Modeling the effects of unsaturated, stratified sediments on groundwater recharge from intermittent streams

Abstract Unsaturated, stratified sediments beneath intermittent stream channels affect groundwater recharge from these streams. Using four different cases of sediment stratification, we simulate transient, variably saturated flow in a two-dimensional (2-D) vertical cross-section between the stream and the underlying water table. These cases include: homogeneous sediments; low permeability streambed sediments; narrow, low permeability lenses; and extensive, low permeability layers. The permeability of the sediments in these cases greatly affects the timing and rate of channel loss and groundwater recharge. Flow patterns and the style of stream/water table connection are controlled by the location and geometry of low permeability sediments. In cases with homogeneous sediments and narrow, low permeability lenses, stream/water table connection occurs by a saturated column advancing from above. In cases with low permeability streambed sediments and extensive, low permeability layers, connection occurs by a water table mound building from below. The style of stream/water table connection suggests simplified physically based interaction models that may be appropriate for these settings. We compared channel loss and groundwater recharge computed using two simplified models, a Darcian seepage equation and the Green-Ampt infiltration equation, with the results from our 2-D simulations. Simplified models using parameters from the 2-D simulations appear to perform well in cases with homogeneous and low permeability streambed sediments. In cases with low permeability lenses or layers, the simplified models require calibrated parameters to perform well.