Modeling hyporheic exchange with unsteady stream discharge and bedform dynamics

Water exchange between streams and hyporheic zones is highly dynamic, and its temporal variation is related to the hydrologic fluctuations of stream discharge and groundwater levels. Unfortunately, predictions of temporal patterns of exchange are difficult due to the many involved hydrodynamic and morphodynamic processes and to their complex nonlinear interactions. These processes include the evolution of streambed morphology in response to changing streamflow as well as feedbacks on surface flow induced by drag resistance due to evolving bedforms. We have employed a stochastic method to analyze the temporal dynamics of bedform-driven hyporheic exchange in streams characterized by subcritical flow and daily discharge variations. The method is an extension of previous studies that includes current-induced alterations of bedform size and celerity and their effect on water exchange. The modeling results show that during high flows stream water penetrates deeper and for longer times in the sediments. The predicted water exchange flux per unit streambed area decreases because the streambed area occupied by each bedform increases faster than the volumetric rate of stream water exchange induced by the same bedform. However, the increase in wetted area with discharge may compensate this reduction by providing additional area for water exchange. One the main finding of the study is that the time-averaged values of exchange flux and depths are quite similar to those modeled for a steady mean discharge, while residence times are somewhat lower. Predicted temporal variations of exchange depths and times around their time-averaged values are moderate compared to steady state values.