Hi:
I am not aware of any free software for this purpose.
The company I work for [MWH Soft] has created InfoSWMM (
www.mwhsoft.com) which will calculate hydrology and other parameters based on GIS data as well as provide access to all other input and output directly in ArcView.
Feel free to email me for more information at:
(Killian.Tobin at mwhsoft.com)
Optionally for a DIY approach you can write your own DB tools to import and export from a geodatabase.
Take Care-