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| Subject: | [gislist] How to calculate the average annual rainfall from 33rd
and 67th percentiles grid data |
| Date: |
08/30/2005 02:30:02 AM |
| From: |
Leonie Meiring |
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Dear Listers
How does one calculate the average annual rainfall if only the 33rd
percentile and 67the percentile is known along with the mean and standard
deviation?
I have 2 grid annual rainfall datasets, one for the 33rd percentile (dry
season) and one for the 67th percentile (wet season). I have a list of
farms for which I need the average annual rainfall. For example:
For one farm the 33rd annual rainfall percentile = 434.3535 mm. There are
492 296 columns and rows in the whole dataset with a mean of 299.9189 and
standard deviation of 135.5186.
The Min value is 60.4778 and the Max value is 981.5317 for the whole
dataset.
The 67th annual rainfall percentile = 552.0655 mm. There are again 492 296
columns and rows in the whole dataset with a mean of 433.3182 and standard
deviation of 173.31943.
The Min value is 80.0178 and the Max value is 1305.12377 for the whole
dataset.
>From the histograms it would appear that the data is not symmetrical -
leaning more towards the left (sorry, I am not a statistician).
Could one still use the following: (67th percentile + 33rd percentile) / 2
to get the annual average rainfall i.e. 50th percentile which would be in
the above-mentioned case (552.0655 + 434.3535) / 2 = 493.2095 mm or is there
another formula one can use?
Your help and advice is much appreciated.
Kind regards
Leonie Meiring
Department of Agriculture
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