Estimation of mean water residence times and runoff generation by180 measurements in a Pre-Alpine catchment (Rietholzbach, Eastern Switzerland)

Stable isotopes (180) are used to investigate hydrological processes in the small Pre-Alpine research catchment Rietholzbach in NE Switzerland. The catchment covers an area of 3.18 km2 and ranges from 680 to 960 m a.s.l. The180 concentration in rainfall and in runoff at various sites, including a lysimeter installed within the catchment, over a 2-a period are investigated. First, conceptual flow models are applied to determine the mean water residence times of different systems of the basin and their storage properties. As a second approach, the separation of runoff components using18O and Mg ions as tracer during some storm flow events was carried out. The following flow mechanisms can be identified from the modelling results: (i) lateral hillslope fluxes, which are faster in the part of the Tertiary Molasse sediments and slower in the part of the Quaternary deposits, and (ii) relatively slow vertical infiltration processes through the soil layer within the lysimeter. Mean residence times of 12.5 months result for the total catchment baseflow component and 24.5 months for the baseflow from the upper part of the catchment, respectively. For the lysimeter seepage water, a mean residence time of about 200 days was obtained. During the intensive storm events, a fast reaction of the stored subsurface water due to heavy precipitation and an expulsion of this storage water amounting to a considerable portion of the total basin runoff can be detected.