Rainfall deciles

If you want to sound like you know what you’re talking about when striking up conversation with a farmer some time, try talking about rainfall deciles. What are they you ask? Hopefully this little explanation will make you feel a little bit smarter.

Farmers often talk about rainfall in ‘deciles’. Those of you who have taken a statistics class at some point in your life will be familiar with that term but for the rest of you I’ll try to explain it.

Take a list of all our annual rainfall totals since records began in the 1880s. Now rank that list in order from smallest to largest. If we then divide the list into 9 equal sized groups, we’ve broken the list up into blocks that represent 10% of the total called ‘Deciles’.

  • The first group ‘decile 1’ is the group of years that are the driest 10% of seasons on record.
  • The fifth ‘decile 5’ group would be the average years in the middle.
  • The last group ‘decile 9’, would contain the wettest 10% of years ever recorded and were probably remembered for flooding.

This might sound like a bit of an abstract concept but it allows farmers from different areas to compare the progress of a season. On twitter I’ve had a couple of people from the low rainfall cropping areas of the Victorian and South Australian Mallee say to me ‘you had 200mm of rain last year? We can grow good crops on that!’ They can. But absolute numbers don’t tell the full story. As farmers the best we can do is plan for the average in our area. When I explained that it was only a decile 1 for our area those guys understood that our poor yields weren’t solely to blame on our poor practice.

Jonathan

Web developer turned farmer. Interests include: my faith, my wife, technology, cricket, farming, ice cream & world events.

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