Scaling of Hyporheic Exchange Processes and Finding the Intermediate Sweet Spot

Hyporheic zones are important regions that reside below and along the sides of streams. Within this region, several ecosystem services are provided including stream temperature regulation, habitats for a large variety of species, pollutant removal, and nutrient cycling. Exchange between the hyporheic zone and stream occurs across multiple scales, but historically studies of hyporheic exchange have been focused on the reach scale, which is typically tens to hundreds of meters in length, and features multiple individual geomorphic features such as steps, pools, or riffles. At the reach scale, the shortest flow paths dominate, being driven by individual geomorphic features. Reach-scale studies also provide the basis for scaling up hyporheic exchange processes in models, combining reaches in series. However, this results in the exclusion of the segment scale. The segment scale is at the length of multiple reaches, having similar morphologies and often defined by major changes in geomorphology such as outcropping bedrock, waterfalls, or stream confluences. At the segment scale, longer flow paths emerge that span multiple reaches. These longer flow paths (coined intermediate-length flow paths for this body of work) are thought to be important for ecological purposes, being an optimal length for hyporheic turnover and increased residence time in the hyporheic zone. Despite their importance, these longer flow paths are often ignored in hyporheic studies.

This dissertation explores the importance of the segment scale and inclusion of intermediate-length flow paths in hyporheic exchange and how incorporating the segment scale and intermediate-length flow paths in studies changes our understanding of hyporheic processes. The first study (Chapter 2) assesses how well reach-scale studies represent hyporheic exchange at larger scales. This study quantitatively assesses how assumptions about representativeness hold up when comparing the reach scale to the segment scale. Results demonstrate that selection strategy and location of a study reach both determine if a study is representative, and that the assumption of representativeness gets propagated into conceptual models and future studies resulting in an understanding of hyporheic exchange being based on reach-scale interactions. The second study (Chapter 3) analyzes the model performance of incorporating the segment scale and intermediate-length flow paths to predict hyporheic exchange. The study compares the truncation of transit time distributions as well as spatial discretization to a model that includes intermediate-length flow paths. The results show that using the segment scale and including intermediate-length flow paths improves prediction for the fate and transport of solutes in the hyporheic zone. The third study (Chapter 4) is a proof-of-concept experiment in measuring intermediate-length flow paths in the field. Using a combination of tracers and channel water balance, this study empirically measures intermediate flow paths and quantifies their relative makeup in fluxes and through the hyporheic zone. Results show that intermediate-length flow paths in the subsurface, which are normally ignored or missed in reach-scale studies, make up a significant portion of the total water balance. Together, this body of work quantifies intermediate flow paths within the hyporheic zone and assesses their importance when modeling and upscaling hyporheic exchange studies.