Scott,

I'm not quite sure I follow what you're asking, but I'll try to answer.  By basin, I'm assuming you mean a detention pond (although I've never heard of a detention pond refered to as a basin).  I suppose it would depend on what is going on upstream of the basin.  Is the area large (i.e. greate than 150 acres)? are there upstream detention ponds? is there a need for water quality storage?  The maximum outflow of the pond is definitely needed however, the peak inflow into the pond has little to do with determining the required volume.  You'll need to do either a mass balance approach (i.e. flow in x time - flow out x time = storage requirement), or route a hydrograph through the pond  (Haestad Methods PondPack, EPASWMM, etc.).  Probably the best thing to do would be to check with your reviewing agency and find out what method they require.

Brian

There is a good article on this subject in the current CE News Magazine by Al Pagan. Go to http://www.cenews.com/columnists.html
to see it online.

Yes there is a quick n dirty way.  It uses the inflow hydrograph.  If you are detaining the water then you also require the outflow discharge.  Plot the hydrograph and then draw a straight line to outflow value and calculate the volume of water detained between the two lines. increase by 10%.  And wala a first trial size of basin.  Once this works then you can tweak it to your desire.  For a retention pond, just calculate the volume of water from the hydrograph.

Here in PA we also call detention basins by that name; I have rarely seen them called "ponds", unless they are providing some extended wet volume.

NRCS TR55 provides a very quick, one page algorithm.  Since you already know the peaks in and out, the only other info you need is the SCS curve number (this requires identifying the Hydrologic Soils Group, a list of which is also in TR55) of the watershed, Type of rainfall distribution (per map on page B2) and the runoff amount (inches, not cfs).  It is free from the NRCS (your local Conservation District might have copies) and I have used it for years without a problem.