Recession discharge from compartmentalized bedrock hillslopes

We used numerical modelling to explore the role of the vertical compartmentalization of hillslopes on groundwater flow and recession discharge. We found that, when hydraulic properties are vertically compartmentalized, streamflow recession behaviour may strongly deviate from what is predicted by groundwater theory that considers the drainage of shallow reservoirs with homogeneous properties. We further identified the hillslope configurations for which the homogeneous theory derived from the Boussinesq solution approximately holds and, conversely, for those for which it does not. By comparing the modelled recession discharge Q and the groundwater table dynamics, we identified the critical hydrogeological conditions controlling the emergence of strong deviations. The three main controls are (i) the contribution of a deep aquifer connected to the stream, (ii) the heterogeneity in hydraulic properties, and (iii) the slope of the interface between a shallow permeable compartment and deep bedrock one with lower hydraulic properties. Our results confirm that a correct physical interpretation of the recession discharge exponent b from the classical equation -dQ/dt = aQ(b), and its temporal progression, requires knowledge of the structural configuration and heterogeneity of the aquifer.

Automated summary

We found that the vertical compartmentalization of hillslopes has a significant impact on groundwater flow and recession discharge. Streamflow recession behavior can deviate from predictions made by groundwater theory when hydraulic properties are vertically compartmentalized.