The Water Cycle
Water is always on the move: ice melts, rivers run to the sea, groundwater percolates, lakes evaporate, and rain falls down. The circulation of water between oceans, atmosphere, glaciers, rivers and through the ground is called the water cycle or hydrologic cycle. This movement has enchanted humans for centuries. “All the rivers empty in the sea, but it never spills over; one by one the rivers return to their source.” (Ecclesiastes 1:7) The first to describe the water cycle was Pallisy in 1580, establishing the theory that the run-off of rivers is due to rainfall. This theory was confirmed a century later when Perrault and Mariotte determined the run-off of the Seine. The precipitation in the river basin appeared to be more than enough to explain the run-off.
The water cycle depends on the Sun as it runs on solar energy. About half of the solar energy that reaches the Earth is involved in evaporating water. Rainfall and other forms of precipitation in large oceanic regions is lower than evaporation, and the surplus water vapor is transported by wind to the different continents. Most of the land’s rainfall is captured by vegetation, the remaining water is absorbed by the soil or runs off to streams and rivers. Part of the rainfall evaporates again or returns to the atmosphere by transpiration from plants. The surface run-off is collected in streams, lakes, and rivers and returns to the sea. The water that infiltrates the soil can percolate into deeper groundwater levels but will ultimately join with surface water and find its way back to the sea.
The “residence time” of water molecules in one body of water varies a lot. The average residence of water in the atmosphere is only 9 days. Similarly, water taken up by crops often doesn’t remain more than a week before it transpires. On the other hand, the average residence time for rivers is 2 to 6 months, as is the case for seasonal snow covers. The residence time in lakes and glaciers varies between 20 and 100 years, but the longest residence times are in the ocean (3,200 years) and in the groundwater (up to 10,000 years).
The importance of the water cycle for our climate is significant. The evaporation process has cooling as its side-effect which is requisite to keep the Earth’s surface temperatures within limits. It is said that without this cooling process, the surface temperature would be 67 °C (153 °F).