Fragmentation of the Hyporheic Zone Due to Regional Groundwater Circulation

Abstract

By use of numerical modeling and field observations, this work quantified the effects of catchment-scale upwelling groundwater on the hyporheic (below stream) fluxes over a wide range of spatial scales. A groundwater flow model was developed that specifically accounted for the hydrostatic and dynamic head fluctuations induced by the streambed topography. Although the magnitudes and relative importance of these streambed-induced fluxes were found to be highly sensitive to site-specific hydromorphological properties, we showed that streambed topographic structures exert a predominant control on the magnitude of hyporheic exchange fluxes in a Swedish boreal catchment. The magnitude of the exchange intensity evaluated at the streambed interface was found to be dominated by the streambed-induced hydraulic head across stream order. However, the catchment-scale groundwater flow field substantially affected the distribution of groundwater discharge points and thus decreased the fragmentation of the hyporheic zone, specifically by shifting the cumulative density function toward larger areas of coherent upwelling at the streambed interface. This work highlights the spectrum of spatial scales affecting the surface water-groundwater exchange patterns and resolves the roles of key mechanisms in controlling the fragmentation of the hyporheic zone.

Is the Hyporheic Zone Relevant beyond the Scientific Community?

“…HEFs (Figure 1) are driven by pressure gradients created by local streambed topographic variations and modulated by subsurface sediment architecture, combined with large-scale geomorphological and hydrogeological characteristics of the river network and adjacent aquifer systems, which can critically impact the spatial variability of HEF patterns [9,[39][40][41]. At the scale of the HZ, very small water level fluctuations drive changes in the hydraulic gradients across streambed bedform structures.…”

“…The absolute gaining or losing of water from streams is a dynamic feature which can vary spatially and temporally. The fragmentation of coherent gaining or losing zones at the streambed interface strongly depends on the regional groundwater contribution [41]. In gaining streams, local regional groundwater systems discharge groundwater through the HZ into surface waters.…”

“…This impedes direct use of the streambed topography to estimate the hydrostatic head, especially at small spatial scales, since very fine topographic features are often not reflected as similar features at the water surface. Recent investigations have suggested a scale (wavelength)-dependent ratio between stream water and bed surface fluctuations, thus providing a way to estimate the hydrostatic head distribution based on the streambed topography [40,41]. Pressure gradients might also be caused by flow of stream water over and around obstacles in the water body, such as woody debris [47][48][49].…”

“…As described above (2.1), hydrostatic and hydrodynamic heads along uneven streambeds have long been known as drivers of hyporheic flow [1,27,45] that pose a hydromechanical (transport) limitation on nutrient biogeochemistry [39,218,219] and impact the regional groundwater discharge patterns [41]. However, there is still uncertainty connected to the importance of these drivers over a wide range of temporal and spatial scales [220,221].…”

The Effect of Stream Discharge on Hyporheic Exchange

“…In addition to the pressure distribution on the SWI, subsurface heterogeneity has a substantial impact on the subsurface flow field [12,13]. Heterogeneity in hydraulic conductivity influences the magnitude of subsurface flow velocity, as well as the size of the hyporheic zone [14,15]. Previous studies have shown that hydraulic conductivity decays with depth following an exponential function that can be estimated based on field measurements [16][17][18].…”