Priyan,

There are many available models that physically based to some degree. One
that is free (model and source code) is Kineros, a distributed, small
watershed, event-based model. You can find more information and download
what you need at:

http://www.tucson.ars.ag.gov/kineros/

Phil

Philip Heilman
USDA-Agricultural Research Service
Southwest Watershed Research Center
2000 E. Allen Rd.
Tucson, AZ 85719
Phone (520) 670-6380 x148
Fax (520) 670-5550
http://www.tucson.ars.ag.gov/
pheilman@...