When the local water table elevation mirrors the stream stage, groundwater flows parallel to the channel and zero-exchange conditions occur (Figures 21a). As no gradient is present between the stream and the groundwater system, water table contours cross the stream at right angles and flow is parallel to the channel (Figure 21b). In this setting, the stream stage represents the local water table elevation. This setting occurs when portions of the stream transition between effluent and influent conditions. Its extent and duration are not commonly discussed in the literature.
Figure 21 – Cross section and map views of a zero-exchange (parallel flow) conditions. The black triangle is the location of the stream stage and the black number is the stage measurement. Equipotential lines and relative head values are shown as blue water table contours. Groundwater flow is in the direction indicated by blue arrows. Aquifer conditions are assumed to be isotropic and homogeneous. Monitoring wells are open only at the bottom. a) Cross section showing zero-exchange. The stream stage and groundwater head are equal at this location and groundwater flow is parallel to the stream. b) Map view showing equipotential lines cross the channel at right angles. Groundwater flow lines are parallel to the stream channel. In this example, the stream stage reflects the local water table. The location of the cross section (C-D) is approximated by the position of the dashed black line (Woessner, 2020).
