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Is there a plug-in or methodology to measure areas under contours, which can be used to develop elevation-area tables?

I know you can create contours as lines from DEMs, but you need to convert them to polygons to measure areas. You also need to trim/crop them with a mask to confine the query to the area of interest.

What I would like to do is draw a polygon around an area of interest and ask a plug-in or method to generate a table of elevations v areas that can be used in hydraulic or hydrological modelling to determine the storage volumes of reservoirs, lakes or dams. (Tables of areas are more useful than tables of volumes because you can apply evaporation to the surface area of the water, although you can differentiate from volume to area, or integrate from area to volume.)

Extermiknit
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    Slicing the raster at the chosen contour value(s) is quicker. For reference you can create hypsometric polygons using GDAL_Contour with -p (polygon) option https://gdal.org/programs/gdal_contour.html and clip your raster https://gis.stackexchange.com/questions/10117/clipping-raster-with-vector-boundaries-using-qgis - this will give you a slightly different value to a classify slice as the lines are slightly generalized. If you want to use existing contours close the ends and polygonize https://gis.stackexchange.com/questions/207463/convert-a-line-to-polygon does that help? – Michael Stimson Nov 09 '20 at 07:04
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    QGIS creates contours with that gdal_contour program. Just write -p into the "Additional command line parameters" box for creating contour polygons. – user30184 Nov 09 '20 at 07:10
  • Thx, but I don't see the solution I am looking for. gdal_contour with -p creates polygons, but does not retain the contour elevation. Contour lines retain elevation, but require manipulation to polygonize, and do not create nested contours. I'm looking for nested contour polygons (like concentric circles), or some such to allow an area calculation at each elevation. – Extermiknit Nov 10 '20 at 03:51

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