Runoff is a key term in water management, but unfortunately one that is used differently in different contexts. Broadly speaking, runoff refers to the movement of liquid water out of an area, but the specific meaning depends on how that area is defined:
- Some sources use runoff to refer to overland flow: the rapid movement of water across the land surface (generally towards a stream) in response to rain. We will avoid this usage, although we will use the terms urban runoff and stormwater runoff to refer to the rapid flow of water from urbanized areas in response to storm events.
- In gridded hydrologic models, runoff from each grid cell is calculated as the excess of precipitation over evapotranspiration, i.e., the amount of liquid (blue) water that is available to form streamflow and/or recharge groundwater.
- At the watershed scale, runoff is typically considered identical with streamflow, since we assume that streamflow represents all the water leaving the watershed; groundwater recharge is not included since we assume that the water that recharges groundwater ultimately ends up discharging to the stream, and we wouldn’t want to double-count it.
- At the global scale, runoff consists mostly of streamflow from the continents to the oceans (and to terminal lakes), but it also includes the flux of ice from Antarctica and Greenland to the oceans and the discharge of groundwater directly to the oceans. In addition, groundwater that discharges to an oasis or playa could, in theory, be included as an additional component of global runoff, but since that discharge never reaches a permanent water body, it is usually not included.