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?
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.