In some areas of the world, rain doesn't fall very often, but when it does it falls quickly in a short period of time. This precipitation will often cause erosion because the soil's surface is very dry. How intensely the soil erodes depends upon how much energy is in the precipitation and how long the dry season was. To quantify this, Fournier created a rainfall distribution index that describes the ratio between the rainfall of the wettest month of the year and the annual precipitation. It was later found that this index wasn't accurate, so it was modified to take into account the rainfall for all of the months of the year.
- Rainfall for each month of year (in millimeters)
- Calculator
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Instructions
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1
Square each monthly precipitation amount. For example, if September had a total rainfall of 10 mm, squaring that rainfall would give you 100 mm.
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2
Add together the results from Step 1. As an example, squaring the total rainfall for each month over a period of one year and adding the results together gives you 1200 mm.
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3
Add together the precipitation amounts for each month in a year. As an example, in one year there was an annual precipitation of 120 mm.
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4
Divide the answer from Step 2 by the answer in Step 3. In the example you would divide 1200 by 120 and get 10. The modified Fournier index for the example year is 10.