Interagency Development of National Watershed and Subwatershed Hydrologic Units

Kenneth J. Legleiter

Hydrologic units through four levels were created in the 1970's and have been used extensively throughout the United States. During the early 1990's the Natural Resources Conservation Service started to delineate hydrologic units to the 5^th (Watershed) and 6^th (Subwatershed) level by using Geographic Information Systems to meet 1:24,000 National Map Accuracy Standards. With increased interest from other federal, state and local entities, this initiative has become an interagency effort. The completion of a verified, single, seamless, nationally consistent, and accurate geospatial database based on hydrologic and mapping principles by 2002 will help in producing a national Watershed Boundary Dataset.

BACKGROUND

A standardized hydrologic unit system, referred to as the Hydrologic Unit Code system, was developed in the mid-1970's by the U.S. Geological Survey (USGS) under the sponsorship of the Water Resources Council. This system divided the country into 21 Regions, 222 Subregions, 352 Accounting Units (hereafter referred to as Basins), and 2,149 Cataloging Units (hereafter referred to as Subbasins) based on surface features. A hierarchical hydrologic unit code containing 2 digits for each of the four levels was assigned to identify the hydrologic units; these four levels are the basis for the 8-digit hydrologic unit code. The underlying concept is a topographically defined set of drainage areas organized in a nested hierarchy by size.

The standardized 8-digit USGS hydrologic units (Levels 1,2,3 and 4) are broadly used; however, the geographical area of the units are too large to adequately serve many water-resource investigations, resource analysis and management needs. For example, the focus of many water resource issues is based upon pollutant loading and land-surface processes, and the cumulative effects of pollution over space and time. Management of these issues requires working with hydrologic units in smaller sizes than those defined by the 8-digit hydrologic units. Programs requiring smaller hydrologic units include State River Basin Management Plans, the United States Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS) conservation and watershed programs, USDA Forest Service land management planning and watershed management programs, and various programs in the Environmental Protection Agency, Office of Water.

The NRCS completed mapping watersheds (Level 5 hydrologic units) in the early 1980's on small-scale state base maps for use in natural resource planning. In the mid-1990's, the NRCS along with State agency conservation partners began a national initiative to delineate and digitize watersheds and subwatersheds on base maps meeting national map accuracy standards. To promote a standardized criterion for hydrologic unit determination and delineation, the NRCS developed National Instruction 170-304 in 1992 that served as the agency's policy for delineating and digitizing watersheds and subwatersheds. The NRCS updated the policy guidelines in 1995 incorporating changes from internal and external reviews.

The U.S. Forest Service and the Bureau of Land Management (BLM) are the primary land management agencies of federal lands in the U.S. Both agencies are delineating and digitizing watersheds and subwatersheds on public lands. The delineation is primarily done on public lands in the western states; however, often all land ownerships are delineated within a subbasin. Earlier delineations of hydrologic units of federally administered public lands served administrative purposes but were often developed without full coordination between federal and state agencies.

INTERAGENCY EFFORTS

The NRCS, USGS, and member agencies of the Federal Geographic Data Committee (FGDC), Subcommittee on Spatial Water Data have been coordinating and conducting a series of regional workshops to promote the development of a nationally consistent hydrologic unit coverage. The Subcommittee on Spatial Water Data was chartered and sponsored by the Advisory Committee on Water Information (ACWI), and the FGDC. The Subcommittee on Spatial Water Data coordinates spatial water data and information activities among all levels of government and the private sector. Spatial water data includes information about streams, hydrologic units, lakes, ground water, coastal areas, precipitation and other hydrologic information related to water resources.

The Subcommittee on Spatial Water Data assists the ACWI and FGDC by facilitating the exchange and transfer of water data, establishing and implementing standards for quality, content and transfer of water data, and coordinating the requirements and the collection of spatial data to minimize duplication of efforts.

During the spring of 2000, it was agreed to by the member agencies of the Subcommittee on Spatial Water Data that a new standard would be written for hydrologic unit delineation to the 5^th and 6^th level. The NRCS, Forest Service, USGS, and BLM have worked with other federal and state agencies, tribes, FGDC, and Subcommittee on Spatial Water Data to author the "Federal Standards for Delineation of Hydrologic Unit Boundaries". This standard builds upon the original NRCS National Instruction 170-304, and establishes the principles and standards that federal agencies should follow in making subdivisions of the 8-digit USGS subbasin hydrologic units. The standard is not agency specific, and has been agreed upon by member agencies of the Subcommittee on Spatial Water Data.

The USGS, USFS, BLM, and NOAA are assisting NRCS in the review and verification of hydrologic units. The USGS, EPA, NOAA, and the National Weather Service are researching techniques to employ digital elevation and bathymetric data for producing hydrologic unit delineations.

STATE-REGIONAL COORDINATION

States have formed Interagency Hydrologic Unit Groups composed of GIS and water resource users. In this group, members representing federal, state, and local watershed groups coordinate and promote within the state and across state borders while developing a statewide HU dataset. Once the statewide datasets are verified, they will be appended together to form one national seamless dataset. The delineation of hydrologic units into "watersheds" and "subwatersheds" is an opportunity to develop a consistent and common nationwide geospatial database for interagency sharing, and for improving federal, state, and local uses of hydrologic units. The "Federal Standards for Delineation of Hydrologic Unit Boundaries" are to be used by agencies and tribes for developing a single, seamless, nationally consistent, and accurate geospatial database of hydrologic units based on scientific hydrologic and mapping principles. The standards are designed not to be specific to one part of the country and to be flexible so it can be practical in all states and island territories. The standard will enable a variety of users from different agencies and programs to contribute to an overall coordinated watershed management approach, efficiently share information and resources, and assure the geospatial database is usable with other related Geographic Information System (GIS) databases.

The utility of hydrologic units of various size and complexity based primarily on natural surface water flow and topographic landforms cannot be underestimated for the potential invaluable analytical and statistical purposes and applications at hydrological and ecosystem scales. Some examples of these programs include watershed management, water quality initiatives, watershed modeling, resource inventory and assessment, and establishing total maximum daily loads.

CRITERIA TO DELINEATE HYDROLOGIC UNITS

The selection and delineation of watersheds and subwatersheds requires good hydrologic judgement. They must be determined solely upon science-based hydrologic principles, not administrative or special projects nor favor any particular program or agency, so to assure a homogeneous national seamless digital data layer. The intent of defining hydrologic units is to establish a base-line drainage boundary framework, which has complete accountability of all land and surface areas. The hydrologic units must be delineated and georeferenced to the USGS 1:24,000 scale topographic base map which meets National Map Accuracy Standards (NMAS).

The four original hydrologic unit levels plus the two new levels are shown in Exhibit 1, Appendix A. Subbasin boundaries (4th level/8-digit) provide the basis for the delineation of watersheds (5th level/10-digit), and watersheds boundaries provide the basis for delineation of subwatershed (6th level/12-digit) hydrologic units. Subbasin boundaries are being edited or adjusted when the existing mapping does not conform to hydrologic and topographic principles.

Watersheds and subwatersheds, like other hydrologic units, are defined along natural hydrologic breaks based on land surface, surface water flow, and hydrographic features. Delineated boundaries define the land area extent of the watersheds and subwatersheds. A hydrologic unit has a single flow outlet except in coastal or lakefront areas. Since watersheds and subwatersheds are subdivisions of a higher level hydrologic unit, they share common boundaries with the existing hydrologic units defined in the next higher level of the hydrologic unit hierarchy.

Some earlier versions of watershed and subwatershed boundaries used administrative boundaries to define hydrologic units. Hydrologic unit boundaries are being determined solely upon hydrologic and topographic features. Hydrologic unit boundaries end at international boundaries of the United States unless agreements are reached with countries neighboring the U.S.

Hydrologic unit boundaries are being delineated on both sides of a stream flowing toward a single point. Boundaries do not follow or run parallel to rivers or streams except where some physical feature prevents water flowing directly to the water surface such as levees, berms, incised channels, etc. Boundaries cross the river or stream perpendicularly at the hydrologic unit outlet (Exhibit 2). By delineating the boundary at a confluence this allows for proper nesting of smaller units within the hydrologic unit for future site-specific planning, assessment, monitoring or inventory activities.

In addition to the primary criteria, there are general criteria for the number of hydrologic units subdivided from a parent unit, the size of hydrologic units, and the treatment of non-contributing and remnant areas. Hydrologic units are generally subdivided into 5 to 15 units from the next lower level. For example, 5 to 15 watersheds (10-digit) are nested in each 8-digit subbasin. This approach accommodates geomorphic or other relevant basin characteristics, and provides fairly uniform size distribution of same-level HUCs within a broader physiographic area. By using this approach, a smooth transition between sizes of same-level hydrologic units is created as topography changes between physiographic areas and helps to maintain consistency of delineations crossing state borders.

Hydrologic units of any given level within a physiographic area are generally the same size. Nationally, the typical size for a watershed is 40,000 to 250,000 acres. The typical size for a subwatershed is 10,000 to 40,000 acres. Exceptions to these national sizes can be found along coastal areas, unusual or extreme topography, karst topography, and remnant areas.

The classic watershed is a pure hydrologically defined surface water drainage area. All of the surface drainage is contained within the classic watershed boundary and converges at a single outlet point, usually at a confluence. The classic watersheds are areas that define major tributaries from a larger "parent" hydrologic unit. Hydrologic units sometimes are divided at a lake outlet when the upstream drainage area size is appropriate for the hydrologic unit level being delineated.

Areas that do not allow for delineation of a "classic watershed" are termed "special situations". These are land areas that fall under the terms of composite, remnant, non-contributing, and diverted areas. They are delineated differently than "classic watersheds". Most of the time, these special situation areas are typically added in with an adjacent hydrologic unit, but occasionally, may exist as very small atypical hydrologic units.

Remnant areas are areas that remain around the main stem of larger streams after they have been delineated. Remnant areas typically occur as wedge-shaped areas along inter-fluvial regions between adjacent watersheds, or as over-bank areas along a stream between junctions with tributaries. Remnant areas also occur in coastal outlet areas to several mainland or island watersheds that are individually smaller than defined for a given hydrologic unit level.

Drainage areas that do not flow toward the outlet of any hydrologic unit are considered non-contributing areas. Such areas may be due to glaciated plains (potholes), enclosed basins, topography, playas, cirques, depression lakes, dry lakebeds, or similar landforms. The largest non-contributing area in the U.S. is the Great Basin which appears in Nevada, Utah, Oregon and California. This closed area is large enough to be considered as a subregion. Non-contributing areas that are small and dispersed are considered to be part of the hydrologic unit, and the area is recorded as non-contributing.

The delineation of hydrologic unit boundaries around reservoirs and lakes is complicated. As a general rule, reservoirs are mapped using the legacy channels by ignoring the reservoir pool. This is done because the pool level fluctuates throughout the year and dams are considered temporary structures. There are situations where this is not feasible and the reservoir is treated as a natural lake.

Natural lakes are being treated differently than reservoirs. Whenever possible, the dam or natural spill point is used as a pour point. This is dependent on the location of the lake relative to adjoining hydrologic units and the size of the lake. There are special situations when a tributary that drains into the lake or reservoir can be mapped down to the water's edge and becomes a 5^th or 6^th level hydrologic unit. An example would be if the drainage area of the tributary were of the recommended size for a particular hydrologic level.

Diverted Waters

Hydrologic units are being delineated based on natural surface water flow and natural topographic land features. Ditches and canals are being used when the man-made feature has permanently altered the natural flow. Many man-made drainage features in the United States were originally either perennial or intermittent channels that local government and private entities converted into a permanent drainage feature. Much of the surface drainage in these areas would "disappear" from local and state drainage maps if permanent man-made diversions were not considered when delineating hydrologic units. If the present day canal or ditch was once a legacy stream channel or has perennial flow, then it may be considered for delineation of hydrologic units.

Coastal Areas

The delineation of hydrologic unit boundaries is often complicated in coastal areas by the presence of large estuaries, bays or sounds. The delineation of hydrologic units in coastal areas is being based on the natural flow of water due to the topography of the land, except where long-term, permanent, large-scale coastal water bodies exist.

The coastal delineation procedure applies to ocean coastal areas, non-ocean coastal areas such as the Great Lakes and large tidal rivers such as parts of the Mississippi, Columbia and Potomac Rives. The coastal delineation procedure does not apply to inland flatlands. The coastal delineation procedure considers estuarine and ocean zones as separate delineation units for differentiating between gravity-based surface flow and tidal or ocean-mixed waters.

Since water levels in coastal water bodies can fluctuate significantly, hydrologic units are being mapped based upon submerged morphologic features including shoals, shore faces and flow channels (legacy channels). Boundaries are not being delineated to the normal, average, or high water levels of the coastal water body. NOAA digital raster graphic charts are being used for delineation in coastal areas.

For a coastal body of water that is of a size consistent with the hydrologic unit level being delineated, it is appropriate to delineate the water body as a separate hydrologic unit. Tributary areas flowing directly into an estuary or sound and that are of a size consistent with the hydrologic unit level being delineated would be delineated as "classic" watersheds with an outlet point at the water body margin.

Areas draining into an estuary or sound hydrologic unit, but which cannot be delineated at a consistent size are being included as part of the estuary or sound hydrologic unit unless the result of the combined unit exceeds the consistent size of other units in that level. In this circumstance, the estuary or sound hydrologic unit is being delineated as a hydrologic unit and the adjacent slope areas are being delineated as remnant hydrologic units.

The seaward boundary of the hydrologic unit coverage includes the drain point of ocean inlets. The type of seaward boundary between inlets depends on whether the ocean shore is of a barrier island or a mainland or large broad island shore. In the case of a barrier island, the seaward boundary is the centerline for ground water flow. In the case of the mainland or a large broad island, the seaward hydrologic unit border is the toe of the shore face.

MAPPING AND DELINEATION PROCESS

The mapping process used to delineate hydrologic unit boundaries is usually either manual or digital techniques or a combination of the two. Hydrologic modeling techniques are being evaluated for use in streamlining and improving the delineation of hydrologic unit boundaries through the use of digital elevation data.

Base Maps, Map Scale and Map Accuracy

The official base map for delineating watershed and subwatershed boundaries is the USGS 1:24,000 scale topographic quadrangles. The delineation is done using either a paper lithograph of the map, Digital Raster Graphic of the 1:24,000 scale topographic map, the 7.5 minute Digital Elevation Model (DEM), or bathymetric data. Digital elevation data equivalent or better to the vertical and horizontal resolutions of either the USGS 30 meter, Level 2 DEMs or National Elevation Dataset (NED) are acceptable for delineating "draft" or "preliminary" maps of hydrologic units. In Alaska and the Caribbean, USGS base maps at 1:25,000 or 1:63,360 scale is being used in the absence of 1:24,000 scale base maps. The 10 and 12-digit hydrologic unit boundaries are being delineated and georeferenced to the USGS 1:24,000 scale base map, which meets National Map Accuracy Standards (NMAS) (Exhibit 3). Adherence to these standards for accuracy, scale and map format will produce a seamless and consistent national hydrologic unit geographic coverage.

Digital Data

Digital geospatial data, such as the USGS 1:24,000 scale Digital Raster Graphics (DRGs) are being used for on-screen digitizing of hydrologic units, and for clarifying line position when using other digitizing methods. Digital Elevation Models (DEMs) are needed if a digital modeling method is used to delineate and digitize the hydrologic unit boundaries. The Environmental Protection Agency (EPA) river reach files and the National Elevation and Hydrography Datasets are other useful databases for identifying primary tributaries within an area being subdivided. The NOAA nautical charts are also available in digital raster form.

Map Delineation Process

Drainage divides are usually determined by bisecting ridges, saddles, and contour lines of equal elevation. Hydrologic unit boundaries follow the middle of the highest ground elevation or halfway between contour lines of equal elevation. The hydrologic unit boundary will cross perpendicular to the stream channel at the outlet. The hydrologic unit boundary has only one outlet point, except in remnant and coastal areas. Hydrologic unit boundaries are not streams or rivers.

In areas of flat terrain, interpolation between contours is being done by referencing to trails, old roads, or firebreaks in forested areas, all of which frequently follow drainage divides. Many times, more detailed information is being obtained from local highway or street profiles, examination of aerial photographs, county drainage or ditch-canal maps, and ground reconnaissance by local employees. In urban areas, delineation to elevation change is being used than relying on storm sewer maps so as to maintain consistency in delineating according to natural features and surface water flow.

Some states are delineating hydrologic units within an 8-digit subbasin at the same time and not stopping at state boundaries. When this is not possible, adjoining states are coordinating the delineation of hydrologic unit boundaries at state boundaries.

Digital Methods

Improvements in GIS software and availability of geospatial data offer increased speed and functionality in hydrologic unit development compared todelineating on paper maps. Two different digital methods are summarized below.

On-Screen Mapping and Digitizing

Many states are delineating hydrologic unit boundaries by using DRGs and heads-up digitizing. For over-water boundaries in coastal areas, boundaries are being delineated to the toe of the shore face and shoal lines using on-screen digitizing from NOAA digital raster nautical charts (Mercator projection). The boundaries are then reprojected to match with the USGS DRGs to complete the over-land hydrologic boundaries.

Hydrologic Modeling from Digital Elevations

GIS, image processing, and hydrologic modeling applications make use of DEMs to represent landform features and drainage network patterns. Digital elevation data equivalent or better to the vertical and horizontal resolutions of either the 30 meter, Level 2 DEMs or National Elevation Dataset (NED) (http://edcnts12.cr.usgs.gov/ned/) are being used to develop a "draft" or "preliminary" delineation of hydrologic units. Depending upon the modeling techniques, data resolution, data consistency, software applications, and other characteristics, the DEMs and NED are in most cases providing a generalized depiction of landforms, especially in areas of moderate topographic variations or flat areas. For this reason, all DEM and NED established boundaries are being independently checked on 1:24,000 DRGs and edited for accuracy.

Updating and Revising Hydrologic Units

Reasons for updating or revising hydrologic unit boundaries may be necessary as a result of a natural phenomena or significant man-made landform modifications. Some examples include the removal of a dam, earthquakes, new reservoirs, man-made embankments or levees, volcanic eruptions, massive landslides, hurricane damage, etc.

The 8-digit subbasin boundaries have been used and referenced so extensively in water resource activities nationwide that changing them occurs only in cases of major delineation error or significant landform changes due to natural phenomena or man-made modifications. When an existing 8-digit subbasin is unusually large or has multiple outlets each exceeding the range of the normal 8-digit subbasin, these are sometimes sub-divided into more than one 8-digit subbasin. Any changes are done in cooperation with the USGS when boundaries are questionable or need to be re-located to be more hydrologically correct.

CODING AND NAMING

Hydrologic Unit Levels

The six different levels of hydrologic units are shown below and in Appendix A, Exhibit 1.

Coding Watersheds and Subwatersheds

A new unique 2-digit code is being added for each 5^th and 6^th level watershed and subwatershed. This is consistent with the coding pattern established in the 1970's by the interagency water data committees. States are coordinating the coding within a subbasin across state boundaries.

States are numbering the hydrologic units sequentially beginning upstream and proceeding downstream within each HU. For example, the uppermost end of the HU is coded 9908020301, the next HU downstream is 9908020302.

Watershed and Subwatershed Naming Protocol

The numeric Hydrologic Unit Code is the primary, unique identifier for each hydrologic unit, however, the numeric identifier alone makes it difficult to relate a hydrologic unit to a geographic area. Hydrologic units are usually named after significant or prominent hydrographic features in an area; however, hydrologic units are sometimes named after non-hydrographic features if better understood by the users and the public.

Every attempt is being made to identify each watershed and subwatershed with a unique name within the area being subdivided. The 1^st, 2^nd, 3^rd and 4^th level hydrologic unit names are not being renamed or changed. The major hydrographic feature within the hydrologic unit from the Geographic Names Information System (GNIS) of the USGS, in cooperation with the U.S.Board on Geographic Names (BGN) http://mapping.usgs.gov/www/gnis/bgn.html, which can be found at http://mapping.usgs.gov/www/gnis is the recommended naming convention. Geographic names are also taken from the USGS 1:24,000 topographic quadrangles or National Hydrography Dataset.

GEOSPATIAL DATA STRUCTURE

Data Capture and Processing

The vector data structure is the required method for the hydrologic unit geospatial database. The following attribute structure was developed for the national review and verification process and to create one national seamless dataset. The attribute structure of the national dataset may be altered once it is converted into the WBD.

Polygon Attribute Scheme

All the following attributes fields are required in the schema specified in the following text. Each attribute field must have the attribute cell populated for each record unless the field is optional. The optional fields can have attribution in one or more cells; this is up to the person(s) doing the delineation. The attribute schema is illustrated for Environmental Systems Research Institute (Esri) software¹. If another software is being used other than ArcInfo or ArcView, the user has been instructed to follow the ArcInfo format. For field width in ArcView, the user has been instructed to create fields according to the width specified under "Output" in the field descriptions. All the listed fields are required in each state dataset, but they may have additional fields for their use. These additional fields will be removed before creating the WBD.

Eight Digit Hydrologic Unit Code - (REQUIRED)

A unique 8-digit code from the USGS map series "Hydrologic Unit Maps" is used as the initial code. The same 8-digit number is used in every record that pertains to a subwatershed that resides within the same 8-digit sub-basin.

Ten Digit Hydrologic Unit Code - (REQUIRED)

This field provides a unique 10-digit code for each watershed. Two digits are added to the end of the existing 8-digit code, therefore resulting in a 10-digit number. This same number is used in every record that pertains to a subwatershed that resides within the same 10-digit watershed.

Twelve Digit Hydrologic Unit Code - (REQUIRED)

This field provides a unique 12-digit code for each subwatershed. Two digits are added to the end of the existing 10-digit code, therefore creating a 12-digit number.

Acres - (REQUIRED)

The area of the subwatershed including non-contributing areas calculated to acres as a whole number. The "Acres" field is calculated from the "Area" field.

States - (REQUIRED)

The "States" field includes the names of all state(s) that the subwatershed falls within. The field is populated with the 2-digit Postal abbreviation separated with a comma between multiple states.

Non-Contributing Area - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)

Drainage areas that do not flow toward the outlet of any hydrologic unit are considered non-contributing. This attribute is the total of the non-contributing areas within a hydrologic unit calculated in acres.

Fifth Level Downstream Hydrologic Unit Code - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)

This field is populated with the 10-digit code of the 5^th level hydrologic unit that is receiving the majority of the flow from the watershed that the 6^th level HU falls within.

Fifth Level Hydrologic Unit Name - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)

The field is populated with the identical name for all 6^th level hydrologic units that fall within the same 10-digit HU. The name used to attribute the watershed is used only once within a 4^th level.

Fifth Level Hydrologic Unit Modifications - (REQUIRED)

This field identifies any type of modifications to natural overland flow present in the 10-digit watershed. The attribute field lists from most significant to least significant modification(s).

Fifth Level Hydrologic Unit Type - (REQUIRED)

This field is populated with the hydrologic unit type that most closely identifies the watershed.

T - "True", "Pure", "Classic" hydrologic unit - All surface drainage flowing to a single outlet, except for non-contributing areas. Usually, the upper end of the hydrologic unit is the headwaters of a stream and the outlet is the confluence.

C - "Composite", "Remnant area" hydrologic unit - Drainage areas that remain after classic hydrologic units are delineated. The most common example of a composite or remnant hydrologic unit is the small triangular wedges between adjacent HU's.

B - "Closed Basin" hydrologic unit - A hydrologic unit where no surface water exits through the pour point, in other words, is 100% non-contributing to the pour point.

F - "Frontal" hydrologic unit - Areas along the coastline of lakes, oceans, bays, etc. that have more than one outlet. These HU's are predominantly land with some water areas at or near the outlet(s).

W - "Water" hydrologic unit - Hydrologic units that are predominantly water with adjacent land areas, ex. lake, easturies, harbors.

I - "Island" hydrologic unit - A hydrologic unit that is one or more islands and adjacent water out to the toe of the shore face.

U - "Unclassified" hydrologic unit - A hydrologic unit that can not be defined or does not fit into one of the types that have been listed.

Sixth Level Downstream Hydrologic Unit Code - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)

This field is populated with the 12-digit code of the 6^th level hydrologic unit that is receiving the majority of the flow from the subwatershed.

Sixth Level Hydrologic Unit Name - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)

This field is populated with official names only. The name used to attribute the subwatershed is used only once within a 5^th level unit.

Sixth Level Hydrologic Unit Modifications - (REQUIRED)

This field identifies any type of modifications to natural overland flow present in the 12-digit subwatershed. The attribute field lists from most significant to least significant modification(s).

Sixth Level Hydrologic Unit Type - (REQUIRED)

This field is populated with the hydrologic unit type for the subwatershed.

T - "True", "Pure", "Classic" hydrologic unit - All surface drainage flowing to a single outlet, except for non-contributing areas. Usually, the upper end of the hydrologic unit is the headwaters of a stream and the outlet is the confluence.

C - "Composite", "Remnant area" hydrologic unit - Drainage areas that remain after classic hydrologic units are delineated. The most common example of a composite or remnant hydrologic unit is the small triangular wedges between adjacent HU's.

B - "Closed Basin" hydrologic unit - A hydrologic unit where no surface water exits through the pour point, in other words, is 100% non-contributing to the pour point. A watershed may be a "closed basin" while all but the highest numbered subwatersheds are not closed basins, except for the 6^th level HU that has a pour point at the same location as the watershed.

F - "Frontal" hydrologic unit - Areas along the coastline of lakes, oceans, bays, etc. that have more than one outlet. These HU's are predominantly land with some water at or near the outlet(s).

W - "Water" hydrologic unit - Hydrologic units that are predominantly water with adjacent land areas, ex. lake, easturies.

I - "Island" hydrologic unit - A hydrologic unit that is one or more islands and adjacent water out to the toe of the shore face.

U - "Unclassified" hydrologic unit - A hydrologic unit that can't be defined or doesn't fit into one of the types that have been listed.

ArcInfo (.pat) and ArcView Polygon Format

Linework Attribute Scheme

This attribute table provides descriptive information on each boundary line.

Hydrologic Unit Level - (REQUIRED)

This field is populated with the highest hydrologic unit level for the line (arc) represented by the record.

Line Spatial Data Source (REQUIRED)

The Linesource indicates the base map source(s) used to delineate at 1:24,000 scale. More than one code can be identified for each arc.

Metadata ID Number - (REQUIRED)

Metadata ID is a code that identifies which metadata file applies to the arc. In many cases there will only be one metadata file. However, in some cases more than one metadata file may be created to identify different groups, procedures, or during mainentance of the dataset used to produce the lines. These separate metadata files may be identified for each separate arc.

ArcInfo Line (.aat) Attribute Format

Comments Info File (FILE OPTIONAL)

A separate info file where comments can be stored for the polygons or lines. The file contains two items, a record number that relates to the .aat or .pat record number (cover#) and an 80-character field called "Comment".

• MO_com_poly
• MO_com_line

QUALITY ASSURANCE AND QUALITY CONTROL

Quality Control

Hydrologic unit boundaries are usually reviewed by the originating agency and/or designated members of an Interagency Hydrologic Unit Group within the state, typically hydrologists, natural resource specialists, and or GIS Specialists with background and experience with hydrologic units. The dataset goes through an extensive review process by the state representatives representing Federal to local agencies before submission for verification.

Geospatial Data Verification

After the 6th level hydrologic units are completed, the originating office will submit the data for final review and verification to the interagency national review group. Archive all verified geospatial files at your agency's geospatial archival center or clearinghouse. At this time the state HU dataset will go through a national review (Exhibit 4) to verify the dataset meets the criteria in "Federal Standards for Delineation of Hydrologic Unit Boundaries." Once the dataset meets the criteria established in the guidelines, a letter will be sent from the Subcommittee on Spatial Water Data to the office that submitted the dataset for national review stating the dataset is verified.

METADATA

According to an Executive Order signed by the President on April 11, 1994, all federal agencies developing geospatial data are required to document newly created data by completing metadata. The FGDC content standards for metadata are required.

CONCLUSION

Interest in a national, seamless, and consistent hydrologic unit dataset at 1:24000 continues to gather support as word spreads of the ongoing activities. An anticipated completion date for all 50 states and U.S. territories are for the end of 2002 (Exhibit 5).

ACKNOWLEDGMENTS

I would like to thank Bruce McCammon, Regional Hydrologist, U.S. Forest Service, Portland, Oregon and Michele May, Cartographer, NRCS, Fort Worth, Texas.

¹ Any use of trade, product, or firm names in this document is for descriptive purposes only and does not imply endorsement by the U.S. Government.

APPENDICES

Appendix A: Illustrations

Exhibit 1. Hydrologic Unit Hierarchy, Source: Bruce McCammon, USFS.

Exhibit 2. Subwatershed boundaries shown in red. Outlets located correctly at tributary side on the bank of the receiving stream at the confluence, boundary crosses perpendicular to river just upstream of confluence.

Exhibit 3. Subwatershed boundary shown in red is delineated correctly along ridgeline. It meets NMAS for 1:24,000 scale maps.

Exhibit 4. Verification Process Flowchart.

Exhibit 5. Hydrologic Unit Status Map.

Appendix B: Acronyms

REFERENCES

Member Agencies of the Subcommittee on Spatial Water Data. 2001. Federal Standards for Delineation of Hydrologic Unit Boundaries (Draft). Subcommittee on Spatial Water Data.

AUTHOR INFORMATION