Water cycle
The water cycle is a biogeochemical cycle that involves the continuous change in form of water on, above and below the surface of the Earth across different reservoirs. The mass of water on Earth remains fairly constant over time. However, the partitioning of the water into the major reservoirs of ice, fresh water, salt water and atmospheric water is variable and depends on climatic variables. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere due to a variety of physical and chemical processes. The processes that drive these movements, or fluxes, are evaporation, transpiration, condensation, precipitation, sublimation, infiltration, surface runoff, and subsurface flow. In doing so, the water goes through different phases: liquid, solid and vapor. The ocean plays a key role in the water cycle as it is the source of 86% of global evaporation.
The water cycle is driven by energy exchanges in the form of heat transfers between different phases. The energy released or absorbed during a phase change can result in temperature changes. Heat is absorbed as water transitions from the liquid to the vapor phase through evaporation. This heat is also known as the latent heat of vaporization. Conversely, when water condenses or melts from solid ice it releases energy and heat. On a global scale, water plays a critical role in transferring heat from the tropics to the poles via ocean circulation.
The evaporative phase of the cycle also acts as a purification process by separating water molecules from salts and other particles that are present in its liquid phase. The condensation phase in the atmosphere replenishes the land with freshwater. The flow of liquid water transports minerals across the globe. It also reshapes the geological features of the Earth, through processes of weathering, erosion, and deposition. The water cycle is also essential for the maintenance of most life and ecosystems on the planet.
Human actions are greatly affecting the water cycle. Activities such as deforestation, urbanization, and the extraction of groundwater are altering natural landscapes all have an effect on the water cycle. On top of this, climate change is leading to an intensification of the water cycle. Research has shown that global warming is causing shifts in precipitation patterns, increased frequency of extreme weather events, and changes in the timing and intensity of rainfall. These water cycle changes affect ecosystems, water availability, agriculture, and human societies.
Description
Overall process
The water cycle is powered by the energy emitted from the sun. There are several ways in which this is accomplished, one of the first ways is through evaporation where the energy from the sun heats the water in oceans, lakes, streams, rivers, seas, ponds, etc. and that water goes through a phase change to become a gas that goes up into the atmosphere. Two other ways that water gets into the atmosphere is through snow and ice sublimating into water vapor and through evapotranspiration which is water transpired from plants and evaporated from the soil.Clouds form because water molecules have a smaller molecular mass than the major gas components of the atmosphere ; this smaller molecular mass leads to water having a lower density which drives the water molecules higher up in the atmosphere due to buoyancy. However, as altitude increases, air pressure decreases which causes a drop in temperature. The lower temperature forces the water vapor to go through another phase change, this time it forces it to condense into liquid water droplets which are supported by an updraft; if there is enough of these water droplets over a large area, it is considered a cloud. Condensation of the water vapour closer to the ground level is referred to as fog.
Atmospheric circulation moves water vapor around the globe; cloud particles collide, grow, and fall out of the upper atmospheric layers as precipitation. Some precipitation falls as snow, hail, or sleet, and can accumulate in ice caps and glaciers, which can store frozen water for thousands of years. Most water falls as rain back into the ocean or onto land, where the water flows over the ground as surface runoff. A portion of this runoff enters rivers, with streamflow moving water towards the oceans. Runoff and water emerging from the ground may be 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 can store freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies as groundwater discharge or be taken up by plants and transferred back to the atmosphere as water vapor by transpiration. Some groundwater finds openings in the land surface and emerges as freshwater springs. In river valleys and floodplains, there is often continuous water exchange between surface water and ground water in the hyporheic zone. Over time, the water returns to the ocean, to continue the water cycle.
The ocean plays a key role in the water cycle. The ocean holds "97% of the total water on the planet; 78% of global precipitation occurs over the ocean, and it is the source of 86% of global evaporation".
Important physical processes within the water cycle include :
- Advection: The movement of water through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land. Atmospheric rivers that move large volumes of water vapor over long distances are an example of advection.
- Condensation: The transformation of water vapor to liquid water droplets in the air, creating clouds and fog.
- 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. 86% of global evaporation occurs over the ocean.
- Infiltration: The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater. A recent global study using water stable isotopes, however, shows that not all soil moisture is equally available for groundwater recharge or for plant transpiration.
- Percolation: Water flows vertically through the soil and rocks under the influence of gravity.
- 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 falls as precipitation each year, of it over the oceans. The rain on land contains of water per year and a snowing only. 78% of global precipitation occurs over the ocean.
- 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 seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.
- Subsurface flow: The flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface 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.
- Transpiration: The release of water vapor from plants and soil into the air.
Residence times
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.
The major ice sheets – Antarctica and Greenland – store ice for very long periods. Ice from Antarctica has been reliably dated to 800,000 years before present, though the average residence time is shorter.
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.
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.