Unsaturated hyporheic zone flow in stream/aquifer conjunctive systems

Saturated flow is typically assumed for seepage from a stream underlain by an alluvial aquifer. However, if the water table falls a sufficient distance below a semipervious streambed, the head losses in this less conductive layer will cause the region beneath the stream, or hyporheic zone, to become unsaturated. Hyporheic zone flow is defined loosely in this research as the flow that occurs underneath the streambed. Unsaturated flow transforms streams from constant head boundaries to constant flux boundaries, impacting the biogeochemistry in the hyporheic zone. The objective of this paper is to discuss the development and implications of unsaturated flow beneath the streambed. Conditions under which saturated or unsaturated flow occurs and the characteristics of each flow regime are discussed. Next, the effect of unsaturated flow is illustrated for the case of stream leakage induced by a well pumping from an aquifer that is hydraulically interacting with a partially penetrating stream. Prior analytical solutions for alluvial well depletions fail to model unsaturated flow between the streambed and water table. An approximating solution is proposed to estimate aquifer drawdown and stream depletion under saturated/unsaturated hyporheic zone flow conditions.