When we turn on the tap, start the washing machine or take a shower, we don’t necessarily think of the sun and the rain, but that is where the water comes from.
The process known as the water cycle begins with energy from the sun reaching water in oceans, seas, rivers and lakes. Water evaporates and becomes water vapour. As the water vapour rises, it cools and condenses into billions of droplets to form clouds.
Vegetation is another source of water vapour. The roots of plants pump water out of the ground and pass it into the atmosphere in a process known as transpiration.
Clouds hold rainwater as long as they stay warm. If the air cools, the droplets merge until they are so heavy that they fall back to Earth as rain, hail or snow. The atmosphere is capable of holding about 10 days’ supply of rain – enough to drop about 25 millimetres of freshwater over the entire surface of the planet.
Rain and snow falling within catchments can take several routes. Some evaporates, some seeps into the ground to become groundwater and some stays on or near the surface to form streams, and ultimately rivers.
The water cycle is also referred to as the hydrological cycle. Both terms describe the solar-powered system that provides freshwater to the land-based ecosystems upon which we depend.
Consumer's Guide to Drinking Water - May 2006 [an error occurred while processing this directive]