Hydrologic Unit delineations are closed polygons that encompass all area draining toward the lowest point (called outlet) in the polygon. Because of nominal sizes for the different hydrologic unit levels (Watershed (40,000 to 250,000 acres) and Subwatershed (3,000 minimum, generally 10,000 to 40,000 acres)), some polygons do not include all areas up to the drainage divide but all areas to one or more other upstream hydrologic units. A unique hydrologic unit code is used to identify each
hydrologic unit. The Watershed hydrologic unit codes start with the 8-digit Subbasin (Cataloging Unit) number that contains the Watershed. Likewise the Subwatershed hydrologic unit codes start with the 10-digit Watershed number that contains the Subwatershed.
A map unit symbol uniquely identifies each closed polygon. Each symbol is linked to a hydrologic unit code. The uniqueness of hydrologic unit codes while applying to hydrologic units does not carry over to polygons. Small islands offshore in coastal and lakeshore areas or in river deltas are given the same code as adjacent hydrologic units on the mainland.
Attribute data associated with a hydrologic unit code includes:
non-contributing area (acres);
names for each level of nesting (region, subregion, basin, subbasin, watershed and subwatershed);
codes for hydrologic units immediately downstream;
name of town where subwatershed outlet occurs;
information about type of subwatershed or watershed;
reference to human-caused modifications to stream flow within the watershed;
notes concerning delineation of hydrologic unit boundary.
Description of attributes by field from the Federal Standards for Delineation of Hydrologic Unit Boundaries
1) .pat attributes
Huc_8 : Eight Digit Hydrologic Unit Code - (REQUIRED)
A unique 8-digit code from the USGS map series "Hydrologic Unit Maps". Use the existing code for the sub-basin where the subwatershed resides; do not change this code. The same number should be used in every record that pertains to a subwatershed that resides within the same 8-digit sub-basin.Numbers were assigned in an upstream to downstream fashion. Where no downstream/upstream relationship could be determined numbers were assigned in a clockwise fashion.
Huc_10: Ten Digit Hydrologic Unit Code - (REQUIRED)
This field provides a unique 10-digit code for each watershed. Add two digits to the end of the existing 8-digit code, therefore resulting in a 10-digit number. This same number should be used in every record that pertains to a subwatershed that resides within the same 10-digit watershed.Numbers were assigned in an upstream to downstream fashion. Where no downstream/upstream relationship could be determined numbers were assigned in a clockwise fashion. It should be noted that the polygons that cross into Idaho were numbered by Idaho and are not consistent with the above methods. Affected huc_8s include: 17040202, 17040204 and 17040104.
Huc_12: Twelve Digit Hydrologic Unit Code - (REQUIRED)
This field provides a unique 12-digit code for each subwatershed. Add two digits to the end of the existing 10-digit code, therefore creating a 12-digit number.Numbers were assigned in an upstream to downstream fashion. Where no downstream/upstream relationship could be determined numbers were assigned in a clockwise fashion. It should be noted that the polygons that cross into Idaho were numbered by Idaho and are not consistent with the above methods. Affected huc_8s include: 17040202, 17040204 and 17040104.
Acres: Acres - (REQUIRED)
Area of subwatershed including non-contributing areas calculated to acres as a whole number, no decimals. The "Acres" field needs to be calculated from the "Area" field.
States: States - (REQUIRED)
The "States" field needs to include the names of all state(s) that the subwatershed falls within. Use the 2-digit postal abbreviation in upper case and sort the state(s) in alphabetical order. If using more then one abbreviation, separate with a comma with no space after the comma.
Ncontrb_a: Non-Contributing Area - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)
Drainage areas that do not flow toward the outlet of any hydrologic unit are considered non-contributing. If a non-contributing area is on the boundary between two or more hydrologic units, determine the low point along the non-contributing area boundary. The non-contributing area should be associated with the hydrologic unit adjacent to the low point on the boundary. This attribute should be the total of the non-contributing areas within a hydrologic unit calculated in acres.
Hu_10_ds: Fifth Level Downstream Hydrologic Unit Code - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)
Populate this field with the 10-digit code of the 5th level hydrologic unit that is receiving the majority of the flow from the watershed that the 6th level HU falls within. Outlets created by ditching or other artificial drainage are not to be considered for this field. If an HU flows into an ocean or the Gulf of Mexico populate this field with "OCEAN" and if a HU flows into one of the Great Lakes use the term "LAKE". If a HU flows across international borders use "CANADA" or "MEXICO" depending on which country the HU drains into. If an HU is a closed basin, then populate this record with the term "CLOSED BASIN".
Hu_10_name: Fifth Level Hydrologic Unit Name - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)
This field is for officially recognized names only. Populate this field by following the directions in subsection 6.3 "Watershed and Subwatershed Naming Protocol". Populate this field with the identical name for all 6th level hydrologic units that fall within the same 10-digit HU. The name used to attribute the watershed should be used only once within a 4th level unit. Some 10-digit HU were assigned the huc_10 number when no GNIS name was identified on the DRG-Es.
Hu_10_mod: Fifth Level Hydrologic Unit Modifications - (REQUIRED)
This field should identify any type of modifications to natural overland flow that alters the location of the hydrologic unit boundary for a 10-digit watershed. In the attribute field, identify from most significant to least significant modification(s). Use one or more of the following abbreviations in uppercase to identify your modification(s). If using more then one abbreviation, separate with a comma with no space after the comma.
SC - Stormwater Canal
ID - Irrigation Ditch
IT - Interbasin Transfer
BC - Barge Canal
SD - Stormwater Ditch
PD - Pipe Diversion
CD - Channel Diversion
NC - Non-Contributing
KA - Karst
LE - Levee
NM - No Modifications
OC - Overflow Channel
DM - Dam at outlet or HU boundary
OT - Other
Hu_10_type: Fifth Level Hydrologic Unit Type - (REQUIRED)
Populate this field with the hydrologic unit type from the list provided that most closely identifies the watershed. Use any of the one-digit abbreviations in uppercase that is provided in the list.
S - "Standard" hydrologic unit - Any land HU with drainage flowing to a single outlet point, excluding non-contributing areas. This includes areas or small triangular wedges between adjacent HU's that remain after classic hydrologic units are delineated. Some examples include "true", "classic", "composite", and "remnant" hydrologic units.
C - "Closed Basin" hydrologic unit - A drainage area that is 100% non-contributing. This means all surface flow is internal, no overland flow leaves the hydrologic unit through the outlet 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't be defined or doesn't fit into one of the types that have been listed.
Hu_12_ds: Sixth Level Downstream Hydrologic Unit Code - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)
Populate this field with the 12-digit code of the 6th level hydrologic unit that is receiving the majority of the flow from the subwatershed. Outlets created by ditching or other artificial drainage are not to be considered for this field. If a HU flows into an ocean or the Gulf of Mexico populate this field with "OCEAN" and if a HU flows into one of the Great Lakes use the term "LAKE". If a HU flows across international borders use "CANADA" or "MEXICO" depending on which country the HU drains into. If an HU is a closed basin, then populate this record with the term "CLOSED BASIN".
Hu_12_name: Sixth Level Hydrologic Unit Name - (FIELD REQUIRED, ATTRIBUTES OPTIONAL)
This field is for officially recognized names only. Populate this field by following the directions in subsection 6.3 "Watershed and Subwatershed Naming Protocol". The name used to attribute the subwatershed should be used only once within a 6th level unit. Some 10-digit HU were assigned the huc_12 number when no GNIS name was identified on the DRG-Es.
Hu_12_mod: Sixth Level Hydrologic Unit Modifications - (REQUIRED)
This field should identify any type of modifications to natural overland flow that alters the location of the hydrologic unit boundary for a 12-digit subwatershed. In the attribute field, identify from most significant to least significant modification(s). Use one or more of the following abbreviations in uppercase to identify your modification(s). If using more than one abbreviation, separate with a comma with no space after the comma.
SC - Stormwater Canal
ID - Irrigation Ditch
IT - Interbasin Transfer
BC - Barge Canal
SD - Stormwater Ditch
PD - Pipe Diversion
CD - Channel Diversion
NC - Non-Contributing
KA - Karst
LE - Levee
NM - No Modifications
OC - Overflow Channel
DM - Dam at outlet or HU boundary
OT - Other
Hu_12_type: Sixth Level Hydrologic Unit Type - (REQUIRED)
Populate this field with the hydrologic unit type from the list provided that most closely identifies the subwatershed. Use any of the one digit abbreviations in uppercase that is provided in the list.
S - "Standard" hydrologic unit - Any land HU with drainage flowing to a single outlet point, excluding non-contributing areas. This includes areas or small triangular wedges between adjacent HU's that remain after classic hydrologic units are delineated. Some examples include "true", "classic", "composite", and "remnant" hydrologic units.
C - "Closed Basin" hydrologic unit - A drainage area that is 100% non-contributing. This means all surface flow is internal, no overland flow leaves the hydrologic unit through the outlet 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 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
Item name Width Output Type (ArcView) N.Dec. Example
Huc_8 8 8 C - 01080201
Huc_10 10 10 C - 0108020103
Huc_12 12 12 C - 010802010310
Acres 8 12 F N 0 026739
States 11 11 C - KS,MO,OK
Ncontrb_a 8 12 F N 0 357
Hu_10_ds 10 10 C - 1710020504
Hu_10_name 80 80 C - Upper Blue River
Hu_10_mod 20 20 C - CB,NC,ID
Hu_10_type 1 1 C - S
Hu_12_ds 12 12 C - 171002050402
Hu_12_name 80 80 C - Drift Creek-Big Bear River
Hu_12_mod 20 20 C - SD,KA,PD
Hu_12_type 1 1 C - C
2) .aat attributes
Hu_level: Hydrologic Unit Level - (REQUIRED)
This field provides the means to create cartographically pleasing maps. Populate this field with the highest hydrologic unit level (smallest number) for the line (arc) represented by the record. Record the level using numbers 1 through 6 representing each level with 1 being the highest and 6 the lowest level. An example would be if a line represents a region, subregion, basin, subbasin, watershed, and subwatershed boundary, then this cell would be populated with a 1 (Region). Example two would be if a line is a subbasin, watershed, and subwatershed boundary, then the cell would get a 4 (Subbasin). Use one of the levels provided in the list below.
Level Digit# Name
1 2 Region
2 4 Subregion
3 6 Basin
4 8 Subbasin
5 10 Watershed
6 12 Subwatershed
Linesource: Line Spatial Data Source - (REQUIRED)
The Linesource should indicate the base map source(s) used to delineate at 1:24,000 scale. Populate the field using one of the standardized code(s) listed below in uppercase. If using more than one code, separate with a comma with no space after the comma.
TOPO24 - Delineation from hardcopy 1:24,000 scale topographic maps
TOPO25 - Delineation from hardcopy 1:25,000 topgraphic maps, only Alaska and Caribbean
TOPO63 - Delineation from hardcopy 1:63,360 topographic maps, only Alaska and Caribbean
DRG24 - Delineation from 1:24,000 scale Digital Raster Graphics
DRG25 - Delineation from 1:25,000 Digital Raster Graphics, only Alaska and Caribbean
DRG63 - Delineation from 1:63,360 Digital Raster Graphics, only Alaska and Caribbean
DEM10 - Derived from 10 meter Digital Elevation Model
DEM30 - Derived from 30 meter Digital Elevation Model
NED30 - Derived from 30 meter National Elevation Dateset Model
EDNA30 - (formally NED-H), derived from 30 meter Elevation Derivatives for National
Applications
BATH"scale" (ex. BATH24) - Interpreted from NOAA 1:24,000 scale bathymetric data
HYPSO"scale" (ex. HYPSO24)- Delineated from 1:24,000 scale contour data
ORTHO"scale" (ex. ORTHO12) - Interpreted from 1:12,000 scale Ortho-imagery
DEDEM10 - Drainage enforced 10 meter Digital Elevation Model
DEDEM30 - Drainage enforced 30 meter Digital Elevation Model
GPS - Derived from Global Positioning System
LIDAR - Derived from LIDAR
IFSAR - Derived from IFSAR data
OTH - Other
UNK - Unknown
All other reference and source maps not listed should be noted in the metadata.
Meta_id: 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 and/or procedures used to produce the lines. These separate metadata files may be identified for each separate arc. The metadata ID should be a 4-character code starting with the 2-letter state postal code, followed by a 2-digit sequence number. For example "OK01", "ID02", etc.
ArcInfo Line (.aat) Attribute Format
Item name Width Output Type N.Dec. Example
Hu_level 1 1 I - 6
Linesource 20 20 C - DEM30,DRG24,GPS
Meta_id 4 4 C - OK01