Water cycle

The water cycle, also referred to as the hydrologic cycle, explains the continuous movement of water within the Earth and atmosphere. While sometimes simplified to a circular cycle of evaporation, condensation, and precipitation, in reality, the paths and influences of water through Earth’s ecosystems are extremely complex and not completely understood. Water is essential to life on Earth, and in its three phases (solid, liquid, and gas), it ties together the major parts of the Earth’s climate system—air, clouds, the ocean, lakes, vegetation, snowpack, glaciers, etc.

Although the total amount of water within the cycle remains effectively constant, its distribution among the various processes is constantly changing. Key processes within the water cycle include the following:

• Liquid water evaporating and becoming water vapor, which condenses to form clouds before precipitating back to Earth in the form of rain and snow
• Water in different phases moving through the atmosphere (transportation)
• Liquid water flowing across the land (runoff), into the ground (infiltration and percolation), and through the ground (groundwater)
• Groundwater moving into plants (plant uptake) and evaporating from these plants into the atmosphere (transpiration)
• Solid ice and snow turning directly into a gas (sublimation). The opposite can also take place when water vapor becomes solid (deposition)

Roughly 119,000 cubic miles (496,000 cubic km) of water evaporates from ocean and land surfaces annually. On average, this water remains in the atmosphere for about eleven days before precipitating to the surface. The water cycle is powered by the sun, and almost half of the total solar radiation hitting the Earth's surface is absorbed by water, causing it to evaporate. Evaporation over the ocean exceeds precipitation, with the net difference representing the transport of water over land. Roughly one-third of the precipitation that hits land returns to the ocean, primarily through rivers. Groundwater discharge to the ocean only accounts for about 0.6% of the total discharge. A small amount of precipitation is temporarily stored in rivers and lakes. The remaining precipitation hitting land, ~17,500 cubic miles (73,000 cubic km) per year, returns to the atmosphere through evaporation.

Representation of the water cycle with estimates in parentheses for the volume in millions of cubic kilometers.

The abundance and timely delivery of water are critical for the needs of society and ecosystems. Humans require water for drinking, industrial applications, irrigation, hydropower, waste disposal, and recreation. In many areas, water supplies are being depleted due to population growth, pollution, and development. These stresses have been made worse by climate variations and changes that affect the hydrologic cycle. The water cycle influences the intensity of climate variability and change. It is a key part of extreme events, such as drought and floods.

The total amount of water on, in, and above the Earth is roughly 332,500,000 cubic miles (1,386,000,000 cubic km). The total amount of freshwater is 2,551,000 cubic miles (10,633,450 cubic km). 71 percent of the Earth's surface is covered in water, and oceans hold 96.5 percent of all the planet's water. Ocean water is saltwater, with freshwater only accounting for 2.5 percent of the Earth's total. Over two-thirds of the freshwater is stored in glaciers and polar ice caps; 30.1 percent is groundwater, with only 1.2 percent on the surface. Of the Earth's surface freshwater, 69 percent is stored as ice and permafrost; lakes hold 20.9 percent; and smaller amounts are stored in the atmosphere, living things, rivers, swamps/marshes, and soil moisture.

Distribution of the Earth's water.

The amount of time water spends in different reservoirs varies. The average length of time water stays in a reservoir before moving to another is known as residence time. Residence time is only meaningful for a reservoir that is at or near a steady-state condition, i.e., the reservoir empties and replenishes at the same rate. Residence time can be found from the amount of water in the reservoir divided by either the inflow or outflow. When multiple inflows and outflows occur, the net sum is used.

The table below shows typical residence times for various reservoirs:

The water cycle is made up of many processes where water moves and sometimes changes phases. While the main three are evaporation, condensation, and precipitation, there are many other important processes involved.

Evaporation is the process of a liquid changing to a gas. In the water cycle, water from oceans, lakes, rivers, and other reservoirs evaporates due to solar radiation, becoming water vapor. Evaporation is also influenced by wind, temperature, and the density of the body of water.

Condensation is the process of a gas changing to a liquid. In the water cycle, this occurs when water vapor in the atmosphere condenses into liquid. This can happen high in the atmosphere (clouds) or near the surface (fog or mist). Clouds form or become denser due to water vapor condensing. Water vapor condenses around tiny particles called cloud condensation nuclei (CCN). These could be particles of dust, salt, or pollutants. Condensation is influenced by temperature and pressure. The dew point of water vapor is highly dependent on both.

Precipitation refers to liquid or solid water falling to Earth as a result of condensation in the atmosphere. Examples include rain, snow, and hail. It is caused when water vapor condenses into bigger and bigger water droplets, becoming heavy enough to fall to Earth. Precipitation is always fresh water, even if the water originally evaporated from the ocean, as sea salt does not evaporate with the water. However, pollutants in the atmosphere can contaminate precipitation. An example of this is acid rain.

Surface runoff is water that flows from the land surface into the ocean. For example, much of the water in rivers comes directly from runoff. When rain hits saturated or impervious ground, it begins to flow overland downhill along channels, moving into larger creeks, streams, and rivers.

Sublimation is the process of solids turning directly into a gas without a liquid stage. In the context of the water cycle, this generally refers to the process of snow and ice changing directly into water vapor. While sublimation is a slower process compared with evaporation, the process accelerates in low-temperature high-pressure environments. The main sources of water from sublimation are ice sheets in the north and south polar regions and ice caps on mountains.

Deposition is the opposite of sublimation, where water vapor changes directly into ice such as snowflakes or frost.

Precipitation falling on the ground can end up being absorbed by plant roots where it is pushed towards the leaves for use in photosynthesis. Transpiration is a process in which liquid water is changed to water vapor by plants. The extra water leaves the plant system as water vapor through tiny openings known as stomata.

The process of water moving deep into the soil is called infiltration or percolation. It seeps down and increases the amount of groundwater.

The concept of a global water cycle has been studied, dating back to the ancient Greeks and Hebrews who documented important hydrological processes. Archaeological evidence from Africa, Sri Lanka, and China shows the sophisticated water management thousands of years ago. The modern theory of water cycling on Earth is credited to Bernard Palissy, who in 1580 theorized an atmospheric water cycle which is a closed system meaning water neither leaves nor enters Earth.

The deep water cycle refers to the movement of water at great depths in the Earth's crust and mantle. At subduction zones, a lighter continental plate and a heavier oceanic plate move into each other. This causes the oceanic plate to bend upwards, and the upper part of the lithosphere is subjected to an extensional regime. The tensional stress produces faulting, and vertical offset of the pieces leads the front fragments to sink downwards toward the deep-sea trench. These bend-faults provide a pathway for marine sediments and seawater to enter the slab.

Depiction of a subduction zone.

As the oceanic plate with its bend faults descends further, it is subjected to rising temperatures and pressures. Some of the water on the surface of the oceanic plate is only loosely bound to clay minerals (fine-grained portion of the marine sediments). This water gets squeezed out of the plate at shallow depths in the forearc of the subduction zone, leaving the system. However, water in the deep faults of the subducting oceanic plate comes into contact with the Earth's mantle producing stable hydrous minerals. Olivine, the major constituent mineral of the mantle, reacts with water forming the mineral serpentine, which can withstand the rising temperatures and pressures to depths of over 100km. At this depth, its crystal lattice becomes unstable, and water molecules are extracted from the mineral to mix with the overlying mantle. The water promotes the melting of the mantle wedge, generating magma that rises towards the surface, producing volcanism. Other water is transported further down into greater depths penetrating areas of the mantle far away from the subduction zone.