The water cycle, also known as the hydrologic cycle or H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth. Water can change states among liquid, vapor, and solid 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, in and out of the atmosphere. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow. In so doing, the water goes through different phases: liquid, solid, and gas.
The hydrologic cycle involves the exchange of heat energy, which leads to temperature changes. For instance, in the process of evaporation, water takes up energy from the surroundings and cools the environment. Conversely, in the process of condensation, water releases energy to its surroundings, warming the environment. The water cycle figures significantly in the maintenance of life and ecosystems on Earth. Even as water in each reservoir plays an important role, the water cycle brings added significance to the presence of water on our planet. By transferring water from one reservoir to another, the water cycle purifies water, replenishes the land with freshwater, and transports minerals to different parts of the globe. It is also involved in reshaping the geological features of the Earth, through such processes as erosion and sedimentation. In addition, as the water cycle also involves heat exchange, it exerts an influence on climate as well.
The water cycle is the continuous movement of water over, above, and beneath the Earth's surface. It is powered by solar energy, and because it is a cycle, there is no beginning or end. As water moves around in the hydrosphere, it changes state between liquid, vapour, and ice. The time taken for water to move from one place to another varies from seconds to thousands of years, and the amount of water stored in different parts of the hydrosphere ranges up to 1.37 billion km³, which is contained in the oceans. Despite continual movement within the hydrosphere, the total amount of water at any one time remains essentially constant.
Movement of water takes place by a variety of physical and biophysical processes. The two processes responsible for moving the greatest quantities of water are precipitation and evaporation, transporting 505,000 km³ of water each year. The flow of water along rivers transports an intermediate amount of water, and sublimation of ice directly to vapour transports relatively very little. The different processes are as follows.
- Canopy interception is the precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground.
- Snowmelt refers to the runoff produced by melting snow.
- Runoff]] includes 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.
- Subsurface Flow is 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 is 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. Approximately 90% of atmospheric water comes from evaporation, while the remaining 10% is from transpiration. Total annual evapotranspiration amounts to approximately 505,000 km³ of water, 434,000 km³ of which evaporates from the oceans.
- Sublimation is the state change directly from solid water (snow or ice) to water vapor.
- Advection is the movement of water—in solid, liquid, or vapour states—through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.
- Condensation is the transformation of water vapour to liquid water droplets in the air, producing clouds and fog.
|Reservoir||Volume of water
|Ice caps & glaciers||29||2.05|
|Streams & rivers||0.0017||0.0001|
In the context of the water cycle, a reservoir represents the water contained in different steps within the cycle. The largest reservoir is the collection of oceans, accounting for 97% of the Earth's water. The next largest quantity (2%) is stored in the ice caps and glaciers. The water contained within all living organisms represents the smallest reservoir.
|Reservoir||Average residence time|
|Glaciers||20 to 100 years|
|Seasonal snow cover||2 to 6 months|
|Soil moisture||1 to 2 months|
|Groundwater: shallow||100 to 200 years|
|Groundwater: deep||10,000 years|
|Lakes||50 to 100 years|
|Rivers||2 to 6 months|
The residence time of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that reservoir (see the adjacent table). It is a measure of the average age of the water in that reservoir, though some water will spend much less time than average, and some much more.
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, water remains in the atmosphere for 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, gaining in popularity particularly for dating groundwater, is the use of isotopic techniques. This is done in the subfield of isotope hydrology.
Changes over time
Over the past century the water cycle has become more intense, with the rates of evaporation and precipitation both increasing. This is an expected outcome of global warming, as higher temperatures increase the rate of evaporation due to warmer air's higher capacity for holding moisture.
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
- construction of dams
- deforestation and afforestation
- removal of groundwater from wells
- water abstraction from rivers
Relationship with climate
The water cycle is powered from solar energy. 86% of the global evaporation occurs from the oceans, reducing their temperature by evaporative cooling. 
Most of the solar energy warms tropical seas. After evaporating, water vapour rises into the atmosphere and is carried by winds away from the tropics. Most of this vapour condenses as rain in the Intertropical convergence zone, also known as the ITCZ, releasing latent heat that warms the air. This in turn drives the atmospheric circulation.
Global warming adds volume and energy to the global hydrological system resulting in an increase in precipitation, if viewed on a global basis. Unpredictable changes in the pattern of atmospheric circulation are possible, including droughts and floods which are not compatible with patters of human settlement and infrastructure.
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.
- U.S. Geologic Survey. Water Cycle. Retrieved on 2006-10-24.
- Arctic Climatology and Meteorology. Precipitation. Retrieved on 2006-10-24.
- Dr. Art's Guide to Planet Earth. The Water Cycle. Retrieved on 2006-10-24.
- National Weather Service Northwest River Forecast Center. Hydrologic Cycle. Retrieved on 2006-10-24.
- Arctic Climatology and Meteorology. Evaporation. Retrieved on 2006-10-24.
- Dr. Art's Guide to Planet Earth. The Water Cycle. Retrieved on 2006-10-24.
- Arctic Climatology and Meteorology. Sublimation. Retrieved on 2006-10-24.
- Arctic Climatology and Meteorology. Advection. Retrieved on 2006-10-24.
- Arctic Climatology and Meteorology. Condensation. Retrieved on 2006-10-24.
- PhysicalGeography.net. CHAPTER 8: Introduction to the Hydrosphere. Retrieved on 2006-10-24.
- Environmental Literacy Council. Water Cycle. Retrived on 2006-10-24.
- U.S. Geologic Survey. Century of data shows intensification of water cycle but no increase in storms or floods. Retrieved on 2006-10-24.
- University of Massachusetts. Reducing Humidity in the Greenhouse. Retrieved on 2006-10-24.
- U.S. Geologic Survey. GLACIER RETREAT IN GLACIER NATIONAL PARK, MONTANA. Retrieved on 2006-10-24.
- Science at NASA. NASA Oceanography: The Water Cycle. Retrieved on 2006-10-24.
- The Environmental Literacy Council. Biogeochemical Cycles. Retrieved on 2006-10-24.
- The Environmental Literacy Council. Phosphorus Cycle. Retrieved on 2006-10-24.
- Ohio State University Extension Fact Sheet. Nitrogen and the Hydrologic Cycle. Retrieved on 2006-10-24.
- NASA's Earth Observatory. The Carbon Cycle. Retrieved on 2006-10-24.
- The Water Cycle, United States Geological Survey
- The water cycle, from Dr. Art's Guide to the Planet.
- Water cycle slideshow, 1 Mb Flash multilingual animation highlighting the often-overlooked evaporation from bare soil, from managingwholes.com.