Water Cycle

Water is the most abundant substance on the Earth’s surface. In fact, roughly 75 percent of the surface of our planet is covered by this important molecule. Water that comes in its liquid and solid forms comprise the world’s oceans, rivers, lakes, glaciers, ice caps, and the groundwater found deep in the earth. In total, this great amount of water amounts to a volume of 1.4 billion cubic kilometres. Water in all its various forms make up the hydrosphere, the discontinuous layer that encloses much of the surface of the Earth.

Water is transferred between the different reservoirs on earth through the water cycle. The entire water cycle is composed of the collection of all reservoirs on earth, the processes that transfer water from one reservoir to another, and how the rates at which these transfers occur. The different paths for water transfer pervade the whole hydrosphere. These avenues for exchange and transfer reach 15 km upwards into the atmosphere and 5 km downwards into the Earth’s crust.

The water cycle first began around 3.8 billion years ago. Water vapour that seeped out of the magma from the Earth’s core rose into the atmosphere. Eventually, the water vapour cooled and condensed, and precipitation in the form of rain fell on the cooling Earth. The sun’s solar energy aided in powering and maintaining this cycle. On the other hand, the Earth’s gravity stopped water vapour from flying out into space. 

The Importance of Water

Water is of utmost importance to many living organisms. Many living things such as plants and animals need water for their survival. Obviously, humans are not exempt from this. More than half of our own bodies are made up of water, and even 70% of each of our cells is water. 

A large majority, 97 percent, of water on Earth is stored in the ocean. 1.7 percent is stored in its solid form in the Earth’s polar ice caps. Water found in rivers, lakes, in the ground, only accounts for an estimate of 1.7 percent. Lastly, water vapour in the atmosphere contributes the smallest portion at 0.001 percent. 

The survival of many living organisms depends on the tiny percentage of fresh water available on the Earth’s surface. A deficiency in the supply of fresh water has serious implications on ecosystems and the various organisms that belong to them. 

Human innovation has resulted in the development of technologies that allow water to be more available and accessible. A few examples of these innovations include the digging of wells to access groundwater, the collection of rainwater, and removing salt from saltwater through desalination. However, despite all of these, many parts of the world still do not have access to clean, potable water. 

Water Cycle Processes

Evaporation

Evaporation is the process in which water in its liquid state changes into its gaseous state. In the water cycle, liquid water stored in the rivers, oceans, lakes, and other bodies of water evaporates, turns into water vapour, and rises into the atmosphere. The rate of evaporation is determined by many factors such as air temperature, the amount of water vapour present in the air (humidity), solar radiation, altitude, and the current state of the ground level. Evaporation occurs the most in places that have high temperatures, brisk air flow, and large surfaces of water. 

Water vapour is all around us, and is even present in the air we breathe. Furthermore, water vapour is a greenhouse gas and plays an important part in keeping our planet warm enough for life to continue to exist. Alongside carbon dioxide and other greenhouse gases, it traps the heat energy the Earth receives from the sun, effectively maintaining the planet’s temperature. 

Condensation

Condensation is the process in which water in its gaseous state changes into its liquid state. Not unlike evaporation, the sun’s energy also has an impact on the condensation process. Air pressure also has an impactful role on condensation. When water vapour cools, it reaches its dew point. This dew point or saturation limit is influenced by the air pressure in an area. 

Condensation can happen at the high altitudes of the atmosphere or even at ground level. In the atmosphere, water vapour condenses into liquid water. Clouds are formed when water vapour condenses around minuscule particles such as specks of dust, salt, or pollutants in the air. These particles are called cloud condensation nuclei (CCN). On the ground, water vapour that condenses and forms clouds is called mist or fog. 

Precipitation

Precipitation is the condensed water vapour in the atmosphere that falls to the ground. However, precipitation is not a process like evaporation and condensation. Instead, it is used to refer to Precipitation may come in the form of rain, snow, hail, ice pellets, and graupel. 

When the atmosphere becomes saturated with water vapour, the water condenses and transforms from its gaseous state to its liquid state. There are two processes that can result in the air being saturated with water vapour–a reduction in the air’s temperature, or the addition of more water vapour. Usually, these two processes occur together and make precipitation possible. Precipitation is heavily reliant on the amount of moisture in the atmosphere, and more moisture increases the chances of precipitation.

Transpiration

The water cycle is affected by development. See the comparison images

Transpiration is the process in which water vapour is released from plants and the soil. Transpiration is often associated with evaporation, as it is also under the influence of the sun. In plants, microscopic pores called stomata open, often due to the presence of light. Water vapour is then released through the plant’s stomata and ascends into the atmosphere. 

Evapotranspiration is the combination of both evaporation and transpiration. Evapotranspiration accounts for all liquid water that changes into water vapour. Additionally, it is also used to assess the movement of water in the atmosphere. 

Runoff

Runoff pertains to a multitude of ways that liquid water flows over land. It occurs when soil has reached its full capacity and can no longer absorb excess water. This water flows over land and drains into nearby water sources. Runoff is the result of a surplus of water from precipitation such as rainfall. The excess water can also come from the melting of snow, ice, and glaciers. Runoff is one of the most important processes in the water cycle. 

Runoff that happens before a channel is reached is called a nonpoint source. This water can be contaminated by the various chemicals and materials that it encounters on the way to a water source. Contaminants that can be possibly found in the soil include pesticides, fertilisers, and petroleum. This contaminated runoff water can potentially pollute water supplies with harmful chemicals.  

Residence Time

A majority of the water on Earth does not cycle quickly in between reservoirs. Instead, water often stays in the same reservoir for extended periods of time. The time in which water molecules stay in one reservoir is called the residence time. Surface water is exposed to the environment around it and cycles quickly. In contrast, water stored underground, in the oceans, and frozen as ice rarely cycles at all.  

Water found in the atmosphere has the shortest residence time; only staying there for an average of 9 days. Water in ancient groundwater reservoirs called “fossil water” have a long residence time and can remain there for thousands of years. In the ocean, water remains for around 3200 years. The residence time of water is longest in the Antarctic ice sheet, roughly 17,000 years. 

The Water Cycle and Climate

Climate is defined as the sum of all weather conditions of an area in a certain period of time. The water cycle’s prevalence in all sectors of the globe allows it to have a definite impact on many natural phenomena, including climate. The water cycle can affect the climate in an area by influencing its humidity and temperature. 

Simply put, humidity is the amount of water vapour in the air. The water cycle does not distribute water vapour evenly throughout multiple regions. Therefore, some sectors end up having a higher concentration of water vapour in the air, making these places more humid. This difference in humidity is what causes two regions to have very different climates. For example, in island and coastal regions, the atmosphere is rich with water vapour, making the air much more humid. On the other hand, the atmosphere in inland regions have less water vapour, making these places less humid. 

The water cycle also facilitates the exchange of heat, which then causes temperatures to fluctuate. Thus, the temperature in a region is partly determined by the water cycle. This is because when water evaporates, it also absorbs the heat energy around it and lowers the surrounding temperature. Conversely, when water condenses it releases heat energy, increasing the temperature of the environment nearby. 

The Water Cycle and The Landscape

The water cycle can also change the features and appearance of the Earth’s landscape. There are two ways in which it alters the physical geography of the earth. Namely, these are erosion and glacial melt.  

Erosion is the process wherein soil and rocks are weathered away by water, ice, or wind. Runoff can be considered a part of erosion. Together, the two phenomena create many of the unique and varied features of the Earth’s landscapes. 

When water flows across a surface, it also strips away some of the surface’s sediment. Over time, more sediment will be washed away, and the water carves a path into the hard surface. Canyons are some examples of how erosion shapes landscapes over long periods of time. The Grand Canyon and the Colorado Plateau were both carved by rivers on top of high plateaus. Over time, their rivers wore away and cut through the rock, and eventually created these marvelous natural formations. Canyons can also be carved by underwater currents found deep in the ocean. The Monterey Canyon is an example of this. 

The movement of large bodies of frozen water also have the capacity to alter the physical features of its surroundings. Glaciers can carve whole valleys, craft mountain peaks, and leave behind rubble the size of boulders. In the United States, Yosemite Valley is a glacial valley created by the movement of glaciers in the past. In the part of the Alps found between Switzerland and Italy, the infamous Matterhorn peak was carved by the collision of glaciers. This event pushed up the earth beneath and created the landforms as we know today. In Canada, erratics called “Big Rock” are boulders left behind by the movement of glaciers in the past. 

The melting of glaciers can also create great changes in the landscape around it. One example of this is the Great Lakes situated in parts of the United States and Canada. These enormous bodies of freshwater are pools of water created by the melting and retreat of a colossal ice sheet. 

The impact of human activities on the water cycle

Modern human activity has produced many effects detrimental to natural systems such as the water cycle. Harmful substances such as toxic chemicals, radioactive materials, industrial waste, fertilisers, herbicides, and pesticides poison both surface and subsurface aquatic systems. The discharge of petroleum, improper disposal of sewage, and thermal pollution also have significant negative effects on the hydrosphere.  

Eutrophication

Eutrophication is the process wherein the increased input of organic matter or nutrients cause high biological productivity in aquatic systems. This increase in biological productivity can lead to the buildup of organic detritus, decreasing the volume of a lake. Eutrophication may occur naturally as organic matter makes its way into aquatic systems. However, human activity may also be the cause of something called cultural eutrophication. Cultural eutrophication is described as the process where sewage, agriculture, and industry leak excess nutrients into aquatic systems. It can also inadvertently bring about drastic changes to aquatic systems, impacting the lives of organisms that live and rely on them. 

Acid Rain

Human activities such as the burning of fossil fuels leads to the release of sulfur dioxide and nitrogen oxides into the atmosphere. In the atmosphere, these chemicals interact with water vapour and produce acid rain. Acid rain is a major proponent in the acidification of freshwater aquatic systems. It is also a problem faced by many countries around the world. 

Greenhouse gas buildup

The increasing levels of atmospheric carbon dioxide is largely the result of the burning of oil, coal, wood, and gas for human consumption. Carbon dioxide is an important greenhouse gas that maintains the Earth’s temperatures. An excess of carbon dioxide can lead to a more pronounced greenhouse effect. This effectively traps heat inside our atmosphere and warms the Earth’s surface.

The ocean is the part of the hydrosphere most affected by these changes. Human activity has caused a shift in the way carbon is cycled. Prior to the burning of fossil fuels, carbon dioxide from the ocean would flow through the atmosphere and to the land. However, human consumption of fossil fuels has reversed this process. The ocean has now become the recipient of carbon that was released from the earth. Carbon dioxide that reacts with the ocean’s surface lowers the pH level of ocean water, making them acidic. In conclusion, this buildup of excessive amounts of carbon dioxide has significant long-term implications on both the carbon cycle and the water cycle. 

Frequently Asked Questions

What is the water cycle?

The water cycle is the continuous process of water movement on Earth, involving evaporation, condensation, precipitation, and the flow of water through rivers, oceans, and the atmosphere.

How does evaporation occur in the water cycle?

Evaporation is the process by which water changes from liquid to vapour, typically from the Earth’s surface, like oceans, lakes, and rivers, into the atmosphere.

What is condensation, and how does it contribute to the water cycle?

Condensation is the conversion of water vapour into liquid water, forming clouds. This process occurs when warm, moist air rises and cools in the atmosphere.

How does precipitation impact the water cycle and Earth’s ecosystems? 

Precipitation is the release of water from clouds in the form of rain, snow, sleet, or hail. It replenishes water sources, supports plant growth, and maintains freshwater ecosystems.

What role do rivers and oceans play in the water cycle?

Rivers transport water from land to oceans, where it evaporates and re-enters the atmosphere. Oceans are major contributors to evaporation and a crucial part of the cycle.

References

Hydrologic Cycle. (n.d.). Retrieved from National Geographic: https://www.nationalgeographic.org/article/hydrologic-cycle/

Hydrosphere. (n.d.). Retrieved from Britannica: https://www.britannica.com/science/hydrosphere/Buildup-of-greenhouse-gases

The Water Cycle. (n.d.). Retrieved from Khan Academy: https://www.khanacademy.org/science/biology/ecology/biogeochemical-cycles/a/the-water-cycle

Water Cycle. (n.d.). Retrieved from National Geographic: https://www.nationalgeographic.org/encyclopedia/water-cycle/

Cite/Link to This Article

  • "Water Cycle". Geography Revision. Accessed on March 28, 2024. https://geography-revision.co.uk/a-level/physical/water-cycle/.

  • "Water Cycle". Geography Revision, https://geography-revision.co.uk/a-level/physical/water-cycle/. Accessed 28 March, 2024.

  • Water Cycle. Geography Revision. Retrieved from https://geography-revision.co.uk/a-level/physical/water-cycle/.