Water Cycle

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.

The movement of water in the global hydrological cycle

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.

• Solar energy: Energy from the sun that heats water, which causes evaporation (water turning into vapour or gas) or transpiration (movement of water through plants).
• Gravitational potential energy: The way in which water accelerates under gravity, which transports it to the river and then to the sea. The water also infiltrates through the soil and then travels through the cracks within the rocks as groundwater.

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 in the water cycle

Gravitational potential energy is what drives the water through the system in a sequence of inputs, outputs, stores, and flows.

• The system is continuous with outputs governing inputs as nothing is lost or gained.
• There are some shifts in the world's climatic zones affected by climate change. This means that some stores are depleting. For example, the ice in the Polar regions and in mountain glaciers is melting without being replenished.
• In the areas that are being warmed, the ground surface dries out as evaporation increases. Global air circulation takes this extra vapour to cooler areas, where it condenses into clouds and precipitation.

Storage in the water cycle

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.

Flows

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

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

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)

Fossil water

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.

What is water insecurity and what is its cause?

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 and the water cycle

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.