Baseflow component of stream flow
Baseflow is the longer-term discharge into a stream from natural storages, notably sustaining flow between rainfall events. Recognising that there can be multiple natural storages in a catchment, the discharge of groundwater to the stream is termed the groundwater component of baseflow.
Of particular interest in water management is groundwater seepage to streams because such discharge can sustain stream flows over extended periods between rainfall. This seepage is commonly referred to as baseflow, differentiated from quickflow which is the direct and short-term response to rainfall that includes flow over the land surface (overland flow or runoff), rapid lateral movement in the soil profile (interflow) and direct precipitation onto the stream surface. Perennial streams that flow continuously throughout the year have a high baseflow component.
The relative contribution of quickflow and baseflow components changes through the stream hydrographic record. The flood or storm hydrograph is the classic response to a rainfall event and consists of three main stages (Figure 1):
- prior low-flow conditions in the stream consisting entirely of baseflow at the end of a dry period;
- with rainfall, an increase in streamflow with input of quickflow dominated by runoff and interflow. This initiates the rising limb towards the crest of the flood hydrograph. The rapid rise of the stream level relative to surrounding groundwater levels reduces or can even reverse the hydraulic gradient towards the stream. This is expressed as a reduction in the baseflow component at this stage; and
- the quickflow component passes, expressed by the falling limb of the flood hydrograph. With declining stream levels timed with the delayed response of a rising watertable from infiltrating rainfall, the hydraulic gradient towards the stream increases. At this time, the baseflow component starts to increase. At some point along the falling limb, quickflow ceases and streamflow is again entirely baseflow. Over time, baseflow declines as natural storages are gradually drained during the dry period up until the next significant rainfall event.
Figure 1: Components of a typical flood hydrograph
It is important to remember that the assumption that stream baseflow solely equates to groundwater discharge is not always valid. Water can be released into streams over different timeframes from different storages such as connected lakes, wetlands, or from snow. Also, temporary storage within the river bank following the passage of high-flow events (bank storage) can also contribute to the baseflow regime, and activities such as stream regulation, industrial discharge or irrigation returns can modify the baseflow signature. Unfortunately, many current definitions assume that baseflow is only derived from groundwater storage.