Modeling the Impact of Stream Discharge Events on Riparian Solute Dynamics

Abstract

The biogeochemical composition of stream water and the surrounding riparian water is mainly defined by the exchange of water and solutes between the stream and the riparian zone. Short-term fluctuations in near stream hydraulic head gradients (e.g., during stream flow events) can significantly influence the extent and rate of exchange processes. In this study, we simulate exchanges between streams and their riparian zone driven by stream stage fluctuations during single stream discharge events of varying peak height and duration. Simulated results show that strong stream flow events can trigger solute mobilization in riparian soils and subsequent export to the stream. The timing and amount of solute export is linked to the shape of the discharge event. Higher peaks and increased durations significantly enhance solute export, however, peak height is found to be the dominant control for overall mass export. Mobilized solutes are transported to the stream in two stages (1) by return flow of stream water that was stored in the riparian zone during the event and (2) by vertical movement to the groundwater under gravity drainage from the unsaturated parts of the riparian zone, which lasts for significantly longer time (> 400 days) resulting in long tailing of bank outflows and solute mass outfluxes. We conclude that strong stream discharge events can mobilize and transport solutes from near stream riparian soils into the stream. The impact of short-term stream discharge variations on solute exchange may last for long times after the flow event.

Solute dynamics across the stream‐to‐riparian continuum under different flood waves

“…The sensitivities of the four parameters to C strˍmax were compared through 20 calculations for each parameter. As shown in Figure , C strˍmax increased linearly with A and t p , whereas it decreased logarithmically with T and r. The sensitivity of C strˍmax to A was approximately two times that of t p , whereas the sensitivity of T was slightly smaller than that of r. The dominant control of A on the solute efflux was in line with the result from Mahmood et al ().…”

“…Afterwards, it required a long time to recover and approach the initial C str , which is mainly in line with the stream concentration–discharge relations found in field studies (e.g., Mei et al, ; Pellerin et al, ; Welch et al, ; Xie et al, ). Similar behaviour of flow and solute movement was modelled by Mahmood et al (), which focused on the effect of fluctuating streams on the SDs residing in the riparian unsaturated zone.…”

“…This fluctuation enhances the hyporheic exchange by altering the direction and magnitude of the surface–subsurface gradient (Arntzen et al, ; Liu et al, ; Storey et al, ; Taniguchi et al, ). Consequently, the changing hydrological regime considerably affects solute mobilization (e.g., Mahmood et al, ; Sawyer et al, ) and hydrochemical evolution (e.g., Gu et al, ; Harvey et al, ) within the riparian zone.…”

“…Fritz and Arntzen () discovered that uranium flux during the process of cyclical changes in water level is in a counter‐clockwise loop curve with water flux. Recently, the impact of stream discharge events of varying peak heights and durations on riparian solute export to the stream was simulated by Mahmood et al (). They indicated that the timing and amount of solute efflux are linked to the shape of the discharge event.…”

“…The preceding studies focused on hyporheic exchange characteristics and conservative/reactive SDs induced by river water fluctuations. However, only a few studies systematically investigated the effects of different flood waves on SDs in riparian zones (Mahmood et al, ; McCallum et al, ; Trauth & Fleckenstein, ). Moreover, beyond the typical parameters of wave amplitude ( A ) and duration ( T ), the effects of two other parameters that characterize the roundness ( r ) and skewness ( t p ) of the wave on SDs have been overlooked.…”