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. Full details about this dataset can be found at https://doi.org/10.5285/1957166d-7523-44f4-b279-aa5314163237