Q capacity represent something like "basin/area above a (pour) point"?
If so, SpatialAnalyst (an ArcGIS extension) has hydrology routines you can use to derive the watershed above any so-called pour point.
To go from a raw DEM to a watershed; however, you have to do some preprocessing first. And I suspect you'll need to do some of these anyway regardless of what your ultimately needing to accomplish.
If memory serves, the workflow to derive a small watershed is this:
- create a DEM without pits/sinks using the fill routine
- derive a flow accumulation raster from your filled-DEM
- define a pour point(s) in a vector layer, then convert it to a raster dataset
- "snap" your pour point(s) to a nearby cell with greatest flow accumulation
- derive a flow direction raster
- derive a watershed for each pour point
Now if you're wanting the software to identify best locations based on some criteria, I believe you'll need write a routine using ArcObjects that "daisy-chains" some of the previous methods together.
Next, if you're interested in comparing alternative GIS platforms for your project, I understand SAGA has a nice set of hydrology tools. Moreover, since SAGA is open-source, you can look under the hood if you need too defend any algorithms used in your analysis, such as would be necessary for a peer-reviewed paper, et cetera.
Finally, if I have missed the point here (i.e. Q is not basin area), let me know and I'll delete my answer. Otherwise, I think you'd do something like I suggested above.