Federal agencies coordinating spatial water data have identified the development of a National Watershed Boundaries Dataset as a top priority for inclusion in the National Spatial Data Infrastructure (NSDI).
The proposed NSDI Watershed Boundaries Dataset will have the following key characteristics:
The National Watershed Boundaries Dataset, the National Elevation Dataset, and National Hydrography Dataset inherently are related. Early maintenance efforts will seek to identify and resolve inconsistencies among these three datasets and use those inconsistencies to help improve the quality of each national dataset.
Federal agencies have been coordinating spatial water data over the past 3 years under the charter of the Federal Interagency Subcommittee on Spatial Water Data (SSWD). The Subcommittee on Spatial Water Data is jointly sponsored by the Advisory Committee on Water Information (ACWI) and the Federal Geographic Data Committee (FGDC). The SSWD has identified the development of a national watershed boundaries dataset as a top priority for inclusion in the National Spatial Data Infrastructure (NSDI).
The most detailed, nationally consistent set of watershed boundaries is the Hydrologic Cataloging Units, which are identified with 8-digit Hydrologic Unit Codes (HUCs). Cataloging Units typically are about 700 square miles in area. Numerous Federal agencies need a greater level of detail to fulfill their resource assessment, management, and regulatory missions. For example, local watershed boundaries are needed to support water management and environmental protection goals, including watershed assessment and restoration.
Watershed boundaries for some areas of the United States already have been mapped by various agencies, principally the U.S. Department of Agriculture, Natural Resources Conservation Service (NRCS). These boundaries will be incorporated into the NSDI Watershed Boundaries Dataset (WBD) where appropriate criteria have been used in the development. Interagency guidelines on the delineation of watershed boundaries are being developed. The guidelines are based primarily on existing NRCS procedures.
Where new watershed boundaries need to be developed, a semiautomated procedure will be used to delineate the boundaries. Where possible, the procedure will use elevation data from the National Elevation Dataset (NED). The boundaries will be checked on-screen and edited for consistency using scanned images of 1:24,000-scale topographic maps called Digital Raster Graphics. When necessary, such as in areas of extremely low relief, boundaries will be verified in the field.
Another dataset inherently related to the WBD and the NED is the National Hydrography Dataset (NHD), which describes the network of streams draining the land. Because the NHD was developed at the 1:100,000 scale and the NED and WBD are being developed at the 1:24,000 scale, there will be some inconsistencies among the three datasets. During early maintenance efforts, instances of such inconsistencies will be identified and used to help improve the quality of the three datasets.
The responsibility for development of the WBD will be shared by several agencies and coordinated by the SSWD.
(Adapted from Seaber, P.R., Kapinos, F.P., and Knapp, G.L., 1987, Hydrologic Unit Maps: U.S. Geological Survey Water-Supply Paper 2294, 63 p. Updated information not from this source is enclosed in square brackets. More information may be found at http://water.usgs.gov/GIS/huc.html)
The United States is divided and subdivided into successively smaller hydrologic units which are classified into four levels, from largest to smallest: Regions, Subregions, Accounting Units, and Cataloging Units. Each hydrologic unit is identified by a unique HUC consisting of two to eight digits based on the four levels of classification.
The first level of classification divides the Nation into 21 major geographic areas, or Regions. These geographic areas contain either the drainage area of a major river, such as the Missouri Region, or the combined drainage areas of a series of rivers, such as the Texas-Gulf Region, which includes a number of rivers draining into the Gulf of Mexico.
The second level of classification divides the 21 Regions into 222 Subregions. A Subregion includes the area drained by a river system, a reach of a river and its tributaries in that reach, a closed basin(s), or a group of streams forming a coastal drainage area.
The third level of classification subdivides many of the Subregions into 352 Accounting Units. These hydrologic Accounting Units nest within, or are equivalent to, the Subregions.
The fourth level of classification is the Cataloging Unit, the smallest element in the hierarchy of hydrologic units. [Efforts are underway to add further levels of subdivisions.] A Cataloging Unit is a geographic area representing part or all of a surface drainage basin, a combination of drainage basins, or a distinct hydrologic feature. These units subdivide the Subregions and Accounting Units into smaller areas. There are 2150 Cataloging Units in the Nation. [Cataloging Units sometimes are called "watersheds." See, for example, the Watershed Information Network site.]
The WBD subdivides 4th-level hydrologic units into 5th- and 6th-level units. Each 4th-level unit will be subdivided into 5 to 10 5th-level units, each comprising 40,000 to 250,000 acres. Each 5th-level unit will be subdivided into 5 to 10 6th-level units, each comprising 10,000 to 40,000 acres. Two additional digits will be assigned to the existing HUC for each level. Thus, the 5th-level units will be assigned 10-digit codes, and the 6th-level units will be assigned 12-digit codes.
Additionally, the coordinating Federal agencies and the SSWD have agreed to change the HUC nomenclature to make the terms more understandable. The terms "Accounting Unit" and "Cataloging Unit" will be changed to "Basin" and "Subbasin." This change has not yet been reflected in the Federal Information Processing Standards (FIPS).
|
HUC level |
Label |
Digits in HUC |
|
1 |
Region |
2 |
|
2 |
Subregion |
4 |
|
3 |
Basin (formerly Accounting Unit |
6 |
|
4 |
Subbasin (formerly Cataloging Unit) |
8 |
|
5 |
Watershed |
10 |
|
6 |
Subwatershed |
12 |
Three primary functions are to be supported by the WBD. The first function, water accounting, is to assign a detailed, unambiguous watershed or subwatershed identifier (address) to any point or area in the United States small enough to be relevant to current national priorities, such as watershed assessment and restoration. The SSWD has determined that the 5th and 6th levels of hydrologic units (Watersheds and Subwatersheds) will meet the need for a nationally consistent watershed addressing system.
The second primary function to be supported by the WBD is to depict the flow of water through the landscape. The network connectiveness of each subwatershed will be identified. This will allow easy identification of all upstream or downstream hydrologic units. This capability allows decision makers to assess the effects of upstream activities on the subwatershed of interest and on downstream areas.
Basin characteristics often are computed for the drainage area upstream from stream measurement sites and used to develop statistical relations. For example, regional regression equations have been developed for every State and Puerto Rico to estimate the magnitude and frequency of floods at ungaged sites. The independent variables used in these regressions include drainage area, main-channel slope, and several other basin characteristics (Jennings and others, 1994). Many of these basin characteristics can be calculated on a hydrologic unit basis and aggregated for all hydrologic units upstream from the site of interest. Thus, the third primary function of the WBD is to provide the framework, using the network connectiveness information, to aggregate basin characteristics for upstream hydrologic units. This capability can dramatically reduce the amount of time required to compute basin characteristics for such applications.
Several Federal and State agencies, along with other groups, are participating in the efforts to build the WBD. The list of Federal agencies includes the U.S. Geological Survey, U.S. Department of Agriculture Natural Resources Conservation Service and Forest Service, U.S. Environmental Protection Agency, Bureau of Land Management, and the National Oceanographic and Atmospheric Administration (NOAA). Each agency is committing resources to the effort, which is being coordinated by the SSWD. The agencies have agreed to a common set of guidelines for delineating hydrologic units, and a draft of the interagency guideline is in progress.
A Coastal Watershed Workgroup, consisting of members from NOAA and other Federal agencies, are drafting guidelines and doing pilot testing for delineation of hydrologic units in coastal areas. These guidelines outline delineation methods for coastal areas such as estuaries and islands.
The NRCS is involved in subwatershed delineation in most States. More information on the status of NRCS efforts, as well as State contacts, are available on the Internet at:
http://www.ftw.nrcs.usda.gov/huc_data.html
A series of regional meetings are being held to discuss hydrologic unit delineation issues throughout the United States. These meetings are sponsored by the SSWD and typically involve representatives from several Federal, State, and other agencies.
The development of the WBD is tied closely to the development of the hydrologic derivatives of the NED-H. The development of the WBD is considered to be the second stage of the three-stage process envisioned to develop the NED-H. The first stage of NED-H development is a blind-pass process. The second stage involves interactive editing of watershed boundaries that help to identify problems within the NED-H. The third stage of NED-H processing is the incorporation of needed changes (identified in stage 2) back into the NED-H dataset. Additional information on the design and status of NED-H is available on the Internet at:
http://edcnts12.cr.usgs.gov/ned-h/index.html
Members of the WBD and NED-H teams are working with the Environmental Systems Research Institute under a Cooperative Research and Development Arrangement (CRADA) to further develop software tools useful for WBD and NED-H processing.
Additional work is proceeding in the Arkansas-White-Red River Basin and other pilot project areas. Integration with NHD, estimation of streamflow statistics, and accuracy assessment of the watershed delineation are among the topics being investigated.
Team members also are participating in the GIS in Water Resources Consortium, which is developing a new Arc Hydrology Data Model. This new data model will work under the new Geo-Object data model Esri is implementing with ArcInfo version 8.
More information about the development of the WBD may be found on the Internet at:
http://water.usgs.gov/wicp/acwi/spatial/spatial_olinks.html
Jennings, M.E., Thomas, W.O., Jr., and Riggs, H.C., 1994, Nationwide summary of U.S. Geological Survey regional regression equations for estimating magnitude and frequency of floods for ungaged sites, 1993: U.S. Geological Survey Water-Resources Investigations Report 94-4002, 196 p.
Seaber, P.R., Kapinos, F.P., and Knapp, G.L., 1987, Hydrologic Unit Maps: U.S. Geological Survey Water-Supply Paper 2294, 63 p.