Spatio-Temporal Variability of Hyporheic Exchange Processes Across a Stream Network

Abstract

Hyporheic exchange processes (HEP) play a critical role in controlling riverine biogeochemical turnover and ecological functioning. Despite the expected scaling of HEP across stream networks, only limited knowledge exists about how HEP changes over the hydrological year and across the stream network. This study investigates spatial and temporal changes in HEP in a second- to fourth-order stream network in southern Germany. We employed radon, an environmental tracer commonly used for quantifying HEP, to study the relationships between HEP and discharge. Numerical mass-balance modelling was applied to quantify HEP, and we specifically focused on the hyporheic area (A_s) and the stream's cross-sectional area (A). Our findings showed a decrease in A_s/A with increasing stream order, indicating changes of HEP across the stream network. The absence of a correlation of A_s with discharge implies that the scaling of HEP may be influenced by a combination of discharge and local heterogeneities in stream geomorphology. Temporal variability in HEP was observed over the hydrological year, with the highest variability in headwater streams. Lower A_s values were noted in headwaters during summer compared to the other seasons and coincided with an increased groundwater contribution to the streamflow and decreased stream discharge. Although neither stream or groundwater discharge were identified as driving factors for reduced HEP in the headwaters during summer, our findings suggest that hydrological processes that lead to decreased streamflow in headwaters may have influenced HEP further downstream in the stream network. This is evidenced by the larger A_s/A ratios observed in higher-order streams during summer compared to other seasons. These findings highlight the necessity for comprehensive investigations of HEP processes over the hydrological year and across the whole stream network.