Kral, F.; Fry, M.; Dixon, H.

Integrated Hydrological Units of the United Kingdom: Sections

This dataset is part of Integrated Hydrological Units (IHU) of the UK, a set of geographical reference units for hydrological purposes including river flow measurement and hydrometric data collection. A Section is the drainage area of a watercourse between two confluences. Only confluences of named watercourses were considered. Each Section carries a name constructed from names of the major river flowing through the Section, the major river flowing into the Section, and the major river into which the Section flows. Sections are spatially consistent with Groups: each Group is made up of one or more Section. Each Section is associated with one Catchment representing the full area upstream from the Section outlet. Identifiers and attributes have been calculated so that direct upstream and direct downstream IHU units can be selected. This layer currently covers Great Britain only as no dataset with river geometries and names with suitable detail is available for Northern Ireland.

Publication date: 2015-02-05

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You must cite: Kral, F.; Fry, M.; Dixon, H. (2015). Integrated Hydrological Units of the United Kingdom: Sections. NERC Environmental Information Data Centre.


This dataset is part of the following


Study area
Temporal extent
2014-08-20    to    …

Supplemental information

Other useful information regarding this dataset:

The National River Flow Archive (NRFA) holds a wide range of hydrological information to assist in the understanding and interpretation of river flows. In addition to time series of gauged river flow, NRFA maintains hydrometric information relating to the gauging stations and the catchments they command and data quantifying other parts of the hydrological cycle.

Provenance & quality

The main inputs for development of Integrated Hydrological Units (IHU) of the UK were data held by the Centre for Ecology and Hydrology. Specifically, the cumulative catchment area raster and outflow raster from the Integrated Hydrological Digital Terrain Model (IHDTM) [1]. The vast majority of the watercourse names come from the subset of CEH UK river centreline network enriched by names from Ordnance Survey 1:50K Gazetteer [2]. Some names were obtained from or verified using other readily available sources like OpenStreetMap ( and WikiPedia ( Bespoke FORTRAN functions, part of the CEH Flood Estimation Handbook code set, were used for catchment delineation. These functions enable sensible interlocking catchments boundaries to be created for the whole IHDTM surface. The option that does not leave gaps between catchments was specified [1]. Except for the catchment delineation, ArcGIS was used as the main GIS platform. Custom Python code was developed to drive the whole process and to perform hierarchical and network tasks. The full name of each IHU Section was derived from three rivers defining the section: the largest (in terms of cumulative catchment area) inflowing river other than the main river in the Section, the main river in the Section, and the largest river flowing into the downstream Section. If no such inflows exist, words 'Source', 'Sea', or the name of the main river in the downstream Section are used as appropriate. A similar approach was adopted for IHU Group names, although group names were edited manually to a great extent to ensure they are simple and readily understood.

[1] Morris, D.G. and Flavin, R.W. 1990. A Digital Terrain Model for Hydrology. Proc 4th Int. Symposium on Spatial Data Handling. Zurich, 1, 250-262.
[2] Moore R V, Morris D G and Flavin R W, 1994. Sub-set of UK digital 1:50,000 scale river centreline network. NERC, Institute of Hydrology, Wallingford.

Correspondence/contact details

Centre for Ecology & Hydrology
Maclean Building, Benson Lane, Crowmarsh Gifford
OX10 8BB
United Kingdom


Kral, F.
Centre for Ecology & Hydrology
Fry, M.
Centre for Ecology & Hydrology
Dixon, H.
Centre for Ecology & Hydrology

Other contacts

Environmental Information Data Centre
NERC Environmental Information Data Centre
National River Flow Archive


Spatial representation type
Spatial reference system
OSGB 1936 / British National Grid


Topic categories
Inland Waters
catchment,  England Great Britain,  hydrology,  Hydrology Rees Section Scotland United Kingdom Wales watershed