The hydrological cycle

The hydrological cycle is a way of describing the storage, transformation and movement of water between the atmosphere, biosphere, lithosphere and the hydrosphere. As with the other cycles described in this Section, the hydrological cycle is one of the fundamental supports on which nature depends. Water covers some 70 percent of the earth's surface so it might be expected that there is plenty of it. However, about 97 percent of the total water on the planet occurs in the oceans and hence is saline (salty). The next largest amount (2.5 percent) occurs in glaciers and ice caps, so is again largely unavailable. Less than 1 percent in total occurs in the atmosphere, lakes and rivers, and the soil and underlying rocks combined.

This is a tiny percentage yet it is this small amount that we rely on to support the land-based biosphere. The hydrological cycle is powered by energy from the sun. There are several mechanisms involved. Heat from the sun causes some of the water from the surface bodies such as oceans, lakes, rivers and, to a lesser extent, the soil surface, to change into water vapour. This process is known as evaporation and is one of the key processes by which waters changes its form and moves back to the atmosphere. Another important process is transpiration. This involves soil water being passed through the plant roots up into the plant itself, from where it leaves the plant as water vapour by a process called transpiration. Where the two processes of evaporation and transpiration occur at the same time it is called evapo-transpiration. These are the key processes by which the water is returned to the atmosphere as water vapour. The total resource of water in the world is constant.

What does change is the form in which water is. It is a very dynamic system, changing as it does from water vapour to snow and ice and to liquid water. In the atmosphere the water vapour will eventually form clouds and become the main source of rainfall, from which water is returned to the land and water surfaces for re-distribution again. The hydrological cycle has been working for millions of years. During this time the total amount of water on earth has remained constant. Water, however, is regularly being cycled between the different 'compartments', e.g. oceans, air, soil, by several different processes. These include precipitation (rainfall, hail and snowfall), evaporation, transpiration, runoff and infiltration.