Recession discharge from compartmentalized bedrock hillslopes

Abstract. We used numerical modelling to explore the role of 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 hold and conversely for which it fails. By comparing the modelled recession discharge and the groundwater table dynamics, we identified the critical hydrogeological conditions responsible for 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 shallow permeable compartment and deep bedrock one with lower hydraulic properties. Our results confirm that a physical interpretation of the recession discharge exponent b from the classical equation −dQ / dt = aQ^b, and its temporal progression, are information that can only be interpreted with knowledge on structural configuration and heterogeneity of the aquifer.