National River Flow Archive

Integrated Hydrological Units of the United Kingdom: Hydrometric Areas with Coastline

This dataset is part of Integrated Hydrometric Units (IHU) of the UK. Hydrometric Areas are used to organise river flow measurement and hydrometric data collection in the UK. Hydrometric Areas are either integral river catchments having one or more outlets to the sea or tidal estuary, or they may include several contiguous river catchments having topographical similarity but separate tidal outlets. In mainland Britain they are numbered from 1 to 97 in clockwise order around the coast commencing in north east Scotland. The larger islands and groups of islands are numbered from 100-108. Ireland has a unified numbering system from 1 to 40 commencing with the River Foyle catchment and circulating clockwise; not all Irish Hydrometric Areas, however, have an outlet to the coast. Only those Hydrometric Areas covering Great Britain and Northern Ireland are included in this dataset. The boundaries between hydrometric areas correspond to catchment boundaries as digitally-derived from CEH Integrated Hydrological Digital Terrain Model (IHDTM) using a catchment definition program. It should be noticed that the Northern Ireland data are clipped to its political boundary so not every Hydrometric Area in this region is completely represented. The naming and numbering convention for the hydrometric areas in Great Britain was originally defined by the Inland Water Survey Committee (and first published in the Surface Water Year-Book of Great Britain 1936-37). For Northern Ireland the system was developed by a multi-agency working group in the 1970s (and first published in Surface Water: United Kingdom 1971-73. Note that full citations of those two publications are provided as additional information source. This dataset represent the same entities as the IHU Hydrometric Areas of the UK without Coastline, however, the outer boundaries of the units follow coastline published by the Ordnance Survey (Meridian 2), rather than the boundaries of the CEH Integrated Hydrological Digital Terrain Model.

Publication date: 2014-06-23

Get the data

Licence, copyright and IPR apply

Format of the dataset : Shapefile

You must cite: National River Flow Archive (2014). Integrated Hydrological Units of the United Kingdom: Hydrometric Areas with Coastline. NERC Environmental Information Data Centre.


This dataset is part of the following


Study area
Temporal extent
2014-06-23    to    …

Supplemental information

This dataset is referenced in:

Harrigan, S., Prudhomme, C., Parry, S., Smith, K., & Tanguy, M. (2018). Benchmarking ensemble streamflow prediction skill in the UK. Hydrology and Earth System Sciences, 22(3), 2023–2039.

Other useful information regarding this dataset:

Ministry of Health and Scottish Office (1939) The Surface Water Year-Book of Great Britain 1936-37. His Majesty Stationary Office. London.
Water Data Unit (1978) Surface Water: United Kingdom 1971-1973. Department of the Environment / Scottish Development Department / Department of the Environment for Northern Ireland. Her Majesty's Stationery Office. London. 193pp.

Provenance & quality

The hydrometric areas are based on watershed derived from the Integrated Hydrological Digital Terrain Model (IHDTM) elevation grid using an in-house catchment definition program. In Great Britain the coastline corresponds to the OS Meridian2 coastline. In Northern Ireland the boundary and coastline was derived from OS Panorama data in times when Ordnance Survey covered Northern Ireland. All boundaries have been processed to ensure they are topologically correct; there are no gaps or overlaps so every point in the country is allocated to one and only one Hydrometric Area

Correspondence/contact details

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


National River Flow Archive
Centre for Ecology & Hydrology

Other contacts

Environmental Information Data Centre
NERC Environmental Information Data Centre
Centre for Ecology & Hydrology


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


Topic categories
Inland Waters
Catchments and Topography England Geospatial / Map Great Britain,  Hydrology hydrometric areas,  hydrometry,  National Northern Ireland Rees Section Scotland United Kingdom Wales Water Resources