The Water Cycle: color graphic showing the movement of water through the water cycle, from evaporation and transpiration to condensation, to water storage in the atmophere, to precipitation, to water storage in ice and snow, surface runoff, snowmelt runoff to streams, streamflow, and freshwater storage. A cut away shows the ground water portion of the water cycle, from infiltration to ground water storage and ground water discharge into springs and freshwater storage. Surface runoff, freshwater storage, ground water storage, and ground water discharge are all shown contributing to water storage in oceans, where the evaporation portion of the water cycle starts again.
SUMMARY OF THE WATER CYCLE
What is the water cycle?
The water cycle describes the existence and movement of water on, in, and above the Earth. Earth's water is always in movement and is always changing states, from liquid to vapor to ice and back again. The water cycle has been working for billions of years and all life on Earth depends on it continuing to work; the Earth would be a pretty stale place to live without it.
Where does all the Earth's water come from? Primordial Earth was an incandescent globe made of magma, but all magmas contain water. Water set free by magma began to cool down the Earth's atmosphere, until it could stay on the surface as a liquid. Volcanic activity kept and still keeps introducing water in the atmosphere, thus increasing the surface- and ground-water volume of the Earth.
A quick summary of the water cycle
The water cycle has no starting point. But, we'll begin in the oceans, since that is where most of Earth's water exists.
The SUN, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air.
Ice and snow can sublimate directly into water vapor.
Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is water transpired from plants and evaporated from the soil.
The vapor rises into the air where cooler temperatures cause it to condense into clouds.
Air currents move clouds around the globe, cloud particles collide, grow, and fall out of the sky as precipitation.
Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years.
Snowpacks in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt.
Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans.
Runoff, and ground-water seepage, accumulate and are stored as freshwater in lakes. Not all runoff flows into rivers, though. Much of it soaks into the ground as infiltration.
Some water infiltrates deep into the ground and replenishes aquifers (saturated subsurface rock), which store huge amounts of freshwater for long periods of time.
Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge, and some ground water finds openings in the land surface and emerges as freshwater springs.
Over time, though, all of this water keeps moving, some to reenter the ocean, where the water cycle repeats.
COMPONENTS OF THE WATER CYCLE
The U.S. Geological Survey (USGS) has identified 16 components of the water cycle:
- Water storage in oceans
- Evaporation
- Sublimation
- Evapotranspiration
- Water in the atmosphere
- Condensation
- Precipitation
- Water storage in ice and snow
- Snowmelt runoff to streams
- Surface runoff
- Streamflow
- Freshwater storage
- Infiltration
- Groundwater storage
- Groundwater discharge
- Springs
This map of sea surface temperatures was produced using MODIS data on the Terra satellite. The red pixels show warmer surface temperatures, while yellow and green are middle values, and blue represents cold water. CREDIT: NASA GSFC
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