Baseflow:
Where Groundwater Meets Surface Water
by Keith E. Schilling
It's late
summer and it hasn’t rained in a month. The lawn has turned brown and farm
fields are dry and cracking. Yet a trip down to the nearby stream channel finds
flowing water. What you are witnessing is the interplay between groundwater and
surface water. Groundwater seepage into a stream channel is called baseflow.
During most of the year, stream flow is composed of both groundwater discharge
and land surface runoff. When groundwater provides the entire flow of a stream, baseflow
conditions are said to exist.
Groundwater
discharges into streams when the water table (top of groundwater saturation)
rises above the streambed. Perennial streams flow because groundwater remains
above the streambed throughout the year. You may notice some streams flow only
part of the year, generally from spring to mid-summer, or only during wet
periods. These intermittent streams occur when the water table rises above or
falls below the base of a stream channel in response to wet or dry weather.
During extended dry periods, the water table falls below the streambed. Only
after rainfall has replenished the groundwater supply does the water table rise
sufficiently to intersect the streambed and resume baseflow discharge.
The amount of baseflow a stream receives is closely linked to the permeability of rock or soil in the watershed. For example, the Floyd River in northwest Iowa flows through a watershed composed of clayey glacial till and silty loess. Based on a 12-year record of stream gaging (1987-1999), each square mile of land in its watershed produced an average of 3.7 inches of baseflow discharge. On the other hand, the Upper Iowa River in northeast Iowa, flowing through a watershed consisting of fractured limestone and dolomite, had more than twice the baseflow discharge (8.0 inches per square mile).
Streams continue to flow during extended dry periods because of contributions from groundwater. Such baseflow conditions affect water temperature, aquatic life, and delivery of pollutants to streams.
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Photo by J.R. Olson.
Baseflow is an
important consideration when evaluating the health of a stream. Arguably, the
most important factor regarding the fate of aquatic organisms in surface water
is the amount of sustainable flow in the channel. Streams with adequate baseflow
can sustain fish and tiny aquatic organisms during prolonged dry periods. Since
groundwater temperatures are nearly uniform year-round, groundwater discharge
also provides a measure of temperature stability in surface water. Streams in
northeast Iowa, home to several indigenous trout species, owe their temperate
conditions to contributions from baseflow and spring discharge. However, in some
cases, discharge of pollutants in baseflow may have detrimental effects on
surface water quality. Nitrate-nitrogen, a common pollutant in Iowa’s streams,
is delivered primarily through groundwater discharge as baseflow or tile
drainage (a type of modified baseflow). Point-source impacts are especially
noticeable when a stream's flow consists nearly entirely of baseflow (see photo
above).
Ultimately it
is the relationship between surface water and groundwater during baseflow
periods that may help solve some of Iowa’s water quality issues. Monitoring
surface water quality when flow consists entirely of baseflow can be used to
identify and locate sources of pollution. In turn, these pollution sources can
be addressed at their origin so that baseflow water quality improves over time.
We can all understand the need for quality baseflow in our streams, especially
during a hot Iowa summer.