IMPORTANT INFORMATION CONCERNING ESTIMATED PEAK FLOWS
StreamStats outputs for ungaged sites provide estimates of both rural and urban peak flows. Use of the urban peak-flow estimates is most appropriate when impervious areas are 10 percent or more. In basins with impervious areas of less than 10 percent, the computed urban peak discharge may be less than the computed rural peak discharge. It is left to the discretion of each user, based on hydrologic judgment and knowledge of the area, to decide which computed peak discharge to use. Furthermore, the urban equations are only applicable in basins with insignificant surface and embankment storage. In a similar manner, the rural equations are only applicable to basins not affected substantially by regulation from impoundments, tidal effects, channelization, levees, or other man-made structures.
Users are cautioned against using the equations outside of the ranges of values of the independent variables used to develop the equations, as potential errors are unknown in these cases. For the rural equations (Weaver and others, 2009), the range in drainage area is from 1 to 9,000 square miles. Impervious areas for streamgages used to develop the rural equations generally were less than 10 percent. For the urban equations (Robbins and Pope, 1996), the range of applicable drainage area is from 0.04 to 41.0 square miles, and the range of impervious areas is from 2 to 54.6 percent.
StreamStats uses digital geospatial data and Geographic Information Systems (GIS) technology to compute the drainage areas and impervious percentages used as explanatory variables in the urban regression equations, whereas Robbins and Pope (1996) used printed topographic maps and manual computation methods. In general, the drainage areas agreed reasonably well, but impervious percentages from StreamStats on average were substantially larger than the published values. Topographic maps used to compute the published values were printed prior to 1996, whereas StreamStats computations used the impervious layer from the 2006 National Land Cover Dataset. Some of the differences in values may be due to differences in the computational techniques, but probably increased land development between the dates of the source data explains most of the differences in the values.
STREAMSTATS FOR NORTH CAROLINA
StreamStats for North Carolina was developed in cooperation with the North Carolina Department of Transportation (NCDOT). The North Carolina StreamStats application is developed from the best available elevation data for North Carolina. Light Detection And Ranging (LiDAR) derived digital elevation models (DEMs) at 30-foot post spacing were used as the basis for basin delineation and derivative topographic basin characteristics. The high-resolution elevation data provides an opportunity to provide the most accurate results from StreamStats for a variety of scientific applications.
In 2007, the State of North Carolina developed a local-resolution National Hydrography Dataset (NHD) product (at an approximate 1:4,800 scale) for 19 western counties in the State. The combination of digital orthophotography, elevation surfaces derived from LiDAR, and existing hydrography was used to create the local-resolution NHD product. Where local-resolution NHD products are not available statewide, StreamStats is implemented by using LiDAR-based topographic data and the best-available hydrography, which currently (2012) is 1:24,000-scale NHD. The local-resolution and 1:24,000-scale NHD are appended to make a continuous streams layer for the entire State and surrounding areas of adjacent States.
The StreamStats application interfaces with the DEMs and the most current regional streamflow regression equations and datasets of landscape characteristics. Basin characteristics, such as rainfall data, topographic variables (such as slope and drainage area), and 1992, 2001, and 2006 National Land-Cover Datasets (NLCD) are incorporated in the North Carolina StreamStats application.
The North Carolina StreamStats application also provides users with access to the most current local areal imagery and NCDOT roads layers for base maps through linkage with NC OneMap, which provides the most up-to-date geospatial data available in the state.