CONCEPTUALIZATION OF THE STREAMBED IN MATHEMATICAL MODELS OF GROUNDWATER-SURFACE WATER INTERACTION: NEW INSIGHTS INTO AN OLD APPROACH

Models of groundwater-surface water interactions commonly incorporate the streambed conductance concept. For example, this concept is essential for modeling the drawdown and stream depletion caused by pumping near partially penetrating streams. Available solutions utilize the Theis solution and an additional streambed leakage term. Using a unique data set from a series of pumping tests performed under various hydrologic conditions at the Prairie Creek site in Nebraska, USA, we tested the following hypotheses: (1) in the analytical solutions, the streambed leakage term accounts for a partially penetrating stream at a certain distance from the pumping well; (2) the streambed conductance coefficient l=(W K’)/m’ contained in mathematical models represents the average stream width W, a streambed of finite thickness m’ and hydraulic conductivity K’. Hypothesis (1) was tested by comparing the results from tests under flow and no-flow conditions of the stream. Hypothesis (2) was tested by comparing the results from two tests under low- and high-flow conditions and thus varying stream widths. The results suggest that the theoretical approximation of a natural stream in the 2D analytical solution has some physical merit. In the studied conditions, however, the streambed conductance coefficient does not accurately account for the average stream width and the streambed. This parameter is better understood as a lumped fitting parameter, which takes into account various factors including the stream’s sinuosity, vertical flow, and aquifer anisotropy.