Long description for the Hierarchy of drainage areas

The SDAC provides unique numeric codes for three types of drainage areas: major drainage areas, sub-drainage areas and sub-sub-drainage areas. The three geographic areas are hierarchically related; a 4 character code is used to show this relationship. In addition to the SDAC classes, an aggregation variant classification of the sub-sub-drainage areas by drainage regions and ocean drainage areas is included. The relationship between these geographic areas is presented in the diagram showing the Hierarchy of drainage areas.