A hydrological code or hydrologic unit code is a sequence of numbers or letters that identify a hydrological feature like a river, river reach, lake, or area like a drainage basin (also called watershed (in North America) or catchment).
One system, developed by Strahler, known as the 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 Otto Pfafstetter, 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.
A comprehensive coding system is in use in Europe. This system codes from the ocean to the so-called primary catchment. The system determines a set of oceans or endorheic systems identified by a letter. These systems are subdivided into a maximum of 9 seas. The seas are numbered 1 to 9. Seas lying far from the ocean, for example the Black Sea receive a higher number. The seas are delimited using the so-called definitions made by the International Hydrographic Organization in 1953. The coasts of these seas are defined clockwise from north west to south east from the strait where the sea connects to the ocean or the other seas.
|Hydrological System||North Sea||A5|
Subsequently every watershed along this coast is assigned a number using the Pfafstetter System. This implies that the four largest watersheds are selected and receive numbers 2,4,6, or 8. The watersheds in between the large systems receive numbers 3, 5, and 7. Numbers 1 and 9 are used for the small watersheds on the edges of the strait. The smaller systems can subsequently be numbered recursively or kept together for grouping purpose. Landmasses (Continent and Islands) are also numbered in a logical manner, along a clock-wise oriented sea. For Europe containing many inner seas this feature helps to read the relative location of a hydrological object in the sea.
United States 
The United States Geological Survey created a hierarchical system of hydrologic units originally called 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. Over time the system was changed and expanded. As of 2010 there are six levels in the hierarchy, represented by hydrologic unit codes from 2 to 12 digits long, called 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.
|Number of HUs
|Example name||Example code
|22,000||Upper Imnaha River||1706010201|
|160,000||South Fork Imnaha River||170601020101|
The original delineation of units, down to subbasins (cataloging units), was done using 1:250,000 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".
The regions (1st level hydrologic units) are geographic areas that 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. Each subregion includes the area drained by a river system, a reach of a river and its tributaries in that reach, a closed basin or basins, or a group of streams forming a coastal drainage area. Regions receive a two-digit code. The following levels are designated by the addition of another two digits. The hierarchy was designed and the units subdivided so that almost all the subbasins (formerly called cataloging units) are larger than 700 square miles (1,800 km2). Larger closed basins were subdivided until their subunits were less than 700 square miles. 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. 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.
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. Also, region and subregion boundaries end at the U.S. international boundary.
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.
"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.
For example, the coastal area along Puget Sound between Seattle and 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 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).
"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 kettles, closed basins, playas, and cirques. 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. Special decisions were required for "semiconfined basins" that contribute flow to another area in wet years but become noncontributing in dry years—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.
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. Therefore, the area of the Sacramento River subregion and the Upper Sacramento River basin, as published by the USGS (27,600 sq mi (71,000 km2) and 7,650 sq mi (19,800 km2) respectively), are too large by at least the size of the Goose Lake subbasin/cataloging unit, 1,080 sq mi (2,800 km2).
Other non-classic drainage issues that have an effect on hydrologic unit delineation and subdivision include reservoirs, diverted waters ranging from small irrigation ditches to interbasin transfers, islands, and coastal areas with large tidal ranges. 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).
- Watershed Topology - The Pfafstetter System, by Jordan Furnans and Francisco Olivera
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