Calibrating GW routine for baseflow and hydrograph recession

SWMM'ers:

I'm working on a continuous SWMM model of a 65 square mile watershed as part of a water quality management study, and have been having difficulty calibrating the model to match observed hydrograph recession and baseflow characteristics. The best I can achieve is a nearly constant baseflow that is considerably higher than the observed dry weather baseflow, yet lower than the post-storm recession. Changing the groundwater parameters has little if any effect on the results. It would be extremely helpful is someone could identify the sensitive parameters for calibration, and suggest which direction and how much to vary them to improve the results.

I'm running XP-SWMM v. 8.52, and have 7 streamflow gages with a 15-minute resolution record from 1999-2001. Rainfall is hourly from the local airport <30 miles away, and the Runoff wet time step is 1 hour, dry time step is 1 day. Transport (Sanitary in XP) time step is 15 minutes. Watershed land use varies from urban core to agricultural and wooded parkland, with an overall imperviousness >30%.

Thank you,

Gene Bosley
Bosley@PBWORLD.COM