Dynamic river networks shaped by surface-subsurface interactions

Hydrological modelling has a long tradition in environmental science, aiming to simulate water flow and storage across a variety of scales, from small catchments to entire river basins.  Over the past decades, new data have been made available which helped supporting the development of numerical models. However, most of them focus on catchment runoff only as it is easier to measure and often represents the more dominant component of the hydrological regime. On the other hand, subsurface processes occurring in the river domains, which govern water storage and regulate the exchange of matter between the superficial and subsuperficial environments, remain relatively underexplored. Moreover, existing models generally neglect the dynamics of expansion and contraction of the network in response to transient hydrological conditions. This numerical simplification is relevant not only when analysing headwater systems but also entire catchments.

Here we propose a novel, physically based, spatially explicit modelling framework which quantitatively represents the interaction between superficial and subsuperficial streamflow dynamics, thus representing the hyporheic zone as the key interface between surface and subsurface compartments. This model combines well-known laws of hydraulics and hydrology into a mass balance that describes how streamflow changes in time in each reach of the river network. The model quantitatively conceptualizes hillslope drainage to focus on the hydrological dynamics taking place on a river network domain in a range of possible scales, from local scale to larger river basins.

This framework aims to i) estimate how flows change in time and space, both in the surface and subsurface domains, and ii) assess the persistency of each reach (i.e. the percentage of time in which the monitoring point is wet) resulting from the interaction between superficial and subsuperficial flows. This approach allows to investigate how subsurface storage controls runoff generation, flow connectivity and network expansion and contraction. Beyond hydrology, the framework provides a basis for investigating water, solute and energy exchanges, thereby offering new opportunities to link hydrologic dynamics with ecological and biogeochemical processes at the catchment scale.