[GRASSLIST:902] Basic volume calculation question

[Patton, Eric]

I have a raster which represents the differenced surface from two
consecutive surveys over a marine disposal site. It is estimated that about
50,000m3 of harbour dredge was disposed of at the site between surveys. I
have further mapcalc'd the difference map so only values greater than 0 are
shown. What is the best approach to calculate the volume of the surface
difference? 

r.volume seems to want a clump-map from r.clump as input....but I'm not
interested in aggregating common raster categories together, I just want to
tally the volume of all the positive, non-null cells in the differenced map.

Which program to use: r.stats/r.statistics/r.report/r.sum/r.volume.....

~ Eric.