drainage basin

Area of land drained by a river system (a river and its tributaries). It includes the surface run-off in the water cycle, as well as the water table. Drainage basins are separated by watersheds. A drainage basin is an example of an open system because it is open to inputs from outside, such as precipitation, and is responsible for outputs out of the system, such as output of water into the sea and evaporation of water into the atmosphere.