Source and Storage
Welcome back! By now you would have successfully completed Module 1 and corresponded and shared your findings with your partner school. Now that we all know something about where you live, and how much water you use, it’s time to enter the urban water cycle.
Right, well we better step back a few paces, and start from the beginning, which can be tricky in a cycle, as it really doesn’t have a beginning or end. Kind of like the chicken and the egg scenario.
Anyway, in a nutshell, the Earth's water is always in movement, and the natural water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states during this cycle between liquid, vapor, and ice.
How do these changes happen? Well, adding or subtracting heat makes the cycle work. If heat is added to ice, it melts. If heat is added to water, it evaporates. Evaporation turns liquid water into a gas called water vapor. If heat is taken away from water vapor, it condenses. Condensation turns water vapor into a liquid. If heat is taken away from liquid water, it freezes to become ice.
In the natural water cycle, water from oceans, lakes, swamps, rivers, plants, and even you, can turn into water vapor. Water vapor condenses into millions of tiny droplets that form clouds. Clouds lose their water as rain or snow, which is called precipitation. Precipitation is either absorbed into the ground or runs off into rivers. Water that is absorbed into the ground is taken up by plants. Plants lose water from their surfaces as vapor back into the atmosphere. Water that runs off into rivers flows into ponds, lakes, or oceans where it evaporates back into the atmosphere.
The cycle continues.
There you go, now you know what the natural water cycle is.
Did you know?
There is a hundred times more water in the ground than is in all the world's rivers and lakes
Ground water in some parts of the world has been aged at over 2 million years since it first seeped into the ground!
Water on the surface is pretty simple to understand. It flows downhill until it reaches the ocean.
Water soaking into the ground, and the concept of ground water, can be a little more difficult to understand. When water soaks into the ground, because of gravity, it passes between particles of soil, sand, gravel, or rock until it reaches a depth where the ground is no longer porous. The area that is filled with water is called the saturated zone and the top of this zone is called the water table. Just like surface water, ground water flows downhill to eventually meet the ocean, though this movement can be very very slow.
Out of Interest
If your school already has, or if you installed a tank to collect rainwater, are you allowed to drink it? If not. Why?
The urban water cycle is kind of like a parasite on the natural water cycle (sounds bad doesn't it, but it's true!). It typically attaches itself to the natural water cycle following precipitation and movement of water along the surface and into the ground. Once water is taken from its natural cycle, it gets stored, treated, distributed, used, dumped, and treated again before being put back into the natural water cycle. Messing with the natural water cycle can have big impacts on the environment, and so understanding it and learning from others (like you guys are doing) is the first step towards making it more sustainable.
This Module is focused on how we access water from the natural water cycle and send it into the urban water cycle.
Humans have devised all sorts of ways to access and store both surface water and ground water. We build dams and weirs to slow (and sometimes stop) the flow of rivers so that water is stored and accessible in times of drought; and we dig or drill wells to access ground water in areas that don’t have surface water.
You learnt in Module 1 that the quantity of suitable water for human use is finite. Unsustainable extraction of water can lead to very dire situations where a community can literally run out of water! By “unsustainable” we mean, taking more water out of our surface and groundwater supplies than the amount that precipitates on the land (termed “recharge”).
What happens, when things get unsustainable in our urban water cycle, well you guessed it, a community, town, city, or country can literally run out of water!
In these situations (they exist!), humans have had to manufacture fresh water by taking the salt out of sea water (desalination) or the waste out of waste water.
Out of Interest
Would you drink recycled waste water?
Out of Interest
Find out how desalination works. What are the positives and negatives of this method of getting fresh water?
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