Water is one of the most important substances on Earth. Water is present in the atmosphere, on the surface of the land and underground. About 70% of the human body is made of water. Humans can survive for weeks without food but only a few days without water.
Water on Earth
Look at the map of the Earth. It shows that there is far more water than land. About 70% of the Earth’s surface is covered by water.
However, almost all 97.5% of the water that covers Earth is in oceans and salt water lakes. This makes it unsuitable for drinking and most other uses. The other 2.5% is fresh water, but almost all of that cannot be used because it is either trapped underground or frozen in glaciers and in the ice caps of the North and South poles.
Only about 0.01% of all water on Earth is renewable fresh water and is available for use.
The water cycle
The water on Earth has been recycled over and over again since the planet was formed.
The natural process of recycling water is known as the water cycle. As water moves through the cycle it changes state. Energy from the Sun causes water to evaporate from bodies of water such as the ocean, rivers and lakes. Liquid water in the ocean has changed into water vapor in the atmosphere.
As the water vapor rises, it cools. At a certain height the air cannot hold any more water vapor and the air is said to be saturated. Any further cooling causes water vapor in the air to condense, changing into tiny drops of liquid water.
When the air cools further, the droplets of water combine to become larger and heavier droplets, which then fall back to earth as precipitation. The precipitation may be in the form of liquid rain or it may be frozen, falling to Earth as snow or hail.
There are two things may happen to the precipitation that falls on land.
- It may flow over the surface as run-off, moving back into rivers, lakes and streams and eventually flow back to the oceans.
- It may soak down into the soil in a process called percolation.
Some of the water that percolated through the soil is taken up by the roots of plants. This water then moves up through the plant. The process of the evaporation of water from plants is called transpiration.
Animals drink fresh water from rivers and lakes. This water is returned to the atmosphere as it evaporates from their bodies, or returned to the ground when it is excreted (removed from their bodies) as urine.
Putting all these processes the water cycle.
Groundwater
Rainwater, rivers and dams are major sources of water for Australia. However more than 20% of the water used in Australia each year comes from groundwater.
Groundwater is water that exists underground. Most groundwater is not in underground lakes or rivers but is trapped in the tiny spaces between grains of sand or within pervious rocks. Pervious rocks are rocks that allow water to soak into them. Pervious rocks contain tiny spaces into which water can soak.
When the water within the pervious rocks can be extracted using a bore or well, then the layer of rock is known as an aquifer. Perth in Western Australia gets about 60% of its water from an aquifer.
The Great Artesian Basin
The great Artesian Basin is one of the largest groundwater basins in the world. About one-fifth of Australia is sitting on top of it. Many millions of years ago there was an inland sea in Australia. Under the sea, rocks formed in alternating layers of pervious rock and impervious rock.
Impervious rock does not allow water to soak into it. Movement of the land has exposed areas of the pervious rock. Water can soak into the pervious rock and flow underground. The impervious rock prevents water from escaping. The result is a very large store of groundwater- the Great Artesian Basin.
Factors affecting the water cycle
Many factors influence the rate at which water moves through the water cycle.
States of water
The water that is trapped as ice at the North and South poles, in glaciers and on the top of high mountains is also part of the water cycle. The water in the ice and snow.
Ice cannot be taken up by plants. Many trees living in areas where water in the soil is frozen for part of the year are deciduous. Deciduous trees lose all their leaves in winter. Without leaves there is very little transpiration. Without leaves the trees are able to survive until the weather becomes warmer and water become liquid again.
Air temperature
Air temperature affects the rate of evaporation from bodies of water and soil. It also affects the rate of transpiration. As the air temperature increases, the rate of evaporation and transpiration both increase. Plants can slow transpiration down if they are losing water more quickly than they can take it through their roots. Plants have special openings through which the water evaporates. Plants are able to close these openings if they are losing too much water. In this way plants are able to reduce the rate of transpiration and slow down the movement of water through the water cycle.
Humidity
Humidity is the amount of water vapour in the air. Water vapour can be added to the air until the air becomes saturated. When this happens, the air cannot hold any more water vapour. As the air above a surface reaches saturation point the rate of evaporation (and transpiration) slows down and stops.
You may have experienced the effect of saturated air. When you sweat on a hot day with low humidity, the sweat dries (evaporates) off your skin quickly and cools you. However, if the air is very humid then the sweat remains on your skin and you feel hot and sticky.
Air movement
Air moving across a wet surface increases the rate of evaporation. Moving air carries away the saturated air, allowing evaporation and transpiration to continue. This is why turning on a fan makes you feel cooler and more comfortable on a hot, humid day: the breeze blows away the saturated air allowing your sweat to start evaporating again.
Landscape
Topography is the hills, valleys, rivers and other physical features of the landscape. The topography of an area affects run-off and percolation. Rain falling on smooth rock and steep slopes will quickly run over the surface and into streams and rivers. These streams and rivers will move the water quickly back into the ocean.
Where there are broken rock surfaces and areas of dense vegetation, run-off will be slower. Slower run-off allows more time for percolation to take place.
Some soils like sand have many large spaces between the particles. Water can easily percolate into these soils and there is very little run-off. Other soils have small particles that are closely packed together. It takes a long time for water to percolate through these soils and more of the water will flow over the surface as run-off.
Hills and mountains experience more precipitation than low-lying areas. As air moves towards hills and mountains it rises to get over them.The rising air cools and the water vapour it holds condenses, resulting in clouds or precipitation. Clouds are often seen over mountains when there are otherwise few clouds around.
SCIENCE FACT:
Boiling water
At sea leavel water boils at 100ºC. At the top of Mt Everest there is less air pressure and water boils at just 68ºC. Deep in the ocean, water pressure increases. Water near deep geothermal vents remains liquid at temperatures much higher than 100ºC.
Vegetation
The type of environment a plant is growing in will affect the rate of transpiration.
For example, cacti and many Australian plants have spikes or small narrow leaves. These are characteristics that help them conserve water so that they can survive in dry climates.They transpire less than plants without these characteristics.
Amount of sunshine
In nature the energy needed to evaporate water comes from the Sun. In parts of the world where there is a lot of sunshine there is more evaporation than in areas with little sunshine and heavy cloud.
