Tuesday, 25 April 2006

How much water falls on a farm each year?

Ever wandered just how much water falls on your own land each year?
I decided to do some math and get my head around the physical quantity that falls on Ochre Arch on average each year. Using metric areas and quantities makes the calculations much easier.
Here are some figures that readers could use to make calculation for their own places.
* 1 mm of rain on a square metre of land produces 1 litre of water.
* A hectare of land is 10,000 square metres. If the shape of a hectare happens to be a perfect square then its dimensions are 100 metres by 100 metres.
* 100 mm of rain on a hectare produces a megalitre (1,000,000 litres) of water (10,000 sq metres X 100).
* 1,000 litres of water takes up a cubic metre of space
* The length of an Olympic swimming pool is 50 metres. If such a pool is completely full and is 10 metres wide and 2 metres deep then it will contain exactly 1 megalitre (50 X 10 X 2 X 1,000 = 1,000,000 litres) of water.
To work out how much rain falls on a property each year one needs to know the average rainfall each year in millimetres (mm) and the total area of the property in hectares. The calculation steps are:
A. Calculate megalitres of rain per hectare per annum.
The easiest way to do this is to divide yearly rainfall in mm by 100. Example: If the yearly rainfall is 550 mm then the number of megalitres per hectare per annum is 5.5 (550/100)
B. Calculate megalitres per annum for the whole property
Multiply the number of megalitres per hectare per annum by the total number of hectares on the property. Example: Megalitres of water per annum that falls on a 385 ha property that averages 550 mm of rain each year = 2,117.5 (5.5 X 385).
Our property, Ochre Arch, happens to be 385 ha and the average rainfall is around 550 mm per annum. Thus one could say that each year the equivalent of 2,117.5 Olympic swimming pools of water fall on the property. That’s a lot of water!!

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