The water cycle (or hydrologic cycle) describes the continuous movement of water above, below, and on the planet. Since the water cycle is in fact a “cycle”, there is no beginning or end. Water exists in three states: liquid, vapor, and ice. Although the balance of water on our planet is fairly constant, individual water molecules may come and go. The water cycle is driven by the sun. The sun heats the oceans and allows water to evaporate into the air. The sun also heats snow and ice which also forms into vapor. Water that transpires (emerges of leaks out) from plants and evaporates into the soil is known as evapotranspiration. Air currents take the vapor up into the atmosphere where cooler temperatures cause it to condense and form clouds. The air currents also move the clouds around the globe. Cloud particles collide and grow into bigger clouds and eventually fall from the sky as precipitation. Some of this precipitation falls as snow and in areas where it is cold enough for long periods of time, the snow can store frozen water for thousands of years.
Snow packs thaw and melt, and the melted water flows as snowmelt or meltwater. Most precipitation falls back into oceans or land. Where precipitation flows over land, it is known as surface runoff. Some runoff flows into rivers and eventually returns to the ocean. Some runoff makes its way to freshwater lakes and ponds. This is known as groundwater. Some water soaks deep into the ground and replenishes aquifers, which store huge amounts of freshwater for long periods of time. Some of this water stays close to surface channels and may eventually find its way back into the oceans as groundwater discharge. Some groundwater finds veins and cracks in land surface and emerges as freshwater springs. Over time, all water eventually returns to the oceans where the cycle starts over and over again.
There are many different processes of Earth’s water cycle. These include precipitation, canopy interception (water that is intercepted by plants and eventually evaporates back into the atmosphere), snowmelt, runoff, infiltration (the flow of water from the surface into the ground), subsurface flow, evaporation, sublimation (the change of solid water directly into water vapor), advection (the actual movement of water, in any of its states, though the atmosphere), condensation, and transpiration.
Groundwater can spend many hundreds or thousands of years beneath the planet’s surface before continuing its cycle. Groundwater that exists for a long time is sometimes referred to as fossil water. Water that is found in soil remains there very briefly, as it spreads thinly across the planet. Most of this water is eventually lost due to evaporation, transpiration, stream flow, or groundwater recharge. Once water evaporates, it spends an average of nine days in the atmosphere before condensing and falling back to Earth as precipitation.
The movement of water through the water cycle varies. Typically, much more water is in storage for long periods of time than what is actually moving through the cycle. The storehouses for the majority of all water on Earth are the oceans. It is estimated that of the 332,500,000 cubic miles of the world’s water supply, close to 321,000,000 cubic miles is found in the oceans (about 95%). The oceans also supply about 90% of the evaporated water that goes through the water cycle.
During colder periods, less water flows through the cycle and more ice caps and glaciers form. When warmer climates prevail, there is more water flow. During the last ice age glaciers covered nearly a third of the planet’s land mass, and resulted in the oceans being about 400 feet lower than they are today. During the last global warming trend (about 125,000 years ago), the seas were about 18 feet higher than they are now. About three million years ago the oceans may have been as much as 165 feet higher than today. A consensus taken in 2007 theorizes that the water cycle will continue to intensify throughout the 21st century. This does not mean that precipitation will increase in all regions. Precipitation is expected to decrease in some regions and possibly bring on severe drought. The drying is projected to be stronger near pole-ward regions and precipitation to intensify in the equatorial regions.
Glacial retreat is one example of the changing water cycle. Glacial retreat is the cycle at which the supply of water to glaciers cannot keep up with the loss of water from melting. Glacial retreat has been extensive since 1850. Some major human activities that are affecting the changing water cycle include: agriculture, industry, construction of dams, deforestation and afforestation, groundwater removal through the use of wells, water removal from rivers and streams, urbanization, and chemical alterations in the atmosphere.