to being discharged, especially via transpiration and evaporation.

The actual time water spends in one form or another can vary greatly from where it is located on the Earth’s surface as well. Once it has evaporated into the atmosphere water generally resides there for around only nine days, at which point it condenses and then falls to Earth once more as rain. As far as the Earth’s principal ice sheets are concerned, namely Greenland and Antarctica, they will generally store water in an ice state for extremely long periods of time. Indeed, ice taken from Antarctica has been dated at around 650,000 years old, although this does outstrip average residence times and is somewhat exceptional.

It is also true to say that water exists outside the actual water cycle itself, as there is far more water lying in storage for very long periods of time, so to speak, than is actually been transported through the water cycle. Much of the water being stored lies in the oceans, and it has been estimated that approximately 95% of the world’s water supply is cycled throughout ocean currents and carried to the car corners of the Earth. Also, it is reckoned that around 90% of the evaporated water supplied into the water cycle comes from the oceans’ supplies.

At times of much cooler climates more glaciers and icecaps are formed, and at this point as more of the water supply is held as ice there is less water in other parts of the water cycle. This is reversed during warmer climatic periods. Looking at it from a climate point of view during the last ice age around one-third of the Earth’s surface was covered by glaciers, resulting in the oceans being around 400 feet lower than they are presently. Conversely, during the last period of climatic warming which occurred around 125,000 years ago the world’s oceans were around 20 ft higher than present levels, although it is reckoned that roughly three million years ago they may have been anywhere up to 165 ft higher, all illustrating how both the climate and the water cycle are interrelated.

The changing water cycle can also be illustrated by glacial retreat, and this phenomenon is where the supply of water to the glaciers from rainfall is unable to keep pace with water loss from such phenomena as melting.

In terms of human activities the water cycle can be affected by such things as industry, agriculture, deforestation, dam building, and general urbanisation. In terms of the effect on climate the sun, as we remember, is the engine of the water cycle. Of all the world’s evaporation, around 86% comes from the globe’s oceans, which serves to lower their temperatures of the planet as a result of evaporative cooling, without which evaporation itself would impact upon the greenhouse effect thereby resulting in a far higher surface temperature of around 67 degrees Centigrade, finally leading to a much warmer Earth.

In terms of the water cycle completing its stages, from evaporation of ground water stored on the Earth in the rivers and oceans all the way to the water changing states through the various stages and eventually falling to Earth once more as rainfall or snow, it generally takes approximately nine days in total to complete. This can of course vary from place to place, though this average is generally held true across the globe.

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