Resolving the Stream Depletion Model Paradox: Theory of Depletion with
Stream Drawdown near a Pumping Well
Abstract
Mathematical models for stream depletion typically use the constant-head
Dirichlet boundary condition or the general Robin boundary condition at
the stream. Both approaches fix stream stage as constant during pumping.
Fixed the stream stage implies the stream acts as an infinite water
source with depletion affecting stream discharge but having no impact on
stream stage. We refer to this depletion without drawdown as the
“depletion paradox.” It is a glaring model limitation, ignoring the
most observable adverse effect of long-term groundwater abstraction near
a stream – dry streambeds. Our field data demonstrate that stream stage
responds to pumping near the stream. This motivates the development of a
model considering transient stream drawdown using the concepts of finite
stream storage and mass continuity at the stream-aquifer interface. The
models include the cases for fully- and non-penetrating the stream.
First-order mass transfer is also assumed across the streambed. The
proposed model reduces to the fixed-stage model as stream storage
becomes infinitely large and the confined flow case with a no-flow
boundary at the streambed when stream storage vanishes. Sensitivity
analysis for hydraulic properties of the stream-aquifer system is also
included. Our results suggest that fixed-stage models (a) underestimate
late-time aquifer drawdown to pumping adjacent to a stream and (b)
overestimate the available groundwater supply from streams to pumping
wells because of the infinite stream storage assumption. This can have
significant implications for the sustainable management of water
resources in interacting stream-aquifer systems with heavy groundwater
abstraction.