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Abstract. Groundwater table dynamics extensively modify the volume of the hyporheic zoneand the rate of hyporheic exchange processes. Understanding the effects ofdaily groundwater table fluctuations on the tightly coupled flow and heattransport within hyporheic zones is crucial for water resourcesmanagement. With this aim in mind, a physically based model is used to explorehyporheic responses to varying groundwater table fluctuationscenarios. The effects of different timing and amplitude of groundwater tabledaily drawdowns under gaining and losing conditions are explored in hyporheiczones influenced by natural flood events and diel river temperaturefluctuations. We find that both diel river temperature fluctuations and dailygroundwater table drawdowns play important roles in determining thespatiotemporal variability of hyporheic exchange rates, temperature ofexfiltrating hyporheic fluxes, mean residence times, and hyporheicdenitrification potentials. Groundwater table dynamics present substantiallydistinct impacts on hyporheic exchange under gaining or losing conditions. Thetiming of groundwater table drawdown has a direct influence on hyporheicexchange rates and hyporheic buffering capacity on thermaldisturbances. Consequently, the selection of aquifer pumping regimes hassignificant impacts on the dispersal of pollutants in the aquifer and thermalheterogeneity in the sediment. 

Abstract Hyporheic exchange is a crucial control of the type and rates of streambed biogeochemical processes, including metabolism, respiration, nutrient turnover, and the transformation of pollutants. Previous work has shown that increasing discharge during an individual peak flow event strengthens biogeochemical turnover by enhancing the exchange of water and dissolved solutes. However, due to the nonsteady nature of the exchange process, successive peak flow events do not exhibit proportional variations in residence time and turnover, and in some cases, can reduce the hyporheic zones' biogeochemical potential. Here, we used a process‐based model to explore the role of successive peak flow events on the flow and transport characteristics of bedform‐induced hyporheic exchange. We conducted a systematic analysis of the impacts of the events' magnitude, duration, and time between peaks in the hyporheic zone's fluxes, penetration, and residence times. The relative contribution of each event to the transport of solutes across the sediment‐water interface was inferred from transport simulations of a conservative solute. In addition to temporal variations in the hyporheic flow field, our results demonstrate that the separation between two events determines the temporal evolution of residence time and that event time lags longer than the memory of the system result in successive events that can be treated independently. This study highlights the importance of discharge variability in the dynamics of hyporheic exchange and its potential implications for biogeochemical transformations and fate of contaminants along river corridors. 

Abstract Coupled groundwater flow and heat transport within hyporheic zones extensively affect water, energy, and solute exchange with surrounding sediments. The local and cumulative implications of this tightly coupled process strongly depend on characteristics of drivers (i.e., discharge and temperature of the water column) and modulators (i.e., hydraulic and thermal properties of the sediment). With this in mind, we perform a systematic numerical analysis of hyporheic responses to understand how the temporal variability of river discharge and temperature affect flow and heat transport within hyporheic zones. We identify typical time series of river discharge and temperature from gauging stations along the headwater region of Mississippi River Basin, which are characterized by different degrees of flow alteration, to drive a physics‐based model of the hyporheic exchange process. Our modeling results indicate that coupled groundwater flow and heat transport significantly affects the dynamic response of hyporheic zones, resulting in substantial differences in exchange rates and characteristic time scales of hyporheic exchange processes. We also find that the hyporheic zone dampens river temperature fluctuations increasingly with higher frequency of temperature fluctuations. This dampening effect depends on the system transport time scale and characteristics of river discharge and temperature variability. Furthermore, our results reveal that the flow alteration reduces the potential of hyporheic zones to act as a temperature buffer and hinders denitrification within hyporheic zones. These results have significant implications for understanding the drivers of local variability in hyporheic exchange and the implications for the development of thermal refugia and ecosystem functioning in hyporheic zones.