Magnitudes and time-scales of hyporheic exchange in streams across a range of morphological environments

The influence of hyporheic exchange on biogeochemical processing of solutes transported through stream networks depends on the time scale of the biogeochemical process, the residence-time distribution of hyporheic exchange flows, and the mass flux of water through the hyporheic zone. To substantially influence the stream's solute load, a large proportion of the total stream discharge must cycle through the hyporheic zone with a residence time distribution substantially longer than the time-scale of the relevant biogeochemical process. Here, I draw examples from the literature to characterize the time-scales and magnitudes of hyporheic exchange driven by a variety of factors, including pumping exchange through stream bedforms, and exchange through larger geomorphic features such as pool-step and pool-riffle sequences and meander bends. While residence time distributions of hyporheic exchange can range from seconds to 10s of days, median residence times tend to be long, so that there is sufficient time for biogeochemical transformations to occur. However, the mass flux of water through the hyporheic zone tends to be small, relative to total discharge, except in very small streams during periods of low flow. Consequently, the effect of biogeochemical transformations are limited by the amount of hyporheic exchange in most streams, even if integrated over many kilometers of channel length. These trends suggest we should re-evaluate how we portray the hyporheic zone. While the hyporheic zone is spatially extensive, it is unlikely to have a major effect on bulk stream water properties such as temperature, nutrient concentrations, or stream solute loads. Rather, hyporheic exchange should create unique environmental patches on the streambed or unique subsurface environments. There is much room for future research to investigate the importance of unique hyporheic environments on stream ecosystems.