Hydrogeological Controls on Stream Discharge Dynamics in Bedrock Catchments: Exploring the Combined Effects of Seepage Development and Heterogeneity

Surface/subsurface interactions and geological heterogeneity have important effects on the dynamics of streamflows. Surface/subsurface interactions speed up transfers through the development of seepage zones, which reduce the response time of the aquifer and increase the proportion of rapid infiltration excess overland flow. On top of it, geological heterogeneity modulates spatially the extent of the seepage zones as well as the intensity of drainage of the underlying aquifer.We investigated the combined effect of the surface/subsurface interactions and geological heterogeneity in a crystalline basement region under temperate climate (Brittany, France), where the limited aquifer capacities, the hydraulic conductivity enhanced by weathering and fracturing and the significant recharge rate promote surface/subsurface interactions. We analysed 40-year of discharge data monitored on two catchments (Arguenon 104 km2 and Aber Plabennec 27.4 km2) using 1D hillslope models (hs1D). The hs1D hillslope model resolves the vertically integrated Boussinesq subsurface flows with a spatially and temporally varying saturation-limited boundary condition on equivalent 1D hillslope structures. We specifically analysed the effect of accounting for heterogeneity on improving the discharge predictions, accounting for the presence of 2 equivalent hillslope with different hydraulic properties. This heterogeneity was defined based of the presence of two main geological lithologies in the catchments. Calibration was performed by a systematic parameter space exploration.The calibrated models display significant differences between the two catchments. In the Aber Plabennec catchment, the homogeneous and heterogeneous hillslope models had very close performances showing an effective geological homogenization of the hydraulic conductivity and porosity. In the Arguenon catchment, the heterogeneous model outperformed the homogeneous model with a 46% increase of the Nash-log criterion showing persistant and significant differences in hydraulic conductivities and porosity. Successful calibration in both cases demonstrated by Nash-log values larger than 0.75-0.8 showed the overall relevance of the hillslope approach and its capacity to check for the presence of hydraulic heterogeneity at the catchment scale. Differences between catchments hints on the potential identification of hydrogeological properties at the regional scale by the combined use of the geological map and stream discharges.