Stream Flow Discharge

The amount of water flowing in a stream past a particular point for a period of time is termed stream flow discharge and can be shown in a hydrograph. As we saw previously, stream flow is a function of precipitation. The influence of precipitation on stream flow can be explored via storm hydrographs, on which both stream flow and precipitation are plotted.

The baseflow of a stream is the sustained amount of flow in a stream when no precipitation event has occurred. As you can probably guess, ephemeral streams have no baseflow; only perennial streams have a measurable baseflow. Following a precipitation event, shown on this graphic as blue bars, the stream flow increases dramatically. The stream flow attributed to the precipitation event is termed stormflow. Depending on the size and intensity of the storm event, the stream can maintain stormflow, or flow above baseflow conditions for days, weeks, or even months.

In a highly urbanized watershed, evapotranspiration (ET) and storage (St) rates are low due to low amounts of vegetation cover and exposed soils. As a result, the amount of discharge is greater than in a watershed with vegetation cover intact. In addition, the peak in-stream flow occurs more rapidly following the precipitation event and is greater in magnitude. Urbanized streams are sometimes called “flashy” due to their rapid response following rainfall.