What is it?
The hydrological water cycle is the mechanism through which all of the world’s water moves around.
It can be viewed as a series of reservoirs or stores which exchange volumes between them by different processes.
The largest of these reservoirs is the Oceans and Seas. These account for around 97% of the Earth’s water reserves.
Second are the frozen wastes of Antarctica and Greenland which contain upwards of 75% of all the freshwater on the planet.
Then there are all the rivers, streams, lakes and ponds which in total equate to less than one tenth of one percent of the Earth’s water.
The processes involved;
These are all driven by the Sun’s energy which moves water between the Oceans, the sky and the land.
It all starts with the oceans;
Heat from the sun causes water to evaporate from the surface. This becomes water vapor, free from salt and most other impurities it rises up into the atmosphere where clinging to tiny dust particles it condenses into droplets.
These droplets form clouds which are carried by the winds for hours or days until changes in temperature or pressure result in precipitation; this can be rain, snow, sleet or hail.
Where this falls over the sea it is simply re-evaporated and the cycle repeats, when it falls on land it enters the part of the cycle which effects us.
High in the hills and mountains snow builds up and eventually slides or melts downwards where it joins the streams and rivers formed by water running off the lower slopes. Some of this is trapped temporarily in lakes and marshlands, here it is once again subject to the sun’s action and is evaporated back to atmosphere.
The rivers grow as they head downhill and some of this water seeps into surrounding land, it sometimes find weaknesses in the rocks and permeates to great depths where it builds up over millennia to form aquifers and the groundwater we tap with wells.
As it continues more will be evaporated and a greater part of it will simply return to the sea but all along the way biological organisms from the smallest bacteria to plants and animals take their share.
Trees intercept rain before it hits the ground and contribute to evaporation, much of what makes it past them lands on the ground and is processed by plants which by absorbing and later evaporating water back to the air complete part of the cycle known as evapotranspiration.
Most of the precipitation which falls on land will return to the sea without any interference, we rely for our fresh water supplies on the small portion which we can capture from the lakes, rivers and aquifers which delay it on its passage.
Whether it takes a few hours or hundreds of thousands of years all water will eventually complete this process and return to the oceans where the cycle repeats endlessly.