Water that keeps moving
The water on Earth today is essentially the same water that has been here for billions of years. It is not created or used up in any meaningful sense; it changes form and location. It evaporates from the sea, falls as rain on a mountain, flows into a river, soaks into the ground, and eventually returns to the ocean. This continuous movement is the water cycle, or hydrologic cycle.
Two forces drive it. Energy from the Sun heats water and causes it to evaporate, and gravity pulls it back down, whether as rain, as flowing rivers or as seeping groundwater. There is no real start or end; every step is a part of a loop.
Where Earth’s water is stored
Most of the planet’s water is in the oceans, roughly 97 percent by commonly cited estimates. Of the remainder, which is fresh water, most is locked up as ice in glaciers and ice sheets, with the rest in groundwater, lakes, rivers, soil moisture and the atmosphere. Only a small fraction of the total is surface fresh water that people can easily reach. These are rounded figures, and different sources give slightly different numbers.
The atmosphere holds only a small amount of water at any moment, but it moves a lot of water because it is turned over quickly. The unusual properties of water, such as its ability to exist as solid, liquid and gas at normal Earth temperatures, make the whole cycle possible.
Evaporation and transpiration
When the Sun warms liquid water, some molecules gain enough energy to escape into the air as water vapor. This is evaporation. Most evaporation comes from the oceans, since they cover most of the planet, but lakes, rivers, wet soil and puddles contribute too.
Plants add to the total. They take up water through their roots and release vapor through tiny pores in their leaves, a process called transpiration. Scientists often combine the two into one word, evapotranspiration, when discussing land.
Ice can also change directly into vapor without melting, a process known as sublimation, which happens slowly from snow and ice under dry, sunny, or windy conditions.
Condensation and clouds
Warm air carries water vapor upward. As it rises, it meets lower air pressure and expands, which cools it. Cooler air cannot hold as much water vapor, and at some point the vapor condenses into tiny liquid droplets, or ice crystals if it is cold enough. Condensation generally needs small particles in the air, such as dust, salt or smoke, for droplets to form around.
Billions of these droplets together make a cloud. The same process happens near the ground when air cools overnight, producing dew or fog. When water vapor condenses, it releases heat, which is one source of energy for large storms such as tropical cyclones.
Precipitation
When cloud droplets or ice crystals grow large enough, they fall as precipitation: rain, snow, sleet or hail. Many drops start as ice crystals high in the cloud and melt on the way down. How precipitation falls, how much, and where depends on local temperature, moisture, and the weather systems that lift the air, such as fronts, mountains and storms.
The distribution is uneven. Some regions receive plentiful rain and others are dry. Mountains can wring moisture out of air as it rises, leaving a drier “rain shadow” on the far side. Global patterns of wet and dry regions are tied to the circulation of the atmosphere, and ocean currents and warm or cold ocean surfaces influence them too.
What happens on the ground
Once water reaches land, there are several possible routes.
- Runoff. Water flows over the surface into streams and rivers, and eventually to lakes and the sea. Runoff is greater when the ground is steep, saturated, frozen, or covered by hard surfaces such as pavement. Heavy runoff is why intense storms can cause flash floods.
- Infiltration and groundwater. Some water soaks into the soil. Part of it is taken up by plants, and part moves deeper to fill the spaces in soil and rock. Rock layers that hold and transmit water are called aquifers. Groundwater moves slowly, and it can take anywhere from days to thousands of years to emerge again at springs or rivers, depending on the path.
- Storage as ice and snow. In cold climates or at high altitudes, precipitation accumulates as snow and ice. It may be held for months, such as in seasonal snowpack that melts into rivers in spring, or for thousands of years in ice sheets and glaciers.
- Storage in lakes and wetlands. These hold water for periods ranging from days to centuries.
Eventually, most water makes its way back to the oceans, from where evaporation starts the cycle again. The time water spends in each reservoir varies enormously: days in the atmosphere on average, but far longer in deep groundwater, ice sheets or the deep ocean.
Water and the rest of the Earth system
The water cycle is closely linked to other Earth processes. Flowing water erodes rock and carries sediment, contributing to the rock cycle. Rivers carve valleys, and over very long periods they can cut deep canyons, like the Grand Canyon. Water carries heat, because evaporation takes up energy and condensation releases it, helping to move energy around the globe in the atmosphere.
Climate also affects the cycle. A warmer atmosphere can hold more water vapor, and this is generally expected to intensify the cycle, with changes in where and how heavily rain and snow fall. The specifics vary by region, and some areas may become wetter while others become drier. Scientists use satellites and ground measurements to track the cycle, including satellite imagery of clouds, snow cover and soil moisture. Changes in snowpack, glacier melt and groundwater levels matter for communities that depend on them for drinking water and farming.
Why it matters
The water cycle supplies fresh water to ecosystems, farms and cities, and it also underlies hazards such as floods and droughts. Human activities, from building dams and paving land to pumping groundwater, change where water goes and how fast. Understanding the cycle is the starting point for understanding why water supply can vary so much from place to place and from year to year.