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
A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...
, river
reach,
lake
A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...
, or area like a
drainage basin
A drainage basin is an area of land in which all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...
(also called watershed in North America) or catchment.
One system, developed by
Arthur Newell 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 , 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.
Watershed Topology - The Pfafstetter System
, by Jordan Furnans and Francisco Olivera
Europe
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
An endorheic basin ( ; also endoreic basin and endorreic basin) is a drainage basin that normally retains water and allows no outflow to other external bodies of water (e.g. rivers and oceans); instead, the water drainage flows into permanent ...
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
The Black Sea is a marginal sea, marginal Mediterranean sea (oceanography), mediterranean sea lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bound ...
receive a higher number. The seas are delimited using the so-called definitions made by the International Hydrographic Organization
The International Hydrographic Organization (IHO) (French: ''Organisation Hydrographique Internationale'') is an intergovernmental organization representing hydrography. the IHO comprised 102 member states.
A principal aim of the IHO is to ...
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.
Subsequently every watershed along this coast is assigned a number using the Pfafstetter Coding 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
See also
* Hydrologic Unit Modeling for the United States
* Water Resource Region
References
External links
*
*
* {{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 , access-date=29 September 2014 , url-status=dead, archive-url=https://web.archive.org/web/20070121122717/http://193.178.1.168/River_Coding_Review.pdf , archive-date=January 21, 2007
Hydrology
Limnology
Water and the environment
Geocodes