Water is in constant motion. Energy from the sun and the force of gravity drive the hydrologic cycle, which is the endless circulation of water between the land, oceans, and atmosphere (air surrounding Earth). Water also changes in form: from gas (water vapor), to liquid, to solid (ice). Rain and snow falling on the land runs off into streams and lakes, or soaks into soil and rocks.
Streams and rivers carry water downhill to lakes and, ultimately, to the ocean. Heat energy from the Sun transforms liquid water at the surface of lakes and oceans and other bodies of water into water vapor. Water vapor in the atmosphere rises and forms clouds. Cooling within clouds causes water vapor to become liquid once again. Rain and snow fall and the cycle begins anew.
The water budget
Earth’s water budget, the total amount of water on the planet, does not change over time. The hydrologic cycle is a closed system. Water is constantly moving and changing form, but it is neither created nor destroyed. With the exception of a very small amount of water added to the hydrologic system by volcanic eruptions and meteors from space, Earth’s total water supply is constant.
In fact, most of the water on Earth today has been recycling through the hydrologic system for billions of years. The same water that comes from a kitchen faucet today could have been drunk by a dinosaur 170 million years ago during the Jurassic Period. It could have been frozen in an ice sheet during the Pleistocene Epoch (a division of geologic time that lasted from 2 million to 10,000 years ago), and could have flowed through a canal in the Roman Empire two thousand years ago.
It could have been snow in the Rocky Mountains last winter, flowed in a river to the city’s municipal water intake, and out of the faucet this morning. Maybe it will return to the river via the sink drain and city sewage system, and then flow to the ocean.
Within the hydrologic system, water resides in environments called reservoirs. Earth’s largest reservoirs, the oceans, contain about 97% of the planet’s total water. Ice, including sheets of ice on the North and South Poles and mountain glaciers (a large body of slow moving ice), and groundwater reservoirs called aquifers hold most of the remaining 3%. Reservoirs of readily useable fresh water— rivers, lakes, soil moisture, atmospheric water vapor, and water in living cells— account for only about 1% of the fresh water, and less than 0.02% of water on Earth.
If a bathtub filled with 100 gallons (379 liters) of water represented Earth’s total water budget, three gallon (11 liter) jugs would hold all the fresh water, and the fresh water available for immediate use by humans would only fill a tablespoon. A microscope would be needed to see the droplet representing the water bound up in plants and animals.
All of Earth’s water molecules are in constant motion. (A molecule is the smallest particle of a substance that has the chemical characteristics of the substance. A water molecule, symbolized by H2O, is made up of two hydrogen atoms and an oxygen atom.) Processes move water from one reservoir to another and within reservoirs. Liquid water flows downhill and circulates within lakes and oceans. Clouds of water vapor, liquid droplets, and ice crystals (snow) move across the sky. Even molecules bound in glacial ice flow downhill.
Energy from the Sun and the downward pull of gravity ultimately drive all the processes within the hydrologic cycle. Water cycle processes include evaporation, condensation, convection, precipitation, freezing and melting, groundwater flow, and runoff.
- Evaporation is the conversion of water from a liquid to a gas. Water moves from bodies of water and land to the atmosphere when heat from the Sun transforms liquid water to water vapor. Most (about 80%) of the water vapor in the atmosphere evaporates from the oceans, especially the tropical oceans near the equator. Transpiration is evaporation of water from the leaves and stems of plants. It contributes about 10% of the water vapor in the atmosphere, and evaporation from inland seas, lakes and rivers accounts for the remaining ten percent.
- Condensation is the conversion of water from a gas to a liquid. As air containing molecules of water rises in the atmosphere, the air cools, and the motion of the water molecules slows. The slower-moving water molecules accumulate as water vapor in the rising air. Water vapor then forms droplets of liquid water that group together into clouds, and eventually can fall as rain.
- Convection is the large-scale circulation of the atmosphere and oceans. Warm air or water rises and cool air or water sinks, creating currents (a steady flow in a dominant direction) that transport water around the globe. Convection causes winds that blow rain clouds over the continents, and ocean currents that transport heat, and affect global climate.
- Precipitation is the transfer of water from the atmosphere to Earth’s surface. Rain, snow, sleet, and hail are all types of precipitation. When condensed water droplets or ice crystals in a cloud become too large and heavy to remain aloft, they fall to the ground as precipitation. Amounts of precipitation vary greatly between locations. For example, the deserts of the American Southwest receive less than 1 inch (2.5 centimeters) of rain per year, while the summit of Mt. Waialeale on the Hawaiian island of Kauai receives more than 400 inches (1,016 centimeters) of rain per year. Heavy precipitation over a short amount of time can cause rivers and groundwater reservoirs to overflow and lead to flooding. Lack of normal levels of precipitation for an extended period of time causes the dried soil and reduced water supplies associated with drought.
- Freezing and melting are the transformations between liquid and solid water. Most freezing occurs in the atmosphere where condensed water vapor forms ice crystals in clouds. Glaciers form in areas near the North and South Poles and in high mountains where more snow falls than melts each year and ice accumulates over many years. In many regions, melting snow and ice replenish river and groundwater flow, as in aquifers, every spring. During the cold winter months in some regions, the surfaces of lakes and rivers freeze. In polar regions, even the seawater and groundwater freeze.
- Groundwater flow is the movement of liquid water through the pores (openings) in soils and cavities in rocks near Earth’s surface. Surface water becomes groundwater by soaking into these tiny spaces, which were filled with air. Groundwater then percolates downward to the surface of the water table, the line where all the spaces are saturated (completely full) with water. Water below the water table flows toward areas of lower pressure where it can be released, such as springs or wells.
- Runoff is the transfer of water from the land surface to the oceans via streams and lakes. (Lakes only hold runoff temporarily, and lake water eventually ends up in the ocean.) Runoff consists of precipitation that neither evaporates back into the atmosphere, nor infiltrates into groundwater. Groundwater discharge can also replenish runoff. Excess runoff leads to flooding.
Dynamic equilibrium and residence times
All water molecules are in motion, but the total volume of water in a particular reservoir stays relatively constant because of a phenomenon called dynamic equilibrium. The processes that remove water molecules from a reservoir are balanced by the processes that add water.
To illustrate, imagine trying to maintain a constant volume of water in a bathtub with an open drain. When the faucet is adjusted to add water at the same rate as it is draining, the water level stays constant, and dynamic equilibrium is reached.
In the same way, sea level stays constant because the amount of water evaporating into the atmosphere matches the amount of water entering from rivers and melting glaciers. Over geologic time (the time from the formation of Earth to the present), this balance changes and the sea level rises and falls.
The atmosphere transfers water from the ocean to the land, but it only holds a tiny portion (.001%) of Earth’s total water. Water has a short residence time in the atmosphere. Almost as soon (usually a few hours) as it evaporates into the air, water vapor condenses and falls again as precipitation.
Water molecules stay in some glaciers, oceans, and groundwater reservoirs for thousands of years, while others only spend a few days or weeks in a reservoir. To maintain dynamic equilibrium, water must leave the reservoir at the same rate that it enters. In reservoirs with very long residence times, a change in the rate of water that enters or leaves can quickly affect the reservoir volume.
For example, the Ogallala groundwater reservoir in the U.S. Great Plains region is a sandstone (rock formed from the compaction of sand) layer that filled with water a thousand years ago when the climate was wetter. In modern times, ranchers in Texas, Oklahoma, Kansas, Nebraska, and other states are using up the stored groundwater by withdrawing it much more quickly than it replenishes in today’s dryer climate.
The hydrologic cycle as a component of the Earth system
The hydrologic cycle is intertwined with the other cycles that make up the Earth system. Moving water chemically and physically erodes (wears away) the solid Earth. It transports sediments (fine soil and other particles) and deposits them in river floodplains (lands near rivers that disperse overflow), deltas (where a river enters a lake or ocean, and continental margins (edges of continents that are underwater). It sculpts the land surface and seafloor.
Water carries dissolved minerals and nutrients that nourish freshwater and marine ecosystems. Water in the oceans and atmosphere regulates Earth’s climate and weather, which makes the planet habitable for biological life. Water is the largest component of most biological organisms. Jellyfish are more than 90% water. If a person weighs 120 pounds (54 kilograms), about 72 pounds (33 kilograms) of his or her weight is water.
Water is Earth’s most essential renewable resource. It is conserved within the Earth system and cannot be “used up.” However, water is very scarce in some regions and overly abundant in others. Deserts, rainforests, canyons, droughts, and floods all result from the uneven distribution of water on Earth.
Scientists have concluded that human activities such as damming rivers, polluting waters, transporting water to arid (dry) regions to grow crops, and contributing to global climate change can alter the hydrologic cycle and change the patterns of water distribution. Water is continuously recycled and is ultimately a renewable resource, but challenges remain to manage water resources as the human population grows.