HR: 11:05h
AN: H12B-04    [Abstracts]
TI: Simulating Stream-aquifer Interactions
AU: * Mehl, S
EM: swmehl@usgs.gov
AU: Hill, M C
EM: mchill@usgs.gov
AB: Accurate simulation of the interaction between ground-water systems, which are typically fully three dimensional, and streams, which are typically linear features that are very narrow relative to the area of the ground-water system, is a challenge even for robust numerical models. This work examines how stream-aquifer interactions are represented in ground-water flow models using both coarse and highly refined grids and single and multi-layer models. Typically, the aquifer and the stream are assumed to be connected through a low-permeability streambed layer. Often, this is formulated as a streambed conductance term that connects the stream and the aquifer, where it is assumed that the dominant vertical head loss occurs in the streambed. This conceptualization is questionable in field situations where the fluxes between the two systems are large; however, it is these situations that are of most interest. For such situations, the assumption that most of the vertical head loss occurs through the streambed is not valid and we show that it leads to streambed conductances that have the unfortunate characteristic of depending on grid size. Alternative conceptualizations need to be considered that account for vertical resistance to flow in the streambed and the aquifer. Here they are explored and compared using numerical simulations. Grid size recommendations based on Dupuit assumptions for representing streams in groundwater models are evaluated. As the grids are refined both horizontally and vertically, the magnitude of stream-aquifer interactions changes. Vertical refinement changes the representation of vertical gradients, and the flux between the stream and the aquifer changes. In this work, simulations using single and multi-layer models produced differences in stream-aquifer flux of 80 percent. This suggests that simulated stream-aquifer fluxes have a significant grid-size dependency. This is problematic for using regional groundwater models, with typically coarse horizontal and vertical grid sizes, to model local-scale stream-aquifer fluxes. Here we use highly refined grids to investigate if there is a grid resolution that, when achieved, the grid-size dependency of stream-aquifer fluxes is eliminated. Local grid refinement is evaluated as a tool for achieving highly refined grids near streams while maintaining coarser discretization elsewhere.
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting