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I usually calculate monthly or 3-monthly Standardized Precipitation Index (SPI) using spreadsheet, based on daily rainfall from weather station. Attached is the spreadsheet example link.

And now I would like to use satellite precipitation estimate (raster data) to calculate the SPI. The problem is in the spreadsheet I use some function like: FREQUENCY, GAMA.DIST, NORMSINV.

Is there any similar above function in ArcGIS Desktop?

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There was a discussion at GeoNet titled How to calculate Standard Precipitation Index (SPI) using ArcGIS which suggests that there is a way to "calculate the SPI using monthly CHIRPS CHG - Data" using the Cell Statistics tool in the Spatial Analyst toolbox.

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  • Yes thank you, I answered that question. At that time, I found that was the simple approach to calculate the SPI using the long-term average and stdev. Then recently I found that method is not perfectly correct, we need to calculate the gamma distribution. So thats why I ask above question in here.
    – user97103
    Dec 5 '19 at 11:10
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    @user97103 You should probably have mentioned this in the question, no?
    – Vince
    Dec 5 '19 at 12:28
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Here you can find ArcGIS toolbox for calculating SPI. This toolbox was made with the same algorithm in the spreadsheet.

https://github.com/Hyun-Woo-Jo/CalculateSPI/releases/tag/v1.0

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