Overview and History of Hydrologic Units and the Watershed Boundary Dataset (WBD)
Hydrologic unit boundaries define the aerial extent of surface water drainage to a point. Hydrologic units through four levels were created in the 1970's and have been used extensively throughout the United States. During that time the U.S. Geological Survey (USGS) developed a hierarchical hydrologic unit code (HUC) for the United States. This system divides the country into 21 Regions, 222 Subregions, 352 Accounting Units, and 2,149 Cataloging Units based on surface hydrologic features. The smallest USGS unit (8-digit HU) is approximately 448,000 acres. During the late 1970's the Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service, initiated a national program to further subdivide HU's into smaller watersheds for water resources planning. A 3-digit extension was added to the 8-digit identification. By the early 1980's this 11-digit HU mapping was completed for most of the U.S.
During the 1980's several NRCS state offices starting mapping watersheds into subwatersheds by adding 2 or 3-digits to the 11-digit HU. By the late 1980's and early 1990's the advent of GIS made the mapping of digital HU boundaries feasible, and in the early 1990's the Natural Resources Conservation Service started to delineate hydrologic units to the 5th and 6th level by using GIS to meet 1:24,000 National Map Accuracy Standards. Subsequently, the NRCS decided to delineate and map the entire U.S. to the 11 and 14-digit level. With increased interest from other federal, state and local entities, this initiative became an interagency effort.
The goal of this initiative is to provide a hydrologically correct, seamless and consistent national GIS database at a scale of 1:24,000, which has been extensively reviewed and matches to a minimum the USGS topographical 7.5 minute quads. The new levels are called watershed (5th level, 10-digit) and subwatershed (6th level, 12-digit). The watershed level is typically 40,000 to 250,000 acres, and the subwatershed level is typically 10,000 to 40,000 acres with some as small as 3,000 acres. An estimated 22,000 watersheds and 160,000 subwatersheds will be mapped to the 5th and 6th level. The GIS coverages are publicly available via the Internet. The database will assist in planning and describing water use and related land use activities.
The mapping is done by the use of GIS, incorporating DEM's, DRG's, and a variety of geospatial data and techniques. A national standard, first called NI-170-304, which is now superseded by the National Interagency Guidelines, established procedures and specifications for delineating and mapping hydrologic units. These guidelines help ensure that HU boundaries are accurate and consistent nationwide and that the digital database is and will be usable with other natural resource, digital data layers in a GIS. The first national standard, NI-170-304, was issued in 1992 and was updated until it was superseded by the interagency guideline, described below.
Over the last ten years, many federal and state agencies have realized current 8-digit hydrologic unit maps are unsatisfactory for many purposes because of inadequate bases or scales. Thus, NRCS worked with other federal and state agencies and with the Subcommittee on Spatial Water Data Federal Geographic Data Committee (FGDC) to establish a federal interagency standard covering mapping and delineation of hydrologic units that would be suitable for all agencies. In cooperation with the FGDC and the Advisory Committee on Water Information (ACWI), a new interagency guideline was written. During December of 2002, this document was presented to the FGDC for review. This document has superseded NI-170-304 as the official standard for delineation of 5th and 6th level hydrologic units.
With the interagency standard, some changes have been made to the criteria for delineation and attribution of the 5th and the level. These changes include coding the 5th level as 10-digit (formerly 11-digit in NI-170-304) and 6th level as 12-digit (was 14-digit in NI-170-304). Another change is that the 3rd level will officially be called "basins" (formerly known as "cataloging units") and the 4th level will be called "sub-basins" (formerly known as "accounting units"). Additional attribute fields have also been added to the dataset. Over the last several years, a series of workshops have been held to promote this interagency effort and to resolve delineation and attribution issues.
This effort to delineate and digitize the HU's is coordinated by federal, state, and local agencies, universities, and others interested in the effort. NCGC provides coordination, verification, and certification of state datasets, and integrates the state coverages into a national Watershed Boundary Dataset.