I am using ArcGIS for Desktop.

I am working with 4km resolution precipitation rasters from the PRISM website because I am trying to calculate the total annual precipitation for each groundwater basin I am interested in.

I have been using the "Extract by Mask" tool to try and accomplish this, but the tool has not been able to clip the raster data to the very small groundwater basins in my dataset.

Is there another tool I can use to accomplish this without altering the resolution of the rasters?

  • Welcome to GIS SE! As a new user be sure to take the Tour. I've assumed that you are using ArcGIS for Desktop and added that to your question. If I'm wrong just use the edit button to remove it and tell us what GIS software you are using.
    – PolyGeo
    Oct 31, 2015 at 3:16
  • What are the size (area) of the groundwater basins in your dataset? Specifically are they smaller than 16 sq km?
    – GeoSharp
    Oct 31, 2015 at 4:53
  • Yes, some of them are smaller than 16 sq km.
    – Jennifer
    Nov 2, 2015 at 21:38

1 Answer 1


I recently did a similar project with NADP deposition rasters (which uses PRISM precipitation results as one input to the model). Using NADP or PRISM 4km gridded model results is arguably taking those data out of context since they are intended to illustrate regional differences rather than local differences, but they were the best estimates available and with that caveat in mind I went for it anyways!

I was also working with somewhat small watersheds/basins, so they would only overlap a couple pixels in some cases. In order to get a deposition (or precipitation) estimate, the easiest solution I had was to convert the basin polygons to rasters with relatively small cells (e.g. 10m), using Polygon to Raster tool. (I had about 30 basins in the same raster, the cells making up each basin had a different value.)

enter image description here

I then used the Zonal Statistics tool to calculate the mean of each of the basin cells based on the values of the deposition raster they overlapped. (I had >200 deposition rasters to iterate through, so the model below generates a table for each deposition raster with means for each basin.) enter image description here

The resulting tables should give you an average precipitation value for your basins. You could then multiply by the area of the basin to generate a volume of precipitation per year, depending on what you need.

  • @Jennifer : I'm wondering if you came to a solution yourself or could post something that worked for you (or if you tried my solution and it worked, please accept this solution!). Also, any GIS users have any feedback on the method I used here? Not sure if this was a weird newbie work-around, or looks like a good solution to more advanced users.
    – Leo
    Nov 4, 2015 at 20:06

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