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The water cycle

Learn how water moves through Earth's ecosystems.

Key points

  • The vast majority of Earth's water is saltwater found in oceans. Only a tiny fraction is readily accessible freshwater, which is what humans need.
  • Water found at the Earth's surface can cycle rapidly, but much of Earth's water lies in ice, oceans, and underground reservoirs; this water cycles slowly.
  • The water cycle is complex and involves state changes in water as well as the physical movement of water through and between ecosystems.
  • Groundwater is found underground between soil particles and in cracks of rocks. Aquifers are groundwater reservoirs often tapped by wells.

Water: Why does it matter?

Water is pretty darn important for living things. Your body is more than one-half water, and if we were to take a look at your cells, we’d find they were over 70% water! So, you—like most land animals—need a reliable supply of fresh water to survive.
Of the water on Earth, 97.5% is salt water. Of the remaining water, over 99% is in the form of underground water or ice. All told, less than 1% of fresh water is found in lakes, rivers, and other available surface forms.
This chart doesn't mention small water reservoirs, such as the atmosphere and the bodies of living organisms. For a more detailed breakdown, see the USGS Water Science School website.1 Image credit: Biogeochemical cycles: Figure 1 by OpenStax College, Concepts of Biology, CC BY 4.0
Many living things depend on this small supply of surface fresh water, and lack of water can have serious effects on ecosystems. Humans, of course, have come up with some technologies to increase water availability. These include digging wells to get at groundwater, collecting rainwater, and using desalination—salt removal—to get fresh water from the ocean. Still, clean, safe drinking water is not always available in many parts of the world today.
Most of the water on Earth does not cycle—move from one place to another—very rapidly. We can see this in the figure below, which shows the average time that an individual water molecule spends in each of Earth’s major water reservoirs, a measurement called residence time. Water in oceans, underground, and in the form of ice tends to cycle very slowly. Only surface water cycles rapidly.
Image credit: "Biogeochemical cycles: Figure 3" by OpenStax College, Biology, CC BY 4.0

The water cycle

The water cycle, or hydrologic cycle, is driven by the Sun’s energy. The sun warms the ocean surface and other surface water, causing liquid water to evaporate and ice to sublime—turn directly from a solid to a gas. These sun-driven processes move water into the atmosphere in the form of water vapor.
Over time, water vapor in the atmosphere condenses into clouds and eventually falls as precipitation, rain or snow. When precipitation reaches Earth's surface, it has a few options: it may evaporate again, flow over the surface, infiltrate into the soil, or percolate—sink down—into the ground.
In land-based, or terrestrial, ecosystems in their natural state, rain usually hits the leaves and other surfaces of plants before it reaches the soil. Some water evaporates quickly from the surfaces of the plants. The water that's left reaches the soil and, in most cases, will begin to move down into it.
In general, water moves along the surface as runoff only when the soil is saturated with water, when rain is falling very hard, or when the surface can't absorb much water. A non-absorbent surface could be rock in a natural ecosystem or asphalt or cement in an urban or suburban ecosystem.
Image credit: The water cycle by NOAA National Weather Service Jetstream, CC BY 2.0
Water in the upper levels of the soil can be taken up by plant roots. Plants use some of the water for their own metabolism, and water that's in plant tissues can find its way into animals’ bodies when the plants get eaten. However, most of the water that enters a plant's body will be lost back to the atmosphere in a process called transpiration. In transpiration, water enters through the roots, travels upwards through vascular tubes made out of dead cells, and evaporates through pores called stomata found in the leaves.
If water is not taken up by plant roots, it may percolate down into the subsoil and bedrock, forming groundwater. Groundwater is water found in the pores between particles in sand and gravel or in the cracks in rocks, and it’s an important reservoir of freshwater. Shallow groundwater flows slowly through pores and fissures and may eventually find its way to a stream or lake, where it can become part of the surface water again.
Some groundwater lies deep in the bedrock and can stay there for millennia. Groundwater reservoirs, or aquifers, are usually the source of drinking or irrigation water drawn up through wells. Today, many aquifers are being used up faster than they're renewed by water that moves down from above.

The water cycle drives other cycles.

The water cycle is important in itself, and patterns of water cycling and rainfall have major effects on Earth's ecosystems. However, rainfall and surface runoff also play important roles in the cycling of various elements. These include carbon, nitrogen, phosphorus, and sulfur. In particular, surface runoff helps move elements from terrestrial, land-based, to aquatic ecosystems.
We'll take a closer look at how this works in the following articles, where we'll examine different elements' biogeochemical cycles.

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