There are two water cycles – The large water cycle which is the exchange of water between ocean and land and the small water cycle which is a closed circulation of water in which water evaporated on land falls in the form of precipitation over this same terrestrial environment.
The large water cycle
Every year approximately 550 thousand km3 of water evaporates each year into the atmosphere. About 86% of the evaporation from the Earth’s surface is from the seas and oceans while about 14% is from land. Of the total amount of atmospheric precipitation originating from this evaporation, 74% falls over the seas and oceans and 26% over land. Therefore it follows that the seas and oceans, through evaporation and precipitation, endow land with a certain volume of water which travels by way of atmospheric thermodynamic flows a great distance over the continents, where it then rains down (or falls in the form of snow).
What happens to the water when it reaches the ground?
Part is absorbed into the ground and, if it reaches the groundwater table, is added to groundwater runoff (except for regions without groundwater runoff). Part is used by vegetation and part evaporates again. The remainder flows away via surface runoff into the network of rivers and back to the seas and oceans. Thus the large water cycle is completed.
If the conditions are balanced the same amount of water lands on the continents as flows to the sea but even relatively small deviations can mean great problems on the continents.
If more water flows from the continents into the oceans than is transferred from ocean to land through precipitation, the land loses water and dries out. This occurs, for example, when humanity systematically lowers the infiltration of rainwater into the soil through its activities (for example deforestation, agricultural activities, urbanization) and channels this water (in the quickest possible way) into rivers and subsequently into the sea. The moisture of the soil decreases, the groundwater table falls, vegetation withers and less evaporation takes place.
If the volume of water flowing from continents to the seas and oceans increases and evaporation of water from the seas and oceans remains unchanged, or does not increase adequately (under the influence of increased evaporation due to global warming), then the increased flow of water from the continents (including the increased melting of glaciers) adds to the rise in ocean levels. Besides the changes in the global water balance which are caused by phenomena beyond the influence of mankind (solar cycles, changes in position of the Earth in relation to the Sun, volcanic activity…), man unconsciously causes further deviations, thus contributing through his activities to the desertification of the continents.
Through conscious activity in the opposite direction, however—namely the deliberate conservation of water on the continents—mankind could stop this ongoing desertification and return the lost water to the continents.
(To be continued)
Ref. Water for the Recovery of the Climate – A New Water Paradigm M. Kravcík, J. Pokorný, J. Kohutiar, M. Kovác, E. Tóth