The land area from which surface runoff drains into a stream channel or system of stream channels, or to a lake, reservoir, or other body of water [6].
In a karst setting, subsurface drainage (internal drainage) may have boundaries defined on the basis of comprehensive ground-water tracing studies.
the total surface land area drained by a stream or river; often used in the sense of the water bodies in the basin. Not a watershed which is strictly an elevated boundary area separating tributaries draining to different river systems