MODELING AND MEASUREMENT OF DYNAMIC HYPORHEIC EXCHANGE AT INTERMEDIATE TIMESCALES
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.