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Water harvesting (activity 4)

Every time it rains, water runs off the roof, along the gutters, down the downpipes, into the stormwater drains, and out to a waterway. If you were able to collect and store the rainwater that falls on your roof at home or at school you could use that water to wash dishes, flush toilets, wash clothes and water gardens.

Collecting rainwater and storing it is called water harvesting.

Under ideal conditions, when 1 millimetre of rain falls onto one square metre of roof area, one litre of water can be harvested (that is, 1 millimetre x 1 square metre = 1 litre).

Not all of the rain that falls is always caught. Some of the rain evaporates, some may be absorbed and some bounces off the surface it falls on. Different conditions have a different wastage factor.

For our purposes, when calculating the amount of rain to be harvested, we will assume that 80 per cent of the total will be caught. References: Water-Harvesting Calculations (Adobe PDF file from the Rainwater Harvesting for Drylands website) page 125 and Roof surface and area from the Rain Harvesting website.

  1. A Victorian primary school has a building roof area of 200 square metre (10 metre x 20 metre = 200 square metre) For every 1 millimetre of rain that falls on the school roof, 160 litres of rainwater can be collected in the rainwater tanks. During a recent storm the school's weather station rain gauge recorded 10 millimetres of rainfall. How much rainwater was collected in the schools rainwater tanks during this storm?
  2. Calculate your school's roof area. How much water could be 'harvested' if 1 millimetre of rain fell on your school roof?
  3. Repeat the calculation for your roof at home.
  4. Find out how much rain fell in your area over the past year (you can obtain this data from your school weather station or from the closest Bureau of Meteorology weather station). How many litres of rainwater could have been harvested at your school during the past year? How many litres of rainwater could have been harvested at your home during the past year?

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