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I have a set of 12 precipitation raster layers downloaded from WorldClim, each one representing monthly precipitation data on a global scale. I've clipped all 12 layers with the area of interest. How can I obtain on QGIS a unique raster representing the average of annual precipitation?

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If you want to take an average of all 12 images, the process is very simple. Within a raster calculator (this can be the calculator in the raster drop-down menu or the GDAL/GRASS/SAGA calculators in the toolbox), create a tool-specific formula that describes an averaging equation:

(Pjan + Pfeb + ... + Pdec) / N

Where N is your number of observations (12).

One thing that you might want to take note of is that because these rasters are created from monthly averages, your it will be difficult to glean much statistical information from them (ie: standard deviation, extremes, etc.), but seeing as the means are roughly composed of the same number of events (28-31), the impact is minimal. Performing an average from daily values is almost always better to remove the slight over(/under)-emphasis of precipitation from shorter(/longer) months.

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    Do you know any data source capable of providing world daily precipitation ? – Giacomo Ponticelli Oct 10 at 18:01
  • It depends on your use case and what limitations/concessions/trade-offs you are willing to deal with. My suggestion would be to try searching within opendata.stackexchange.com as earth observation raster data is a pretty common questions. For simple calculations, you're probably fine with WorldClim; For more in-depth analysis you might want a composite product or a climate reanalysis. It really depends. – Trevor J. Smith Oct 10 at 18:07
  • Thank you very much – Giacomo Ponticelli Oct 11 at 9:06

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