Effects Of Time-Varying Streambed Hydraulic Properties On Stream Depletion
Neupauer, Roseanna M 1 ; Lackey, Greg 2 ; Pitlick, John 3
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Stream depletion is the reduction in flow rate in a stream due to pumping in an aquifer that is hydraulically connected to the stream. The amount of stream depletion is related to the pumping rate, the distance between the well and the stream, and the hydraulic properties of the aquifer and streambed. The properties of the streambed that affect stream depletion include the hydraulic conductivity of the stream bed sediment, the thickness of the sediment, and the channel width. These properties are often lumped into a single property called streambed conductance.
If streambed conductance is high, water can flow easily between the aquifer and the stream. Thus, during pumping in an aquifer, a substantial amount of water in the stream will be drawn into the aquifer, leading to high stream depletion. On the other hand, if streambed conductance is low, water cannot be exchanged easily, so stream depletion is lower. Natural streambeds are heterogeneous, with stretches of higher conductance alternating with stretches of lower conductance. Pumping near a high-conductance stretch produces more stream depletion than pumping near a low-conductance stretch. In natural channels, the patterns of streambed heterogeneity can vary temporally due to scour and deposition during high flow and low flow periods.
Through numerical simulations, we demonstrate that these temporal variations in heterogeneity patterns lead to exaggerated amounts of stream depletion. Specifically, if a well is pumping near a stream stretch that alternates between periods of high conductance and periods of low conductance, the amount stream depletion during the high conductance periods may be higher than the amount of stream depletion that would be expected if the streambed sediment permanently had high conductance. Similarly, the amount of stream depletion during the low conductance periods may be lower than the amount of stream depletion that would be expected if the stream bed sediment permanently had low conductance. These results show that numerical simulations of stream depletion may underestimate stream depletion if the time-varying behavior of streambed conductance is not modeled.