AP®︎/College Environmental science
The water cycle
Learn how water moves through Earth's ecosystems.
- 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.
The pie chart shows that 97.5% of water on Earth, or 1,365,000,000 kilometers cubed, is salt water. The remaining 2.5%, or 35,000,000 kilometers cubed, is fresh water. Of the fresh water, 68.9% is frozen in glaciers or permanent snow cover. Groundwater—such as soil moisture, swamp water, and permafrost—account for 30.8%. The remaining 0.3% is in lakes and rivers.
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.
Bars on the graph show the average residence time for water molecules in various reservoirs. The residence time for glaciers and permafrost is 1,000 to 10,000 years. The residence time for groundwater is two weeks to 10,000 years. The residence time for oceans and seas is 4,000 years. The residence time for lakes and reservoirs is 10 years. The residence time for swamps is one to 10 years. The residence time for soil moisture is two weeks to one year. The residence time for rivers is two weeks. The atmospheric residence time is 1.5 weeks. The biospheric residence time, or residence time in living organisms, is one week.
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.
Water evaporates form the ocean surface and forms clouds by condensation. Water in clouds may fall as precipitation over either the land or the sea. Clouds formed over the sea may move over the land. When rain falls over the land, it may flow along the surface, infiltrate the soil—move into it from above ground—and percolate through the soil, moving downward to become groundwater. Groundwater in upper levels may flow into rivers, lakes, or oceans. Water near the soil surface may be taken up by plants and move out of their bodies through transpiration from the leaves. Snowmelt runoff and sublimation of snow and ice are other processes that contribute to the water cycle.
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.
Want to join the conversation?
- I am wondering, how can we empty aquifers faster than they replenish? Shouldn't they be filled up continuously since it's all a cycle? Or is the water that we use from these aquifers raining down, or flowing down, into other areas or the ocean?(20 votes)
- That is very interesting i actually didnt know that ;-)(0 votes)
- Can water pollutants and air pollutants interfere with the water cycle and the ecosystem around it?(6 votes)
- No it’s scientifically impossible(0 votes)
- See the image: what is the difference between infiltration and percolation?(7 votes)
- Infiltration is defined as the downward entry of water into the soil or rock surface and percolation is the flow of water through soil and porous or fractured rock.
So percolation is form of infiltration.
- how do aquifers help the water cycle ?(5 votes)
- Aquifers show up as springs in some places. Sometimes, they even form underground rivers which eventually may join a surface river. In this way, they contribute to the water cycle. Sometimes, they may choose to stay confined in the underground regions.(2 votes)
- What is the role of moisture for bringing rain? Does low pressure or high pressure affect the amount of rain?(6 votes)
- Most of time low pressure is better for rain fall.(0 votes)
- why is clouds white colour?(4 votes)
- because vapor is kind of white and combined with dust i guess it pretty much makes a white thing(2 votes)
- Why is there more salt water than fresh water(4 votes)
- Linked to VeganBackpacking's question, if we drained too much water from aquifers near the ocean, what effect would that have on the plants there?(4 votes)
- ("some groundwater lies deep in the bedrock and can stay there for mileninnia") What happens with this water that never comes out?Don't we use it?Thanks😊(3 votes)
- what's the difference between permeability and porosity?(3 votes)
- Porosity is a measure of how much surface is open space.
Permeability is a measure of the ease with which a fluid (water in this case) can move through a porous surface.(1 vote)