The water cycle.
The
water cycle, also known as the
hydrologic cycle, describes the continuous movement of water on, above and below the surface of the
Earth. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among
liquid,
vapor, and
ice at various places in the water cycle. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go.
Description
The sun, which drives the water cycle, heats water in the oceans. Water evaporates as vapor into the
air. Ice and
snow can
sublimate directly into water vapor.
Evapotranspiration is water
transpired from plants and evaporated from the soil. Rising air currents take the vapor up into the atmosphere 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 can thaw and melt, and the melted water flows over land as
snowmelt. Most precipitation falls back into the oceans or onto land, where 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
groundwater are stored as freshwater in lakes. Not all runoff flows into rivers. Much of it soaks into the ground as
infiltration. Some water infiltrates deep into the ground and replenishes
aquifers, 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. Some groundwater finds openings in the land surface and comes out as freshwater springs. Over time, the water returns to the ocean, where our water cycle started.
Different Processes
Precipitation:Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as
rain, but also includes
snow,
hail,
fog drip,
graupel, and
sleet. Approximately of water fall as precipitation each year, of it over the oceans.
[Dr. Art's Guide to Planet Earth. Retrieved on 2006-10-24.]Canopy interception:The precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground.
Snowmelt:The runoff produced by melting snow.
Runoff:The variety of ways by which water moves across the land. This includes both
surface runoff and
channel runoff. As it flows, the water may infiltrate into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.
Infiltration:The flow of water from the ground surface into the ground. Once infiltrated, the water becomes
soil moisture or
groundwater.
Subsurface Flow:The flow of water underground, in the
vadose zone and aquifers. Subsurface water may return to the surface (eg. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of
gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.
Evaporation:The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. The source of energy for evaporation is primarily
solar radiation. Evaporation often implicitly includes
transpiration from
plants, though together they are specifically referred to as
evapotranspiration. Total annual evapotranspiration amounts to approximately of water, of which evaporates from the oceans.
:The state change directly from solid water (snow or ice) to water vapor.
Advection:The movement of water — in solid, liquid, or vapor states — through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.
Condensation:The transformation of water vapor to liquid water droplets in the air, producing
clouds and
fog.
Transpiration:The release of water vapor from plants into the air. Water vapor is a gas that cannot be seen.
Residence times
The
residence time of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that reservoir (
see adjacent table). It is a measure of the average age of the water in that reservoir.
Groundwater can spend over 10,000 years beneath Earth's surface before leaving. Particularly old groundwater is called
fossil water. Water stored in the soil remains there very briefly, because it is spread thinly across the Earth, and is readily lost by evaporation, transpiration, stream flow, or groundwater recharge. After evaporating, the residence time in the atmosphere is about 9 days before condensing and falling to the Earth as precipitation.
In hydrology, residence times can be estimated in two ways. The more common method relies on the principle of
conservation of mass and assumes the amount of water in a given reservoir is roughly constant. With this method, residence times are estimated by dividing the volume of the reservoir by the rate by which water either enters or exits the reservoir. Conceptually, this is equivalent to timing how long it would take the reservoir to become filled from empty if no water were to leave (or how long it would take the reservoir to empty from full if no water were to enter).
An alternative method to estimate residence times, which is gaining in popularity for dating groundwater, is the use of
isotopic techniques. This is done in the subfield of
isotope hydrology.
Changes over time
The water cycle describes the processes that drive the movement of water throughout the
hydrosphere. However, much more water is "in storage" for long periods of time than is actually moving through the cycle. The storehouses for the vast majority of all water on Earth are the oceans. It is estimated that of the 332,500,000 mi
3 (1,386,000,000 km
3) of the world's water supply, about 321,000,000 mi
3 (1,338,000,000 km
3) is stored in oceans, or about 95%. It is also estimated that the oceans supply about 90% of the evaporated water that goes into the water cycle.
[http://ga.water.usgs.gov/edu/watercycleoceans.html USGS, The Water Cycle: Water Storage in Oceans - Retrieved on 2008-05-14]During colder climatic periods more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age glaciers covered almost one-third of Earth's land mass, with the result being that the oceans were about 400 ft (122 m) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about higher than they are now. About three million years ago the oceans could have been up to 165 ft (50 m) higher.
The scientific consensus expressed in the 2007
Intergovernmental Panel on Climate Change (IPCC) Summary for Policymakers is for the water cycle to continue to intensify throughout the 21st century, though this does not mean that precipitation will increase in all regions. In subtropical land areas — places that are already relatively dry — precipitation is projected to decrease during the 21st century, increasing the probability of
drought. The drying is projected to be strongest near the poleward margins of the
subtropics (for example, the
Mediterranean Basin,
South Africa, southern
Australia, and the
Southwestern United States). Annual precipitation amounts are expected to increase in near-equatorial regions that tend to be wet in the present climate, and also at high latitudes. These large-scale patterns are present in nearly all of the
climate model simulations conducted at several international research centers as part of the 4th Assessment of the IPCC.
Glacial retreat is also an example of a changing water cycle, where the supply of water to glaciers from precipitation cannot keep up with the loss of water from melting and sublimation.
Glacial retreat since 1850 has been extensive.
Human activities that alter the water cycle include:
- alteration of the chemical composition of the atmosphere
- removal of groundwater from wells
Effects on climate
The water cycle is powered from solar energy. 86% of the global evaporation occurs from the oceans, reducing their temperature by
evaporative cooling. Without the cooling effect of evaporation the greenhouse effect would lead to a much higher surface temperature of , and a warmer planet.
Effects on biogeochemical cycling
While the water cycle is itself a
biogeochemical cycle, flow of water over and beneath the Earth is a key component of the cycling of other biogeochemicals. Runoff is responsible for almost all of the transport of
eroded sediment and
phosphorus from land to
waterbodies. The
salinity of the oceans is derived from erosion and transport of dissolved salts from the land. Cultural
eutrophication of lakes is primarily due to phosphorus, applied in excess to
agricultural fields in
fertilizers, and then transported overland and down rivers. Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to waterbodies. The
dead zone at the outlet of the
Mississippi River is a consequence of
nitrates from fertilizer being carried off agricultural fields and funnelled down the
river system to the
Gulf of Mexico. Runoff also plays a part in the
carbon cycle, again through the transport of eroded rock and soil.
See also
