STREAM-AQUIFER INTERACTION-MODEL WITH DIFFUSIVE WAVE ROUTING

A practical approach to modeling the hydraulic interaction of a stream and aquifer via streambed leakage is based on the established U.S. Ge ological Survey (USGS) model, MODFLOW. To represent flood-wave propaga tion and the associated bank storage, MODFLOW's STREAM module is repla ced by the Muskingum-Cunge diffusive-wave-routing scheme. The diffusiv e wave model closely approximates a dynamic model of a hood wave's spe ed, shape, and streambed leakage. Because the stream responds more rap idly to disturbances than the aquifer, streambed leakage is calculated at the flood routing time scale in order to properly represent the st ream-aquifer coupling. However, both the relative magnitude and timing of aquifer response to a flood wave depend on the strength of this co upling. We find discrepancies in both the flood wave and the streambed leakage when the wave and ground-water motions are evaluated at diffe rent time scales. These discrepancies are significant in the case of a strong stream-aquifer coupling, for which equal aquifer and flood-rou ting time steps may be required. Wave diffusion and bank storage are s hown to be comparable in magnitude and should, therefore, be included in stream-aquifer interaction models. Diffusive wave routing more accu rately represents wave propagation, bed leakage, and aquifer response if short aquifer time steps are taken, and is preferable to the STREAM module for simulating short time transients. However, the STREAM modu le is useful for simulating large time frames if accurate modeling of the flood-wave propagation is not required.