1 A GIS Approach to Watershed Modeling in Maricopa County, Arizona By E. James Nelson, Christopher M. Smemoe, and Bing Zhao Abstract: Most conventional rainfall-runoff modeling methods are tedious and time- consuming. The Watershed Modeling System (WMS) is a comprehensive computer software application for watershed characterization and rainfall-runoff modeling in a graphical user interface environment. Through several GIS operations and tight integration with GIS databases, WMS enables hydrologists and water resource engineers to perform rainfall-runoff modeling more efficiently than conventional modeling methods. In this paper, methods of computing hydrologic parameters for the Flood Control District of Maricopa County are discussed, and demonstrated by applying them to the Gavilan Peak Watershed. This watershed is located in the vicinity of the community of New River in northern Maricopa County, Arizona. Introduction Hydrologic modeling is one of the many natural applications of Geographic Information Systems (GIS). Using GIS software, digital elevation data can be used to define the drainage basin boundaries, create stream networks, and compute drainage basin data. Once computed, several important variables, such as area, slope, and runoff distances, important to hydrologic analysis, can be determined. With grid-based elevation data readily available, it is a relatively simple task to define the extents of drainage basins and compute sub-basin parameters. However, drainage basin delineation and basic parameter computation is only the first step in developing data for a lumped parameter model such as HEC-1. Further processing of land use, soil, and rainfall data are equally important to developing the model. The HEC-1 model has been the standard for hydrologic analysis in Maricopa County for years. To use lumped parameter watershed modeling programs like HEC-1, the sub- basin data values at each grid cell in a GIS must be "lumped" into a few data values (such as a runoff coefficient and watershed response time) for the entire sub-basin. Aggregation of GIS data to a format that is usable by a lumped parameter watershed model is a daunting task. A program called the Watershed Modeling System (WMS) has been developed that has the capability of automating watershed delineation and parameter calculation from digital elevation data, importing GIS data, extracting important watershed information from the GIS database, running hydrologic modeling programs, and viewing the results of those programs. The focus of this research was to create hydrologic models using available GIS data and the standard Flood Control District of Maricopa County hydrograph computation procedures. The methods described here were implemented for use in the WMS so the entire model could be developed within a single application program.