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The water cycle (also known as the hydrological cycle) is the constant movement of the water on, above, and below the surface of the earth. It is made up of a series of processes: the water evaporates from the oceans and eventually condensates and precipitates on the land before returning again to the oceans through various pathways such as river runoff and direct groundwater discharge.
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Jetzt kostenlos anmeldenThe water cycle (also known as the hydrological cycle) is the constant movement of the water on, above, and below the surface of the earth. It is made up of a series of processes: the water evaporates from the oceans and eventually condensates and precipitates on the land before returning again to the oceans through various pathways such as river runoff and direct groundwater discharge.
The amount of water on earth is finite and that is why it is a closed system. This means water cannot leave or enter the earth and its atmosphere. The three components for the cycle are stores, flows, and processes.
Fig. 1 - Global water cycle diagram
As you can see from the diagram above, water's nature and form change all the time. Global water stores include lakes, oceans, aquifers (underground lakes), and the cryosphere (glaciers, ice sheets). Global water can take shape as water, vapour, ice, saline, or freshwater. There are two processes that drive the cycle.
As there is more evaporation due to global warming, there is more moisture in the atmosphere. This can lead to increased condensation as the air cools which turns into greater precipitation. In some places, it leads to increased cloud cover and precipitation as climate changes.
Gravitational potential energy is what drives the water through the system in a sequence of inputs, outputs, stores, and flows.
Fig. 2 - Where the water is in the world water stores
Although the amount of water that is in the stores fluctuates, each store has a relative size. As seen in the graph above 96.5% of water is in the oceans, 2.5% is freshwater, and 0.9% is in other saline water sources. Most of the freshwater is in the cryosphere (glaciers and ice sheets) and most of the surface water is in ground ice and permafrost.
The transfer of water globally by flows from one store to another is known as fluxes. These fluxes vary with the temperature and the season. The variation is known as annual fluxes.
The ocean loses more water to the atmosphere from evaporation than it receives from precipitation, whereas it is the opposite for landmasses. Surface runoff makes up the difference known as the balance. If the balance was disturbed, the oceans would receive more water and the continents would dry. This balance is called the global water budget and stops this from happening. Water does not stay in the atmosphere for long. Its residence time is short there, whereas it resides in the ocean for longer periods.
The importance of the Tropics comes from the fact that most of the world's rainfall is created there, in the Intertropical Convergence Zone (ITCZ). This means that it is the biggest flux transferring water from oceans to land. The steep angles of the sun over the tropical oceans cause high evaporation. Trade winds carry the water vapour towards the ITCZ where strong convectional currents lift the air that leads to cooling and condensing into clouds. These atmospheric flows of moisture are called tropospheric rivers.
Depending on the place of the ITCZs, there can be drastic changes in the rainfall in equatorial nations. This can lead to droughts or floods, such as the catastrophic drought in Brazil during 2014-2015, which will be touched upon in the article about droughts.
Fig. 3 - Location of the ITCZ on the world map in July vs. in January
The importance of the polar regions comes from the fact that two thirds of the earth's freshwater are locked up there, in the cryosphere, in the form of ice sheets and glaciers. These regions contribute to the circulation of the water and transfer of heat around the world, driving the global hydrological cycle. This triggers ocean circulation known as thermohaline circulation, also known as the global conveyor belt.
Thermohaline circulation. Source: Robert Simmon, NASA (Public Domain)
Beneath Greenland's ice sheet and under the Kenyan desert are water stores known as aquifers. These are untapped ancient stores of freshwater that exist in the polar regions and beneath many deserts.
Water insecurity is the lack of a reliable source of water that has the appropriate quality and quantity to meet the local human population and environment. The cause is often linked to water scarcity, which is the imbalance between demand and supply. This could be due to physical scarcity or economic scarcity. One of the ways to tackle water insecurity is with water supply management using hard engineering. This will be explored further in the article about water supply management.
Climate change is affecting rainfall and the risk for flooding in the UK is rising, as can be seen in the article about floods. Globally there are areas that will have surpluses within the water cycle and others will have a deficit. Case studies such as the Sahel receiving wetter years could develop to re-greening the desert, whereas the drought that is happening in California could lead to land that cannot provide agriculture. There has been research to try to predict climate change and map the uncertainties, comparing past years of weather patterns and pressure systems. This will be explored further in the climate change article.
Evaporation in the water cycle is the process of water being heated by the sun and turning into vapour or gas.
Condensation in the water cycle is the process of air cooling and turning into precipitation.
Infiltration in the water cycle is the process of water travelling through the soil and then the cracks within the rocks.
The water cycle is driven by solar energy and gravitational potential energy. Energy from the sun heats the water, causing evaporation or transpiration. Gravitational potential energy accelerates the water and transports it to rivers and then to the sea.
The water cycle (also known as the hydrological cycle) is the constant movement of the water on, above, and below the surface of the earth. It is made up of a series of processes where the water evaporates from the oceans and eventually condensates and precipitates on the land before returning again back to the oceans through various pathways such as river runoff and direct groundwater discharge.
What is a closed system?
A closed system is when water cannot leave or enter the earth and its atmosphere.
What are the three components of the global hydrological cycle?
The three components of the cycle are stores, flows, and processes.
Describe how solar energy drives the water cycle.
The energy from the sun heats the water, which causes evaporation or transpiration.
What is gravitational potential energy?
Gravitational potential energy is what drives the water through the system in a sequence of inputs, outputs, stores, and flows.
Give an example of gravitational potential energy working within the system of continuous outputs governing inputs as nothing is lost or gained.
In the areas that are being warmed, ground surface dries out as evaporation increases. Global air circulation takes this extra vapour to cooler areas, where it condenses into clouds and precipitation.
What is the biggest water store?
The ocean.
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