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A{{Short '''hydrological code''' or '''hydrologic unit code''' is a sequencedescription|Sequence of numbers or letters that identify a hydrological feature like a [[river]], river [[Reach (geography)|reach]], [[lake]], or area like a [[drainage basin]] (also called watershed (in North America)) or catchment.}}
A '''hydrological code''' or '''hydrologic unit code''' is a sequence of numbers or letters (a ''[[geocode]]'') that identify a hydrological unit or feature, such as a [[river]], river [[Reach (geography)|reach]], [[lake]], or area like a [[drainage basin]] (also called watershed in North America) or catchment.
One system, developed by [[Arthur Newell Strahler|Strahler]], known as the [[Strahler number|Strahler stream order]], ranks streams based on a hierarchy of tributaries. Each segment of a stream or river within a river network is treated as a node in a tree, with the next segment downstream as its parent. When two first-order streams come together, they form a second-order stream. When two second-order streams come together, they form a third-order stream, and so on.
One system, developed by [[Arthur Newell Strahler]], known as the [[Strahler number|Strahler stream order]], ranks streams based on a hierarchy of tributaries. Each segment of a stream or river within a river network is treated as a node in a tree, with the next segment downstream as its parent. When two first-order streams come together, they form a second-order stream. When two second-order streams come together, they form a third-order stream, and so on.
Another example is the system of assigning IDs to watersheds devised by [[:pt:Otto Pfafstetter{{ill|Otto Pfafstetter]]|pt}}, known as the [[Pfafstetter Coding System]] or the Pfafstetter System. Drainage areas are delineated in a hierarchical fashion, with "level 1" watersheds at continental scales, subdivided into smaller level 2 watersheds, which are divided into level 3 watersheds, and so on. Each watershed is assigned a unique number, called a Pfafsetter Code, based on its ___location within the overall drainage system.<ref>[http://proceedings.esri.com/library/userconf/proc01/professional/papers/pap1008/p1008.htm Watershed Topology - The Pfafstetter System] {{webarchive|url=https://web.archive.org/web/20110710202642/http://proceedings.esri.com/library/userconf/proc01/professional/papers/pap1008/p1008.htm |date=2011-07-10 }}, by Jordan Furnans and Francisco Olivera</ref>
==Europe==
==United States==
{{main|Hydrologic unit system (United States)}}
The [[United States Geological Survey]] created a hierarchical system of hydrologic units originally called [[Water Resource Region|regions]], sub-regions, accounting units, and cataloging units. Each unit was assigned a unique '''Hydrologic Unit Code''' (HUC). As first implemented the system had 21 regions, 221 subregions, 378 accounting units, and 2,264 cataloging units.<ref name="USGS HUCs">{{cite journal|last=Seaber, Paul R., F. Paul Kapanos, and George L. Knapp|title=Hydrologic Unit Maps|journal=United States Geological Survey Water-supply Papers|year=1987|volume=No. 2294|pages=i-iii, 1–63}}</ref><ref name=usgs>{{cite web|title=Hydrologic Unit Maps - What are Hydrologic Units?|url=http://water.usgs.gov/GIS/huc.html|publisher=[[USGS]]|accessdate=2010-10-27}} {{PD-notice}}</ref> Over time the system was changed and expanded.<ref>{{cite web |title= Overview and History of Hydrologic Units and the Watershed Boundary Dataset (WBD) |publisher= [[Natural Resources Conservation Service]] |url= http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/history.html}} {{PD-notice}}</ref> As of 2010 there are six levels in the hierarchy, represented by hydrologic unit codes from 2 to 12 digits long, called [[Water Resource Region|regions]], subregions, basins, subbasins, watersheds, and subwatersheds. The table below describes the system's hydrologic unit levels and their characteristics, along with example names and codes.<ref>{{cite web |title= Watershed Boundary Dataset (WBD) Facts |publisher= [[Natural Resources Conservation Service]] |url= http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/facts.html}} {{PD-notice}}</ref>
{| class="wikitable" border="1" style="margin: 1em auto 1em auto;"
|- valign="top"
! scope="col" | Name
! scope="col" | Level
! scope="col" | Digits
! scope="col" | Average size<br>(square miles)
! scope="col" | Number of HUs<br>(approximate)
! scope="col" | Example name
! scope="col" | Example code<br>(HUC)
|- valign="top"
! scope="row" | [[Water resource region|Region]]
| align="center" | 1 || align="center" | 2 || align="right" | 177,560 || align="right" | 21 || align="center" | Pacific Northwest || 17
|- valign="top"
! scope="row" | [[Water resource subregion|Subregion]]
| align="center" | 2 || align="center" | 4 || align="right" | 16,800 || align="right" | 222 || align="center" | Lower Snake || 1706
|- valign="top"
! scope="row" | [[Water resource basin|Basin]]
| align="center" | 3 || align="center" | 6 || align="right" | 10,596 || align="right" | 370 || align="center" | Lower Snake || 170601
|- valign="top"
! scope="row" | [[Water resource subbasin|Subbasin]]
| align="center" | 4 || align="center" | 8 || align="right" | 700 || align="right" | 2,200 || align="center" | Imnaha River || 17060102
|- valign="top"
! scope="row" | [[Water resource watershed|Watershed]]
| align="center" | 5 || align="center" | 10 || align="right" | 227<br>(40,000–250,000 acres) || align="right" | 22,000 || align="center" | Upper Imnaha River || 1706010201
|- valign="top"
! scope="row" | [[Water resource subwatershed|Subwatershed]]
| align="center" | 6 || align="center" | 12 || align="right" | 40<br>(10,000–40,000 acres) || align="right" | 160,000 || align="center" | North Fork Imnaha River || 170601020101
|}
The original delineation of units, down to subbasins (cataloging units), was done using 1:250,000 [[Scale (map)|scale]] maps and data. The newer delineation work on watersheds and subwatersheds was done using 1:24,000 scale maps and data. As a result, the subbasin boundaries were changed and adjusted in order to conform to the higher resolution watersheds within them. Changes to subbasin boundaries resulted in changes in area sizes. Therefore, older data using "cataloging units" may differ from newer, higher resolution data using "subbasins".<ref name="WBDstandard">{{cite web|url=https://pubs.usgs.gov/tm/11/a3/pdf/tm11-a3.pdf|title=Federal guidelines, requirements, and procedures for the national Watershed Boundary Dataset: U.S. Geological Survey Techniques and Methods 11–A3|last=|first=|date=|year=2009|website=|publisher=[[Natural Resources Conservation Service]] and [[United States Geological Survey]]|accessdate=4 November 2010}}</ref>
[[Image:Huc region-edit.png|thumb|right|The 21 top-level 2-digit "region" hydrologic unit boundaries. The names are as follows: 01 New England, 02 Mid-Atlantic, 03 South Atlantic-Gulf, 04 Great Lakes, 05 Ohio, 06 Tennessee, 07 Upper Mississippi, 08 Lower Mississippi, 09 Souris-Red-Rainy, 10 Missouri, 11 Arkansas-White-Red, 12 Texas-Gulf, 13 Rio Grande, 14 Upper Colorado, 15 Lower Colorado, 16 Great Basin, 17 Pacific Northwest, 18 California, 19 Alaska, 20 Hawaii, 21 Caribbean.]]
The regions (1st level hydrologic units) are geographic areas that contain either the [[Drainage basin|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. Each subregion includes the area drained by a river system, a [[Reach (geography)|reach]] of a river and its tributaries in that reach, a [[Endorheic basin|closed basin]] or basins, or a group of streams forming a coastal drainage area.<ref name=HUmaps>{{cite web |title= Hydrologic Unit Maps, U.S. Geological Survey Water-Supply Paper 2294 |last1= Seaber |first1= Paul R. |last2= Kapinos |first2= F. Paul |last3= Knapp |first3= George L. |publisher= [[United States Geological Survey]] |url= http://pubs.usgs.gov/wsp/wsp2294/pdf/wsp_2294.pdf |accessdate= 3 November 2010}}</ref> Regions receive a two-digit code. The following levels are designated by the addition of another two digits.<ref>{{cite web|url=https://www.nrcs.usda.gov/programs/rwa/Watershed_HU_HUC_WatershedApproach_defined_6-18-07.pdf |title=Watersheds, Hydrologic Units, Hydrologic Unit Codes, Watershed Approach, and Rapid Watershed Assessments |publisher=[[USDA]] |accessdate=2010-10-27 |url-status=dead|archiveurl=https://web.archive.org/web/20101012233329/http://www.nrcs.usda.gov/programs/rwa/Watershed_HU_HUC_WatershedApproach_defined_6-18-07.pdf |archivedate=2010-10-12 |df= }}</ref>
[[Image:HUC subregions.png|thumb|right|A map of the "subregion hydrologic unit boundaries. Note this updated map updates boundaries to include the ocean as well as the portions of the basins that cross international boundaries.]]
The hierarchy was designed and the units subdivided so that almost all the subbasins (formerly called cataloging units) are larger than {{convert|700|sqmi|km2}}. Larger closed basins were subdivided until their subunits were less than 700 square miles.<ref name=HUmaps/> The 10-digit watersheds were delineated to be between 40,000 and 250,000 acres in size, and the 12-digit subwatersheds between 10,000 and 40,000 acres.<ref name=WBDstandard/> In addition to the hydrologic unit codes, each hydrologic unit was assigned a name corresponding to the unit's principal hydrologic feature or to a cultural or political feature within the unit.<ref name=HUmaps/>
The boundaries of the hydrologic units usually correspond to drainage basins with some exceptions; for example, subregion 1711, called "Puget Sound", includes all U.S. drainage into not only Puget Sound but also the [[Strait of Georgia]], [[Strait of Juan de Fuca]], and the [[Fraser River]].<ref>{{cite web |url= http://wa.water.usgs.gov/data/wuse/huc.names.txt |title= List Hydrologic Unit Codes (HUCs) - USGS Washington |publisher= [[USGS]] |accessdate= 19 July 2011}}</ref> Also, region and subregion boundaries end at the U.S. international boundary.<ref name=HUmaps/>
In general, hydrologic units were delineated such that all surface drainage within each unit converges at a single outlet point—a type of hydrologic unit called a "classic hydrological unit". It was not always possible to delineated units in this way while adhering to the size and subdivision standards of the system. There are several "non-classic" types of drainage areas, each requiring special criteria for delineation and subdivision.<ref name=WBDstandard/>
"Remnant areas" occur along coasts where individual streams are too small for the given subdivision type. Such remnants were combined into a single unit if they were adjacent and could be combined. These "composite" units are called "frontal units". They are non-classic because they have more than one outlet.<ref name=WBDstandard/>
For example, the coastal area along [[Puget Sound]] between [[Seattle]] and [[Mukilteo, Washington|Mukilteo]], is delineated at the finest "subwatershed" level as "Shell Creek-Frontal Puget Sound", HUC 171100190203. This hydrologic unit includes numerous small streams that drain directly to Puget Sound, including [[Pipers Creek (Seattle)|Pipers Creek]] and [[Boeing Creek]]. As a consequence of the smallest "subwatershed" being non-classic, every higher level unit containing it are also non-classic "frontal" units—"Lunds Gulch-Frontal Puget Sound" (HUC 17110019), "Puget Sound" (HUC 171100 and 1711), and "Pacific Northwest Region" (HUC 17).<ref name=WBD>{{cite web|title=Watershed Boundary Dataset|publisher=[[United States Department of Agriculture|USDA]], [[Natural Resources Conservation Service|NRCS]], National Cartography and Geospatial Center|url=http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/|accessdate=September 4, 2010}} [[ArcExplorer]] [[Geographic information system|GIS]] data viewer.</ref>
"Noncontributing areas" are drainage areas within a hydrologic unit that do not drain to the unit's outlet. They can be caused by such things as potholes and [[Kettle (landform)|kettles]], closed basins, [[Dry lake|playas]], and [[cirque]]s. If a noncontributing area is large enough, it was designated as a hydrologic unit of its own. The largest such example is the [[Great Basin]], designated a hydrologic unit the Region level. When a noncontributing area was not large enough to be designated a hydrologic unit, it was merged into the surrounding or bordering larger hydrologic unit.<ref name=WBDstandard/> Special decisions were required for "semiconfined basins" that contribute flow to another area in wet years but become noncontributing in dry years—[[Goose Lake (Oregon-California)|Goose Lake]], for example. The USGS instructed the people doing the delineation work to take extra care in the case of semiconfined basins and to seek assistance from others, but to ultimately make their own decision on whether the semiconfined basin should be designated a noncontributing area or not. Another special case occurs when noncontributing areas very small and dispersed, or scattered throughout a drainage area. These were considered part of the encompassing hydrologic unit. In short, noncontributing areas cannot be subject to strict criteria for delineating, and methods vary from state to state, landform type to type, and special cases. The effect of noncontributing areas on specific hydrologic units is explained in [[metadata]] as best it can.<ref name=WBDstandard/>
The Goose Lake example illustrates how USGS hydrologic units do not always conform strictly to drainage basins. Despite being part of the Upper Sacramento River basin (or accounting unit), HUC 180200, and the Sacramento River subregion, HUC 1802, the Goose Lake subbasin (or cataloging unit), HUC 18020001, was defined as a closed basin during the watershed and subwatershed delineation process.<ref name=WBD/> Therefore, the area of the Sacramento River subregion and the Upper Sacramento River basin, as published by the USGS ({{convert|27600|sqmi|km2|abbr=on}} and {{convert|7650|sqmi|km2|abbr=on}} respectively), are too large by at least the size of the Goose Lake subbasin/cataloging unit, {{convert|1080|sqmi|km2|abbr=on}}.<ref>{{cite web |title= Boundary Descriptions and Names of Regions, Subregions, Accounting Units and Cataloging Units |publisher= [[United States Geological Survey|USGS]] |url= http://water.usgs.gov/GIS/huc_name.html |accessdate= 16 November 2010}}</ref>
Other non-classic drainage issues that have an effect on hydrologic unit delineation and subdivision include [[reservoir]]s, diverted waters ranging from small irrigation [[ditch]]es to [[interbasin transfer]]s, islands, and coastal areas with large [[tidal range]]s. The 5th and 6th level hydrologic units, called "watersheds" and "subwatersheds", were assigned one of seven attribute codes to indicate drainage type: standard (classic, one outlet), closed basin (no outlet), frontal (multiple outlets), water (predominately water with adjacent land areas), island (one or more islands and adjacent water), and unclassified (an area that cannot be defined or does not fit one of the other types).<ref name=WBDstandard/>
==See also==
==External links==
* {{cite web |url= http://www.dcr.virginia.gov/soil_and_water/hu.shtml |title= Hydrologic Unit Geography |publisher= Virginia Department of Conservation & Recreation |accessdateaccess-date= 21 November 2010}}
* {{cite web |url= http://ccm.jrc.ec.europa.eu |title= CCM2 Catchments and Rivers of Europe |publisher= Joint Research Center of the European Commission |accessdateaccess-date= 13 September 2012}}
* {{cite web|url=http://193.178.1.168/River_Coding_Review.pdf |title=Review of Existing River Coding Systems |publisher=European Coding Systems WFD GIS Working Group |accessdateaccess-date=29 September 2014 |url-status=dead|archiveurlarchive-url=https://web.archive.org/web/20070121122717/http://193.178.1.168/River_Coding_Review.pdf |archivedatearchive-date=January 21, 2007 }}
[[Category:Hydrology]]
[[Category:Limnology]]
[[Category:Water and the environment]]
[[Category:Geocodes]]
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