Contributing Area

Conceptual graphic showing larger and smaller watersheds, and relative runoff amount from each

The size of the contributing area of the rainfall in a basin has a direct influence on the total volume of runoff that drains from that basin.

It probably comes as no surprise that when rain falls in a uniform manner over a larger basin and a smaller basin, the larger basin produces more runoff volume. All things being equal, a drainage area that is twice as large can generate twice as much runoff volume as its smaller counterpart.

conceptual graphic showing smaller storm footprint than size of  watersheds

Of course many storms will cover only part of a basin. So for most situations, the runoff volume will be determined by the contributing area – that part of the basin covered by a storm – not the total size of a basin.