The EU Water Framework Directive (2000/60/EC) (WFD) establishes a framework for the protection, improvement and management of surface waters and groundwaters. The Subcatchment dataset is built on clusters of river water body polygons and are entirely contained within the Catchment polygons dataset. All Subcatchments are represented as polygons.

The Subcatchment dataset forms part of a three tier hierarchy. The base unit and tier 1 are the WFD River Waterbody (RWB) basins. The middle tier 2 will consists of the WFD Subcatchments. And final tier 3 layer will be the WFD Catchments. The Subcatchments dataset is build using clusters of these RWB basins. No RWB polygons, or Subcatchment polygons, will cross the boundaries of each Catchment feature. All RWBs and Subcatchments will be completely nested within a single Catchment polygon. In some instances, across main river channels, a RWB basin will exist in two Subcatchment polygons