The VAAs include greatly enhanced capabilities for upstream and downstream navigation, analysis and modeling. Examples include: retrieve all flowlines (predominantly confluence-to-confluence stream segments) and catchments upstream of a given flowline using queries rather than by slower flowline-by-flowline navigation; retrieve flowlines by stream order; subset a stream level path sorted in hydrologic order for stream profile mapping, analysis and plotting; and, calculate cumulative catchment attributes using streamlined VAA hydrologic sequencing routing attributes.
The VAAs include results from the use of these cumulative routing techniques, including cumulative drainage areas, precipitation, temperature, and land cover distributions. Several of these cumulative attributes are used to estimate mean annual flow and velocity as part of the VAAs.
NHDPlus contains a snapshot (2005) of the 1:100,000-scale NHD that has been extensively improved. While these updates will eventually make their way back to the central NHD repository at USGS, this will not have happened prior to distribution of NHDPlus because the update process for the central NHD repository is still in development. Consequently, the NHDPlus will contain some temporary database keys and, as a result, NHDPlus users may not make updates to the NHD portions of NHDPlus with the intent of sending these updates back to the USGS. Once the NHDPlus updates have been posted to the central NHD repository, a fresh copy of the improved data can be downloaded from the central NHD repository and that copy will be usable for data maintenance. Note that the NHDPlus products are tightly integrated and user modifications to the underlying NHD can compromise this synchronization.
A water-quality model developed by the U.S. Geological Survey (USGS) called SPARROW (Spatially Referenced Regressions on Watershed Attributes), can utilize the NHDPlus network functionality to track the downstream transport of nutrients, sediments, or other substances. NHDPlus water bodies and estimates of streamflow and velocity are used in SPARROW to identify reservoir retention and in-stream loss factors. NHDPlus climatic and land surface attributes can be used in SPARROW to identify potential factors in the delivery of nutrients from the land surface to streams. NHDPlus land cover information is useful for SPARROW in determining potential sources of nutrients.
NHDPlus data is also being used in select areas for a USGS Web-based application, called StreamStats. StreamStats provides tools to interactively select any point on the NHDPlus to obtain streamflow and watershed characteristics for the selected point.
NHDPlus has been designed to accommodate many users needs for future applications. NHDPlus provides the framework and tools necessary to customize the behavior of the network relationships as well as building upon the attribute database, for which the user can assign its own data to the network.
Acknowledgement of the originating agencies (USEPA and USGS) would be appreciated in products derived from these data.