A catchment is an area of land or other type of surface draining to a point in a drainage system, defined by its topography and the catchment boundaries or divides. For most hydrological applications, catchments are horizontal projections of the actual surfaces (as they appear on a map). However, for some small catchments such as roofs, the actual sloping surface may be used.

A large natural catchment may also be called a watershed or a river basin.

Many hydrological models divide catchment areas into sub-catchments or sub-areas, which can take various forms due to hydrological models being formulated differently.  Catchments are complex and their geometry cannot be taken for granted. Boundaries can change depending on the severity of a storm, as overflows may occur between sub-areas when flowrates exceed the capacities of drainage components.  

In DRAINS urban sub-catchments, boundaries are usually defined by road centrelines and property boundaries (which are assumed to have solid fencing or walls that will direct water along them). Through its use of storages and overflow routes, DRAINS can model boundaries that alter with the size of floods  For example, overflow routes out of sag pits operate when the storages associated with such pits become full.  In smaller storms there may be no overflows, while in larger ones flows can occur along defined overflow routes.