The Hydrologic Cycle - A Summary

Hydrogeology, Geology, & Environmental Issues

Every day, over one thousand cubic kilometres of water evaporates from the land and sea. The water vapour condenses into the tiny droplets of liquid that make up clouds, and eventually it falls back to the earth as rain, hail or snow. Every three millennia, this overturning of water from ground to sky and back again recycles an amount of water equivalent to all the world's oceans. It is called the hydrological cycle, and all life on the planet depends on it.

The hydrological cycle creates fresh water from the salty stuff of the oceans, because the salt is left behind when sea water evaporates. Much of the water vapour simply falls as rain back into the oceans. But some falls over the land surface, where it gathers into streams and rivers, making its gradual journey back to the sea. Some water takes other routes through the hydrological cycle. If it falls as snow over the polar ice caps, it can become locked away in the ice sheets for hundreds of thousands of years. And some rain water percolates through porous soil and rock until it meets an impermeable rock layer. Then it seeps down subterranean slopes as ground water, carried in aquifers that break through to the surface as springs or which can be tapped through wells.

Groundwater can be pumped back into the air before it can find its way back to the sea. Plants suck it up through their roots and release it as vapor from their leaves a process called transpiration. A typical birch tree transpires about eighty gallons of water each day. It is because of this plant-induced humidity that tropical rain forests swelter under a moist climate. So the hydrological cycle is really a set of wheels within wheels, each turning at a different pace each ferrying water between land, sea and sky, between salt and fresh.

Original URL Off-Line: http://www.nature.com/news/1998/000127/full/news000127-12.html