The importance of static and dynamic head drivers for hyporheic exchange: evaluation of a spectral modelling approach

Groundwater surface water interactions can greatly impact the ecohydrology on a wide range of spatial scales, ranging from biogeochemical reactions under local bedforms to alteration of regional groundwater discharge patterns. Hyporheic exchange fluxes (HEF) are controlled by the streambed geology and driven by hydraulic head fluctuations at the stream bottom, consisting of a static and a dynamic part. Currently, few studies have investigated the relative importance of these two drivers of HEF in the field, which hinder a holistic understanding of the governing processes and may affect predictions of hyporheic exchange intensities.

This study is based on an extensive field survey of 9 stream reaches located in small, pristine streams in Sweden, with varying hydromorphological characteristics such as slope, bottom material, morphological complexity and stream discharge. The field survey included distributed measurements of the hydraulic head and the hydraulic conductivity along the streambed, as well as tracer tests with Rhodamine WT. The overall aim of the study was to evaluate the relative importance of HEF driven by dynamic and static head fluctuations in streams by usage of a spectral model that decomposes the observed hydraulic head fluctuations on distinctive spatial scales. As a validation, the advective storage path (ASP) transport model was calibrated against the conducted in-stream tracer tests and its parameters compared to the equivalent gained from the spectral model.

The results showed that the average exchange velocity evaluated by the two models were comparable in most observed cases, validating the usage of the spectral model in small alluvial streams with high slope, and low discharge and stream depth. However, a sensitivity analysis of the two models revealed some degree of equifinality for some of the independent model parameters. Detailed results from the spectral model indicated that the static head was dominating the HEF in all reaches, both on average and when distributed over separate spatial scales. Uncertainty in the results was found, predominantly effecting calculations of the dynamic HEF and connected to (1) the approximation of streambed topography at spatial scales <0.5 m, where the dynamic exchange is assumed to dominate and (2) the use of Fehlman’s constant for estimating the hydrodynamic exchange under complex streambed topographies. Despite those uncertainties, the spectral model approach gives a deeper understanding of the phenomena of HEF by incorporating its multiscale nature and illustrating the fact that static and dynamic drivers might be equally important, only acting on different scales.