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 land cover controls on streamflow generation. Streamflow responses to precipitation were highly variable. Runoff coefficients (Rc) were typically higher in catchments dominated by less permeable bedrock (i.e., marls and claystones, Rc=0.43 to 0.52) than in catchments with a high fraction of permeable bedrock (i.e., sandstones and conglomerates, Rc=0.19 to 0.40). The fraction of new water (Fnew, water younger than ∼ 16 d in this study) determined via ensemble hydrograph separation was strongly related to differences in bedrock geology.Fnew was highest in impermeable bedrock catchments (i.e., with a dominance of marls and claystone, Fnew=4.5 % to 11.9 %), increasing with higher specific daily streamflow (Fnew up to 45 % in one catchment). In catchments with an important fraction of permeable sandstone and conglomerates, high Fnew variability with specific streamflow (Fnew as high as 25 % in one catchment) was also found, despite a damped and delayed hydrograph response to precipitation and low Fnew (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 %).Fnew variability with streamflow was near zero, contrasting with the rapid response of the hydrograph to precipitation events. At high specific streamflow, Fnew 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.