The Arc Hydro Data Model can be defined as a geographic database containing a GIS representation of a Hydrological Information System under a case-specific database design which is extensible, flexible, and adaptable to the user requirements.

It takes advantage of the next generation of spatial data in Relational Database Management Systems (An RDBMS-based GIS System), the geodatabase model. Conceptually, it is a combination of GIS objects enhanced with the capabilities of a relational database to allow for relationships, topologies, and geometric networks.

The best source of information about the model is the Arc Hydro book:

Arc Hydro
GIS for Water Resources
Edited by David Maidment
ESRI Press, 2002
ISBN 1-58948-034-1
Item 88713 140 pp.