Modelling hyporheic exchange: From the boundary layer to the basin

Hydrological connections in streams occur longitudinally along the stream channel, laterally with the floodplain and vertically with the hyporheic zone. These connections are an important control on freshwater ecosystem processes at the basin-scale including nutrient cycling and retention; movements of organisms to complete life stages; and the provision of refugia during high and low flow periods. They also influence human and farm health by creating pathways for the sequestration and mobilization of microbial communities including human and animal pathogens. Methods are available to observe and model lateral and longitudinal hydrological connectivity at the basin-scale but this is not true for the vertical dimension. Understanding the strength of vertical hydrological connectivity across river basins is important for freshwater science and will lead to better catchment management for human and ecosystem health outcomes. Hyporheic exchange is fundamental to vertical connectivity, transporting mass, energy, and momentum between the sediment and the water column. Recent work by the authors has led to the development of new resistance model of sediment-water interfacial flux at the patch scale (ca., 1 to 10 m) including processes of hyporheic exchange. The model parameterizes patch-scale hyporheic exchange in terms of a mass transfer resistance coefficient R, and a scaling law for R has been developed based on a meta-analysis of previously published hyporheic exchange experiments in recirculating laboratory flumes. For this study, we adapt this scaling law to natural stream channels in the Murray-Darling Basin using reach-averaged values of key hydraulic variables that are assumed to be fixed throughout the stream network or modeled using hydraulic geometry relations. Our model of patch-scale hyporheic exchange predicts much more frequent exchange between the water column and the streambed in steeper upland streams. A molecule of water transported along a 100 km length of upland stream may journey into the streambed more than 1000 times. In contrast, the same molecule might only pass into the streambed once while being transported a similar distance in a lowland river. This suggests that any hyporheic processes influencing the character of the water column (through biogeochemical transformations or source-sink dynamics) will have a much stronger effect in steeper gradient rivers. The stronger hydrological connectivity between water column and hyporheic zone in steeper rivers is likely to promote buffering of solute and suspended contaminants delivered as a pulse from headwater catchments. This suggests an interesting interaction between vertical and longitudinal hydrological and biogeochemical connectivity at the basin-scale. Upland rivers may be characterized by strong vertical and weak longitudinal connectivity whereas the reverse may be true in lowland rivers.