A catchment is the geographical zone in which water is captured, flows through and eventually discharges at one or more points. The concept includes both surface water catchment and groundwater catchment.
A surface water catchment is defined by the area of land from which all precipitation received flows into and through a sequence of streams and rivers towards a single river mouth and into the sea or a larger river. The catchment of a tributary river is often called a sub-catchment.
A groundwater catchment is defined by a combination of geological structure and groundwater flow paths. Its boundaries often do not match the boundaries of the overlying surface water catchments.
Surface water / Groundwater interaction depending on local conditions, surface and groundwater catchments may be physically separate or interconnected. For example, where surface water overlies a layer of impermeable clay, it is separated from groundwater. However, where surface water overlies permeable rock such as sand and gravel, water can move freely between surface and groundwater in either direction.
The catchment as presented in the Waterplan Risk Framework Platform is a surface water catchment. It may differ from the catchment as defined by local agencies or experts.
Explaining similar terminology
There are a number of similar terms relating to catchment sometimes used interchangeably.
River basin or basin. Is equivalent to surface water catchment. For tributary rivers they may be called a sub-catchment or sub-basin.
Watershed. Is also a common term for surface water catchment. In US and International English, watershed is the land surface of the catchment. In UK English, it is the boundary around the catchment (what in US English is called the divide). Waterplan favours the terms catchment or river basin.
Groundwater basin. Similar to groundwater catchment. But it may group aquifers and catchments together.
Surface water divide. The boundary of the surface water catchment is the line or ridge of highest elevation running along a chain of hills or mountains. The name derives from the fact it ‘divides’ falling rainfall to flow into one catchment or the other, either side of the ridge. It is more difficult to define on flat topography.
Groundwater divide. The imaginary boundary separating two groundwater catchments. Like for a surface water divide, the water flows in different directions either side of it.