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Natural water cycle

The water cycle, also known as the hydrologic cycle, describes the continual movement of water between the surface of the earth, underground and the atmosphere.

The water cycle has no start or end, and water can exist in different states throughout the various stages. The sun is what keeps the water cycle going round and around.

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Atmosphere

The air contains 0.001% of the Earth's water. Water spends, on average, 9 days in the air before it returns to the land.

The main gases in the atmosphere are nitrogen (78%) and oxygen (21%). Many other gases including carbon dioxide and argon make up the remaining 1%.

The only gas which change in the amount found in the air at any time is water vapour. There can be between 0 and 4% water vapour in the air.

When we talk about how much water vapour is in the air we are talking about relative humidity. Relative humidity is a measure of how much water vapour is in the air compared to the maximum amount that can be held in air at that temperature.

For example, a kilogram of air at 25ºC would be completely saturated if there were 20 grams of water in it (100% relative humidity). Therefore if there were only 10 grams of water in that kilogram of air at 25ºC, the relative humidity would be 50%.

Evaporation

This is the process by which energy from the sun (solar energy) turns liquid water into water vapour. As the water molecules get hotter, they begin to move faster in the liquid. They collide more often with each other, and, gradually some water molecules move fast enough to break away from the others. This water vapour rises into the atmosphere.

Over 80% of the water vapour in the atmosphere is from water evaporated from the oceans. When this water evaporates the substances dissolved in it (like salt) remain behind.

Transpiration

Water is absorbed through the roots of plants and moves up through hollow tubes to the leaves. The movement of water up the plant is due to capillary action.

Leaves have small holes in them through which they can absorb carbon dioxide and release oxygen. 

Water vapour is also lost through evaporation from the pores of leaves. The process of water movement and loss from plants is called transpiration. 

Condensation

Air temperature drops as you move up through the atmosphere so the water vapour, carried upwards by the air, cools as it rises and it turns into liquid water again.

This process, condensation, is aided by the presence of tiny particles (for example, dust and smoke) in the air on which droplets of water vapour can collect and grow.

Precipitation

As condensation occurs the water droplets grow in size until eventually they become too heavy to remain suspended in clouds and they fall to the ground. If water droplets in clouds join together to form drops between 2 and 6.5 mm in diameter, they may fall as rain.

Fog or mist are really just low lying clouds. 

Hail forms when droplets of water freeze around a small core of ice. Hail stones increase in size by adding extra layers of ice as they are tossed up and down by rising and falling air currents common in thunderstorms.

Sleet forms when raindrops fall through air below 0°C and freezes.

Snow forms when water vapour in the air changes directly to ice. Snowflakes are usually 6-sided ice crystals found in many different patterns. The air temperature must be less than 0°C for snow to form.

Dew is formed when water vapour condenses near the ground.

On clear nights, the land will rapidly lose heat to the air. Any moist air near the surface will cool and condensation will occur. 

Frost forms when the surface temperature falls below freezing (0°C) resulting in ice crystals directly forming from the saturated air.

Infiltration

The process of water soaking into the ground is called infiltration or seepage. How much water runs off the ground and how much soaks in depends on many factors including:

  • soil type
  • soil conditions (is it already saturated?)
  • slope of land
  • amount and heaviness of rain
  • amount of vegetation cover

Water which soaks into the ground can be stored as groundwater. Some of this water may also gradually find its way back to the oceans flowing under the ground.

The movement of water down through the soil is called percolation.

Some water may flow down below the water table and become 'lost' from the water cycle. This is known as deep percolation.

Groundwater

Groundwater is derived from rain which percolates down through the soil or fractures in rock, so filling up the pores between sand grains or the fissures in rocks. Anything from none to half of the rainfall in a given area may reach the water table and thus recharge the groundwater. Geological formations such as those composed of sand, sandstone and limestone which contain usable quantities of groundwater are called aquifers.

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The water table at any given point shows an annual variation related to the seasonal pattern of rainfall and evaporation. Other factors being equal, the changes are likely to be greatest in clayey soils (several meters) and least in coarse sandy soils (typically less than one metre). The water table tends to follow the ground surface, but is constrained by the ocean, rivers and streams, so groundwater mounds tend to develop in regions where the topography is higher than surrounding areas.

Surface runoff

Only some of the water soaks or infiltrates into the ground, a lot of it can run off either into lakes, rivers or into the oceans.

Nearly 40% of all precipitation flows back across the land to seas and oceans. 

Ice

Just over 2% of the Earth's water is in the form of ice - mainly as glaciers, ice caps and icebergs.

Water can stay trapped as ice for tens of thousands of years or longer.

Lakes

Freshwater lakes contain just under 0.01% of the Earth's water. Water can remain in lakes, on average for tens of years or longer. 

Ocean

The world's total water supply has a volume of about 13,700 million cubic kilometres. If this was spread evenly over the Earth's surface it would cover it to a depth of around 2.5 kilometres. However 97% of this is salt water in the oceans and the remaining 3% would only form a layer about 70 metres in depth. 

Rivers

About 0.01% of the Earth's water is found in rivers and streams at any time. Water stays an average of about 14 days in a river or stream.

Melting

A large amount of energy is required to melt water. It would take a small radiator (1 kW) about 6 minutes to melt 1 litre of ice. In the same time, one litre of liquid water would increase in temperature by 80°C.

New water

New water is added to our atmosphere when water, trapped in ancient rocks, is released during volcanic eruptions.

Some water is also lost from the Earth when radiation strikes water molecules in the upper layers of the atmosphere. 

The radiation can 'split' the water molecules into oxygen gas and hydrogen gas, which escapes into outer space. 

Fortunately, 'New' water is 'created' at about the same rate as water is lost into space so the total amount of water we have stays pretty much the same.

Role of the sun

The sun provides heat energy for evaporation to occur. It also is a major contributor to winds by causing uneven heating of the Earth's surface.

As air (containing water vapour) close to the ground is heated by the sun, it rises and then begins to cool. Cool air is denser than normal or warm air so it begins to "fall" back towards the ground. 

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This movement of warm and cold air is called a convection current.

Urban areas

The building of cities and their surrounding suburbs has disrupted the local water cycle in many ways:

  • rainfall is higher over cities due to the higher temperatures and more particles in the air (see condensation).
  • runoff is higher and faster as much of the surface is impermeable to water (e.g. roofs and roads). As a result many creeks have been altered to handle the increased water flow.
  • rural areas have been altered by the construction of dams for the urban population.