When astronomers search the galaxy for life, they look for signs of water. Water supports life, but it can also change form, wear down mountains, build clouds, and warm the poles. Regardless of the amount, location, or form, water is in constant motion. The water cycle includes places where water is stored (reservoirs) for short and long periods and processes that move water (flows) and other substance, shaping the face of the planet and making life on Earth possible.
Water is stored in different parts of the water cycle anywhere from minutes to millennia. These natural reservoirs contain more than 330 million cubic miles (mi3), or 1.4 billion cubic kilometers (km3), a small portion of which touches every life on Earth every day. (One cubic mile (mi3) is equal to more than 1 trillion gallons. On cubic kilometer (km3) is exactly 1 trillion liters.)
Ocean: The ocean contains 97 percent of Earth's known water supply, or 0.32 billion mi3 (1.335 billion km3). Because of its size, the ocean accounts for most of the planet's evaporation and precipitation and also helps transport heat, nutrients, and other materials around the globe.
Atmosphere: The atmosphere holds merely 0.001 percent, or 3.05 mi3 (12.7 km3), of Earth's water. Much of this remains in the atmosphere for only a few days on average, but this reservoir is the primary means of moving water great distances, particularly far inland.
Cryosphere: The frozen places on Earth, in particular ice in Antarctica and Greenland, account for 2 percent, or 6.1 million mi3 (25.5 million km3), of water and about two-thirds of fresh water. Many people rely on runoff from mountain glaciers and snowfields for their water supply.
Land: Water on and beneath the land (surface- and groundwater) account for just one percent, or 3.7 mi3 (15.55 km3), of Earth's water. This crucial source makes up the majority of what is known as "available water"—water that can be easily tapped for human use.
Lithosphere: Earth's crust and mantle, contains a vast, but unknown amount of water. This takes two forms: "free" water in cracks and pores, and chemically bound water—hydrated minerals formed in the presence of water and that contain hydrogen and oxygen.
Water is the only natural substance that can exist as a solid, liquid, or gas under everyday conditions. It is also the "universal solvent," capable of dissolving more substances than any other liquid. Water's ability to take many forms and to transport other materials, including heat, is a key reason why life exists on Earth. (One Sverdrup (Sv) is a flow rate of 1 million cubic meters, or 264 million gallons, per second.
Currents & Circulation: Winds create surface currents, such as the Gulf Stream which ranges from 30 to 150 Sv. Differences in ocean temperature and salinity also drive thermohaline circulation (the ocean conveyor), parts of which are estimated to be more than 50 Sv.
Ocean evaporation: Evaporation over the ocean is 85 percent of the global total, more than 3 times what occurs over land and equivalent to between 11.7 and 14.3 Sv.
Ocean precipitation: More than three-quarters of global precipitation occurs over the ocean, a flow of water equal to between 10 and 13.4 Sv.
Evapotranspiration: Evapotranspiration combines evaporation (liquid water changing to vapor) and transpiration (water released by plants). Evapotranspiration over land totals about 2.5 Sv, or 15 percent of the water entering the atmosphere from Earth's surface.
Land precipitation: Less than one-quarter of global precipitation occurs over land, a flow of water from the atmosphere equal to about 3.5 Sv.
Sublimation: Under certain conditions, water can change directly from a solid (snow and ice) into a gas (water vapor) without melting, a process known as sublimation.
Wind transport: Winds move water vapor and heat energy long distances across the face of Earth in a relatively short period of time.
Groundwater: Water flowing from the surface of the land into the ground forms groundwater, one of the most important, and least renewable, water resources on Earth. Some deep aquifers hold water for tens of thousands of years.
Human use: Agriculture currently accounts for 70 percent of water use worldwide, 22 percent is used by industry, and 8 percent is consumed as drinking water.
Rivers and runoff: Rivers carry sediment, nutrients and other materials, making them an important link between land and ocean. The entire global input of fresh water from rivers to the ocean is equal to about 1 Sv. Currently, more than half of global river flows and surface runoff are diverted for human use.
Hydrothermal vents: In some places, water seeping into the seafloor is heated by magma. Some of the hot water reacts with rocks to form hydrated minerals; some rises back to the seafloor at hydrothermal vents, carrying with it chemicals and trace elements that mix into the ocean.
Subduction: Hydrated minerals, which contain chemically bound water, are carried beneath the seafloor at subduction zones, where the material is recombined into the crust and mantle. By some estimates, there is as much water in Earth's mantle as in all other parts of the water cycle combined.
Volcanoes: Volcanic eruptions release large amounts of steam into the atmosphere (or, in the case of seafloor volcanoes, water into the ocean). Volcanic dust particles also act as "condensation nuclei" that provide a starting point for clouds to form.