HYPORHEIC EXCHANGE FLOWS - CHALLENGES FOR FURTHER UNDERSTANDING

In the stream's hyporheic zone, water entering a stream channel from the surrounding catchment continues to have connections with the catchment. It is well-documented that hyporheic exchange flows significantly influence nutrient dynamics. Additionally there is evidence of hyporheic exchange flows influencing the processes establishing (1) the concentrations of major-ions and metals in stream-catchment systems, (2) spatial variation in stream water temperature and (3) a variety of aquatic habitats. The Transient Storage Model (TSM), a pseudo-two-dimensional representation of stream and hyporheic zone solute transport implemented in the USGS ‘OTIS' code, is often used to identify characteristics of solute residence times and hyporheic zone physical size. Continuing advances in knowledge of the hydrological processes of hyporheic zones beyond the analysis and interpretation of the TSM are critical to quantitative analysis of stream ecosystems. Current research directions, set by numerous investigators, suggest that much work remains in interpreting the basic hydrologic mechanisms by which streams and their catchments are connected. Example challenges emblematic of the widely ranging scales include: (1) Can we determine the time scales of the physical mixing and transport along a hyporheic flowpath?, (2) What physical and hydrometric properties of a stream-catchment system determine the characteristics of transport within a hyporheic zone, (2') Can these properties be routinely measured along a stream at scales of 10s of meters, 100s of meters, or a kilometer?, and (3) How does transport through the hyporheic zone influence the timing and reaction of solutes moving from the catchment to a stream?. Hyporheic zones form connections of the stream to its catchment in manners distinct from those of a stream cutting through an aquifer from which ground water discharges into the stream much as water from an array of pipes. Interpretations of hyporheic exchange flows are needed at a variety of scales to in-turn fully interpret stream-catchment biogeochemistry.