Bedrock geology controls on new water fractions and catchment functioning in contrasted nested catchments

Abstract. We still lack substantial understanding on how landscape characteristics shape the storage and release of water at the catchment scale. Here we use 13 years of fortnightly precipitation and streamflow δ¹⁸O measurements together with hydrometeorological data from 12 nested catchments (0.5 to 247.5 km²) in the Alzette River basin (Luxembourg) to study bedrock geology and landcover controls on streamflow generation.

Streamflow responses to precipitation were highly variable. Runoff coefficients were typically higher in catchments dominated by less permeable bedrock (i.e. marls and claystones, R_c = 0.43 to 0.52) than in catchments with a high fraction of permeable bedrock (i.e. sandstones and conglomerates, R_c = 0.19 to 0.40). The fraction of new water (F_new, water younger than ~16 days in this study) determined via ensemble hydrograph separation was strongly related to differences in bedrock geology. F_new was highest in impermeable bedrock catchments (i.e. with a dominance of marls and claystone, F_new = 4.5 to 11.9 %), increasing with higher specific daily streamflow (F_new up to 45 % in one catchment). In catchments with an important fraction of permeable sandstone and conglomerates, high F_new variability with specific streamflow (F_new rising to 25 % in one catchment) was also found, despite a damped and delayed hydrograph response to precipitation and low F_new (means of 1.3 to 2.7 %). In the weathered bedrock catchments (i.e. dominated by schists and quartzites), rapid infiltration led to large fractions of water that was older than 12 weeks (~80 %) and very small fractions of water younger than two weeks (~3.5 %). F_new variability with streamflow was near zero, contrasting with the rapid response of the hydrograph to precipitation events. At high specific streamflow, F_new was also correlated with bedrock geology and certain land use types. The extensive data set of streamflow δ¹⁸O enabled us to link water storage and release to bedrock geology. Such information is key for a better anticipation of water storage and release functions under changing climate conditions, i.e. long dry spells and high-intensity precipitation events.