Data Integration Speeds Water Modeling

Engineers and hydrologists performing large-scale water resource analysis in the United States can now access a virtual treasure trove of public domain data that was previously unavailable in one location. Through ongoing collaboration of two federal agencies, the NHD (National Hydrography Dataset) has been integrated with elevation and watershed data to provide a new, cohesive suite of data called NHDPlus, which could vastly shorten the process of modeling watersheds.

The NHD is a basic set of digital spatial data containing information about lakes, rivers, streams and other water bodies, and has been available for most of the United States since 2000 to serve as a foundation in building water resource models. Developed collaboratively by the EPA (U.S. Environmental Protection Agency) and the USGS (U.S. Geological Survey), the NHD combines elements of USGS digital line graph hydrography files and EPA reach files. Although useful as a building block in mapping and watershed studies, the original NHD contained no detailed information about drainage areas, stream velocities or other flow characteristics, so users had to generate or import those parameters from other sources.

NHDPlus, which was publicly introduced in 2005 and expanded to cover the entire United States in late 2006, combines the NHD with the National Elevation Dataset and the Watershed Boundary Dataset. The linked datasets provide access to attributes such as elevation-derived drainage areas, land cover, flow direction and streamflow volume and velocity estimates. By using linked data, “you can chain things together” and relate upstream and downstream watersheds, said Jeff Simley, hydrography coordinator with the USGS in Denver. For example, users can track flow from a watershed in Montana to a downstream watershed in Missouri to determine travel time for flows in the Missouri River basin, a process that “would take weeks of effort” using traditional methods, according to Simley.

NHDPlus integrates three previously separate datasets. (Figure courtesy of EPA.)

The enhanced data suite has enabled environmental engineers to model water contamination incidents and predict how contamination would affect downstream water resources. Water quality modeling of this type requires integrating a variety of landscape data and watershed characteristics, and “previously there was no good way to relate them,” said Tommy Dewald, NHD project manager for the EPA in Washington, D.C. The U.S. Forest Service and a group of partners have used NHDPlus to build an Incident Command Tool for Drinking Water Protection, called ICWater, that helps evaluate contamination incidents and track substances that pose public health threats. Dewald said other users also have analyzed various water quality and discharge scenarios using NHDPlus.

NHDPlus has been used to model water contamination scenarios and develop the ICWater tool. (Figure courtesy of EPA.)

Developing NHDPlus required months of research, data collection and software development to integrate the various datasets. “We spent almost two years evaluating different techniques to determine drainage areas,” Dewald said.

The linking of disparate data sources, such as vector stream data and raster elevation data, also presented challenges, according to Cindy McKay, senior developer with Horizon-Systems Corporation, a Herndon, Virginia-based software consultant that assisted EPA and USGS in developing NHDPlus. “The datasets were not spatially integrated and elevation data was not always located accurately [horizontally],” she said. Also, large water bodies such as lakes are typically depicted only as flat surfaces on topographic maps, while water modeling usually requires more accurate depiction of lake and river bottoms.

Further complicating the task was the diverse nature of U.S. watersheds, said McKay. The country is divided into 18 major watersheds, and characteristics such as slope and soil type in Colorado, for example, differ greatly from those in Florida. The team had to account for these factors in building NHDPlus, she said.

The United States consists of 18 major drainage areas. (Figure courtesy of Horizon Systems Corp.)

Users can access NHDPlus data at no charge from the EPA web site and find additional NHD data on the USGS web site. GIS software capable of reading shape files can be used to view the data. NHD data is available at both 1:24,000 and 1:100,000 scales, and NHDPlus files are currently available only at 1:100,000. A higher resolution scale might be pursued in the future if funding can be found, although no plans are currently in the works, said USGS’s Simley, adding that individual states with specific needs might be able to spur development of higher resolution datasets and other features.