Hopefully this will be one of the equations Autodesk solves for in their next release of Civil 3D. I'm telling you...wouldn't it be nice to be able to overlay land use information over drainage area polygons to extract weighed "C" values per basin? Then add in the terrain surface model to determine the most hydraulically distant point within each catchment area, the travel paths, and have the software figure out time of concentrations (Tc) for overland, shallow concentrated, and open channel sheet flow conditions. Then input rainfall amounts to determine flow amounts per area. Simple rational method calculations you know. Then have a print out in a nice LandXML formatted report. Now that my friends would be a huge step closer to the Holy Grail and would begin to make that second "D" in CADD grow just that much larger.
Oh BTW, I apologize for not posting more often. You folks who read the Civil 3D newsgroup may have seen a thread concerning the issues that I've been having with Service Pack 2 and the Pipes options. Unfortunately, I think my final answer is to wipe my system clean and start fresh with a clean Windows OS install. Go figure. And out of a few computers which have been upgraded at my office, mine is the only one not to work. Must be that mediaWiki install that I did a while back using Apache web server, PHP, and MySQL. C3D Pipes may have not liked that, but at this point, that is only a guess. It's pretty much the only thing I haven't uninstalled from the system to troubleshoot my Pipes problem. If I find out what is causing the issue, I'll post it here (and the newsgroup).