Hi Andy I just went through this process and didn't really find a great automated approach. Even the Hydro tools didn't seen to give me exactly what I needed. I ended up doing a combination of arc hydro and manual editing to delineate tributary area to each inlet. I essentially started at the larger watershed scale (huc 12) and then worked my way back from every outfall. 2nd Nature llc. outlines, nicely, a methodology for doing this. They have a tutorial on their site under MAP MS4 TO MEET PHASE II PERMIT REQUIREMENTS- http://www.2ndnaturellc.com/client-access/central-coast-stormwater-ms4-support/ .
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