MODELING AND MEASUREMENT OF DYNAMIC HYPORHEIC EXCHANGE AT INTERMEDIATE TIMESCALES

Traditional hyporheic exchange models characterize short-term exchange (minutes to hours) during periods of steady baseflow. However, unsteady processes ranging from evapotranspiration (ET) to seasonal flood pulses have the potential to drive substantial hyporheic mixing. Unsteady forcing is a particular challenge at intermediate timescales, where temporal averaging or assuming constant flow conditions is not appropriate. Here we use a new simple modeling scheme, based on a piston model and measurement of vertical head gradients, to evaluate the effects of flood pulses and ET on hyporheic residence times and fluxes. Analyses were applied to data collected over a decade in the Everglades floodplain and compared to residence times and fluxes estimated through inverse advection-dispersion modeling of chloride concentration profiles.

Residence times and hyporheic fluxes estimated from the simple piston model were similar to those estimated from the modeled chloride concentration profiles. The finer temporal resolution of the piston model provided more detailed insight into dynamic hyporheic exchange processes. The model revealed that flood pulses drive relatively deep (~1 m) flow paths that produce nonstationarity in hyporheic exchange statistics over the multi-year time frame and residence time distributions that do not conform to standard types. ET increased monthly-timescale effective diffusion by orders of magnitude and decreased multi-year mean residence times by 15% but had negligible effect on multi-year hyporheic fluxes. Overall, hyporheic mixing induced by ET on monthly timescales and by flood pulses on annual to multi-year timescales was of similar magnitude to that induced by shear or pumping-driven flow in streams (effective diffusion ≈10-100x molecular diffusion). Results suggested that time-varying exchange rates need to be accounted for in models to characterize the dynamic flow paths that arise from variable forcing at intermediate timescales.