Water Cycle

The water cycle, also called the hydrologic cycle, is the continuous circulation of water within the Earth-atmosphere system, and its most important processes are evaporation, transpiration, condensation, precipitation, and runoff. It is a biogeochemical cycle, one defined by the continuous change in form of water across different reservoirs.

The total amount of water within the cycle remains essentially constant, yet its distribution among the various processes is continually changing.

Water is stored in the atmosphere, on the land surface and below the ground, and it can be a liquid, a solid or a gas. The vast majority of water on the Earth's surface, over 96 percent, is saline water in the oceans, and the Earth's water content is about 1.39 billion cubic kilometers (331 million cubic miles), with about 96.5% of it in the global oceans. Water is the only common substance that can exist naturally as a gas, liquid, or solid within the relatively small range of temperatures and pressures found at the Earth's surface. Viewed from space, water is the most striking feature of our planet: in both liquid and frozen form it covers 75 percent of the Earth's surface. The volume of all water on, in and above the Earth is about 332,500,000 cubic miles (1,386,000,000 cubic kilometers), a sphere about 860 miles in diameter.

There is much more freshwater stored in the ground than there is in liquid form on the surface, and some of the water flowing in rivers comes from seepage of groundwater into river beds. The upper layer of the soil is the unsaturated zone, and below it lies the saturated zone, called groundwater, where liquid water is stored in pores, cracks and spaces between rock particles. The idea of rivers flowing underground is largely a myth: the vast majority of underground water occupies the spaces between rocks and subsurface material. Water may travel long distances or remain in groundwater storage for long periods before returning to the surface or seeping into other water bodies such as streams and the oceans. Ocean circulation is the largest water flux, estimated at about 7,100,000 cubic kilometers per year, followed by ocean evaporation at about 420,500 cubic kilometers per year. The transport of moisture from ocean to land and streamflow to the ocean are each estimated at about 46,000 cubic kilometers per year.

Almost 90 percent of Earth's ice mass is in Antarctica, while the Greenland ice cap contains 10 percent of the total global ice mass. Lake Baikal is the world's oldest, largest, and deepest freshwater lake, nearly a mile deep and holding over 23,000 cubic kilometers of water. Water vapor in the atmosphere is a powerful greenhouse gas and a major driver of weather and climate.

Water occurs naturally on Earth as a gas, a liquid, and a solid, and it is essential to life on the planet. Heat is absorbed as water transitions from liquid to vapor through evaporation, and released when water condenses or freezes.

Evaporation is the process that changes liquid water into gaseous water, or water vapor, moving water from the Earth's surface into the atmosphere. About 90 percent of the moisture in the atmosphere comes from water evaporating from oceans, seas, lakes, and rivers, while the other 10 percent comes from transpiration from plants. The ocean is the source of 86 percent of global evaporation.

Only about 10 percent of the water evaporated from the oceans is transported over land and falls as precipitation. Once evaporated, a water molecule spends about 10 days in the air.

Condensation is the process by which water vapor in the air is changed into liquid water, and it is responsible for the formation of clouds. Whether it occurs is not a matter of one particular temperature but of a difference between two temperatures: the air temperature and the dewpoint temperature, the latter being the temperature at which air becomes saturated and cannot hold any more water vapor. As condensation proceeds and liquid water forms from vapor, the water molecules become more organized and heat is released into the atmosphere.

Clouds exist in the atmosphere because of rising air, and since they drift over the landscape they are one of the ways water moves geographically around the globe. They float because the density of the same volume of cloud material is less than the density of the same amount of dry air. One estimate of cumulus cloud density is about 0.5 gram per cubic meter, so a cloud of 1 cubic kilometer contains roughly 500,000,000 grams of water droplets.

Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail, and it is the main way atmospheric water returns to the surface of the Earth.

Millions of cloud droplets are required to produce a single raindrop. Ice crystals can grow rapidly at the expense of water vapor in a cloud in the Bergeron-Findeisen process.

One process by which precipitation forms is collision and coalescence, or the warm rain process, in which cloud droplets of varying size collide and stick together into larger drops.

Surface runoff, especially when it runs across surfaces like roads, can pick up and then deposit particulate matter and sediment into rivers, which is not good for water quality.

Some water that infiltrates will remain in the shallow soil layer, where it moves gradually vertically and horizontally through the soil and subsurface material.

Gravity and pressure move water downward and sideways underground through spaces between rocks, and it eventually emerges back to the land surface, into rivers and the oceans. The estimated groundwater pool is about 22,600,000 cubic kilometers, and groundwater recharge is estimated at about 13,000 cubic kilometers per year.

At high elevations or latitudes where snowpack persists year-round, the weight of the accumulated snow compresses it into ice. When ice forms in a steep mountain environment, it slowly flows downhill, forming a glacier. The Greenland ice sheet is about 600,000 cubic miles (2.5 million cubic kilometers) in volume and averages about 5,000 feet (1,500 meters) in thickness.

A lake forms where surface-water runoff and some groundwater seepage have accumulated in a low spot, with water entering faster than it can escape. A reservoir, by contrast, is a manmade lake created when a dam is built on a river, with river water backing up behind the dam.

Not all lakes are fresh. Some lakes, such as the Great Salt Lake in Utah, are saltier than the oceans, and the Dead Sea is too salty for aquatic life.

The atmosphere functions as a superhighway in the sky, carrying water across the globe. Were all of that water to rain down at once, it would cover the globe to a depth of only 2.5 centimeters, about 1 inch.

One estimate of the volume of water held in the atmosphere at any one time is about 3,100 cubic miles (12,900 cubic kilometers), only about 0.001 percent of Earth's total water volume.

The water cycle is often taught as a simple, circular cycle of evaporation, condensation, and precipitation, but the reality of the cycle is much more complicated.^{12The water cycle · National Oceanic and Atmospheric Administration ↗}

Human water use, land use and climate change all impact the water cycle, and understanding these impacts helps work toward using water sustainably. The water in your watershed quenches thirst, grows food, washes clothes, and powers industry. Too much water can cause raging floods and flush pollutants and soil into rivers and streams.

• 1997–2015
  TRMM
  TRMM was a research satellite in operation from 1997 to 2015 and was designed to improve understanding of the distribution and variability of precipitation within the tropics.^{18The Tropical Rainfall Measuring Mission (TRMM) · NASA Global Precipitation Measurement ↗} By covering the tropical and sub-tropical regions of the Earth, TRMM provided information on rainfall and its associated heat release, which helps power the global atmospheric circulation.^{18The Tropical Rainfall Measuring Mission (TRMM) · NASA Global Precipitation Measurement ↗}
• 27 February 2014
  GPM Core Observatory
  Building upon the success of TRMM, the GPM concept centers on a Core Observatory satellite carrying an advanced radar and radiometer system to measure precipitation from space.^{19The Global Precipitation Measurement Mission (GPM) · NASA Global Precipitation Measurement ↗}
• 22 May 2018
  GRACE-FO
  The GRACE-FO orbiter launched on May 22 in 2018 with the objective of tracking Earth's water movement.^{20GRACE-FO · NASA Science ↗}

GRACE-FO is a successor to the original GRACE mission, which orbited Earth from 2002 to 2017 and tracks Earth's water movement.

Measurement of the water cycle relies on instruments that track where water is and how it moves. Rain gages are used to keep track of precipitation, while stream gages keep track of how much water is in rivers. These observations support a broader effort: NOAA is striving to expand understanding of the water cycle at global to local scales to improve the ability to forecast weather, climate, water resources, and ecosystem health.