By David R. Maidment, Director, Center for Research in Water Resources, University of Texas at Austin

The advent of ArcGIS has opened a window of new technological possibilities for geospatial data storage and analysis. These possibilities have shown much potential in the field of water resources, and at the Center for Research in Water Resources (the Center), University of Texas at Austin, we focused our effort on the design of a model for storing water resources data in ArcGIS, formally termed the ArcGIS Hydro data model but more widely known as simply Arc Hydro.

To involve government, industry, and academic partners in this effort, Esri and the Center jointly formed a Consortium for GIS in Water Resources (see "Consortium for GIS in Water Resources to Design New Object Models," ArcNews Winter 1999/2000, Vol. 21, No. 4).

This three-year research and development effort is coming to a close, and I'd like to outline what we've learned and describe some exciting new capabilities now available to water resources users of ArcGIS.

Three aspects of ArcGIS emerged as critical factors in building Arc Hydro: networks, the geodatabase, and object modeling.

Networks

First, networks are fundamental to water resources analysis because the river and stream network is the key pathway for the transport of water through the landscape. ArcGIS has a new structure called a geometric network, which topologically connects points and lines, and a set of network analysis tools, which allow tracing of flow upstream and downstream through such networks. These network tracing capabilities are available as standard tools in ArcView 8.x, so it is no longer necessary to have a special extension of ArcView to use network analysis. Our experience in prototyping Arc Hydro on many large river and stream networks has shown that the new ArcGIS network capabilities work very effectively for water resource networks. We've developed a Hydro Network as part of Arc Hydro by adding to the geometric network a set of relationships that link network junctions to watersheds, waterbodies, and hydro point features such as stream gauges.

The Geodatabase

Second, the geodatabase has proven to be a robust structure for cataloging and storing data. A key advantage of the geodatabase over the coverage data model in workstation ArcInfo and the shapefile data model in ArcView 3 is that relationships between individual geographic features can be permanently stored as part of a geodatabase. This is of great assistance in following the path of water movement from watersheds through streams, lakes, rivers, and bays. We have adopted a universal identifying number, the HydroID, that is unique throughout all features of an Arc Hydro geodatabase, and by storing the HydroID of a HydroJunction as the JunctionID attribute of a watershed it is easy to associate these features, so that we know to which junction on the hydro network the drainage from a particular watershed discharges.

Object Modeling

Third, the concept of object modeling is central to ArcGIS. This allows us to build special objects and features for water resources that are derived from generic ArcObjects applicable to any spatial analysis in GIS. Moreover, the adoption in ArcGIS of the Component Object Model standard from Microsoft allows the close integration of ArcGIS with other applications such as Access, Excel, and Word. Indeed, the personal geodatabase version of ArcGIS stores all its data in Microsoft Access. This is convenient when dealing with time series data of water flows, rainfall, and water quality that are readily handled in a relational database.

The Key Advance

With all these new capabilities of ArcGIS, what can we now do with Arc Hydro that we couldn't do before? The key advance that Arc Hydro makes to the application of GIS in water resources is that it makes very effective use of vector data. The main success in this field during the 1990s was the use of raster digital elevation models for watershed delineation. Within a raster grid, the use of a single cell for a point, a sequence of cells for a line, and a zone of cells for an area provides an approximate way of linking point, line, and area features for water resources analysis. In Arc Hydro, vector features for points, lines, and areas are linked using the hydro network and feature-to-feature relationships. This allows for a wealth of detail to be stored compactly in a single geodatabase. Indeed, we have built a prototype geodatabase called Arc Hydro USA that contains 1:500,000 scale representation of the watersheds, river network, stream gauges, and waterbodies of the entire continental United States.

However, we haven't forgotten the importance of raster analysis. The Arc Hydro tool set comes with a very comprehensive set of tools for watershed and stream network delineation from a digital elevation model, so if you don't have vector data for your project area you can start with a digital elevation model and use the Arc Hydro raster tools to generate a connected network of watersheds and streams for which standard Arc Hydro attributes are added automatically as the raster analysis proceeds. The Arc Hydro raster analysis tools allow the partitioning of a large basin into regions. The regions' digital elevation models can be analyzed separately and then the results combined into a regional watershed description, rather than having to do the raster analysis using a single grid for the whole basin.

Model Application

How do you apply the Arc Hydro data model? All you need to begin is a set of streams, watersheds, waterbodies, and points such as stream gauge locations. In ArcCatalog, you create a new, empty geodatabase containing a feature data set called Arc Hydro, copy your data layers into it, then use ArcCatalog software's geometric network wizard to build your Hydro Network. You need to do this in ArcInfo as geometric networks cannot be built using ArcView. Then, you add the standard Arc Hydro attributes to your data using the Schema Creation wizard in ArcCatalog, and fill in the values of these attributes using the Arc Hydro tools. An important step is labeling all your features uniquely using the assigned HydroID tool. Once you've done that, you can connect your watersheds, waterbodies, and gauges to the network with other Arc Hydro tools, and you'll end up with a nice Arc Hydro framework data set.

Once you've established your Arc Hydro framework data set, you can add additional Arc Hydro components such as a three-dimensional river channel description and time series of water resources measurements.

Data Application

What can you do with data built into Arc Hydro format? First, there are many core features of ArcGIS that can be used immediately such as the network tracing tools on the hydro network. We've added a new tracing capability to Arc Hydro-the ability to trace upstream or downstream across drainage areas so you can determine the region of hydrologic influence of any location on the landscape. Also, you can determine watershed properties, such as average precipitation or runoff, and accumulate them going downstream so that you can estimate the flow in streams and rivers. It is possible to select United States Geological Survey (USGS) stream gauging stations in ArcMap and automatically download the stream flow measurements made at those stations into Arc Hydro's time series data storage without ever leaving ArcMap. Arc Hydro is a hydrologic information system because it combines both geospatial and temporal water resources data. As the use of Arc Hydro continues to develop, further hydrologic analysis and modeling capabilities will become available.

Visit the Consortium Web site (www.crwr.utexas.edu/giswr) or see arconline.esri.com/datamodels/water.cfm for more information about Arc Hydro development. I wish to acknowledge the contributions of the National Hydrography Dataset group within the U.S. Geological Survey and the U.S. Environmental Protection Agency.