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Class 12 Biology (India)
The water cycle
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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.
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.
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 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?(21 votes)
it's all a cycle, but there are factors influencing precipitation such as global warming or deforestation etc, pollutants can also be another way of disrupting the fresh ground water.(3 votes)
- Can water pollutants and air pollutants interfere with the water cycle and the ecosystem around it?(6 votes)
- Why is there more salt water than fresh water(5 votes)
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(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.(2 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?(3 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)- How is aquifer faster then they replenish.(2 votes)
Groundwater can become depleted if we use it at a faster rate than it can replenish itself. The replenishment of aquifers by precipitation is called recharging. Depletion of aquifers has increased primarily due to expanding agricultural irrigation(2 votes)