Integrated Hydrological Units of the United Kingdom: Groups
(IHU Groups)

The main inputs for development of Integrated Hydrological Units (IHU) of the UK were data held by the Centre for Ecology & 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 (openstreetmap.org) and WikiPedia (wikipedia.org). 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.