Realistic models of municipal water distribution systems pose problems due to their size and complexity. Traditional approaches are to build and run systemwide models that are highly abstract (skeletonized), and more detailed, limited extent, independent models (e.g., single pressure zones). This compromise makes it impossible to assess detailed impacts in the context of overall system operation. This paper discusses the use of hierarchical organization techniques implemented primarily with GIS tools and data structures that allow detailed models to be created and simulated within the context of a complete systemwide model. A three-level structure is used that allows considerable detail at the local level. This approach is being used at the City of Tucson's Water Department to allow investigation of water quality issues in detailed networks. The models are built in conjunction with spatially accurate maps maintained with CAD (AutoCAD/ArcCAD) and GIS (ARC/INFO, ArcView GIS, MapObjects) tools. Data structures are used to track the association of elements between hierarchical levels. This amounts to formalizing how elements at one level are aggregated into fewer elements at the next higher level. Appropriate attributes are maintained for modeling for elements at each level. The couplings, or connections, between model subsystems are critical in this approach as they define where a collection of model elements at one hierarchical level may be removed and replaced with a more detailed representation at the next lower hierarchical level. The goal is to be able to "drill down" to the appropriate level of detail for the question at hand (water hydraulics or quality, extent of problem, time frame) and then have the model built automatically by various programs. The process of specifying where detailed structures are to be substituted will be handled through an interface combining both database and GIS technologies (MapObjects with Visual FoxPro 6.0). Examples of these interfaces will be presented as part of the paper. Outline: · Problem description · Spatial structure of the service area · Supporting data structures and schema · User interface design · Sample model output