Spatial and temporal runoff generation processes in a Swiss pre-alpine headwater catchment

Abstract

In mountainous headwater catchments, with their high variability of hydrological processes and catchment properties together with the limited spatial and long-term data, it remains unclear how the different spatiotemporal controls affect runoff generation during base and stormflow. In this doctoral thesis environmental tracers are used to identify sources contributing to baseflow, observe linkages between different landscape units and discern how rainfall becomes stormflow in a Swiss pre-alpine headwater catchment, the Zwackentobel (4.25 km2), with high precipitation amounts (P>2000 mm y−1) and heterogeneous catchment properties. The spatially collected rainfall and its isotopic composition are used to test whether it is reasonable to assume that in small headwater catchments, there is spatially homogenous rainfall and isotopic composition, implying that one sampling location is sufficient. Important tracers in this thesis are the stable isotopes 2H and 18O. The collected water samples were analysed with a laser spectroscope. For this analysis it was tested how many measurements are needed to reduce the memory effect and obtain a desired accuracy. For the Zwackentobel water samples it was sufficient to measure each sample six times while for highly depleted samples the memory effect was reduced only after seven to eight injections. The water samples collected during three snapshot sampling campaigns were analysed for their isotopic (δ2H) and hydrochemical components (Ca, DOC, AT, pH, SO4, Mg and H4SiO4)andcontainedusefulinformationonthedifferentsourcesandhelpedtoidentifythe flowpath of the water during baseflow. Although the six subcatchments had different landscape units, the inter- and intra catchment variability of the isotopic and hydrochemical components was generally small and statistically not significant. Streamwater samples at the subcatchment outlets were more similar to springs near the water divide than to groundwater from observation wells and wetlands. The wetlands, with 30-60% of the subcatchment area and large storage capacity, were less connected and acted as passive features with negligible contribution to baseflow runoff. In five subcatchments of the Zwackentobel headwater catchment, rainfall and streamwater of 13 different rainstorms were sampled to perform a two-component isotope hydrograph separation. Pre-event water contributions based on δ18O or δ2H computations were similar. The pre-event water contributions of headwaters depended largely on rainfall (amount and intensity) and varied more between events than between catchments, despite clear differences in land cover between the catchments. With increasing rainfall amount, the proportion of rainfall in runoff increased and changed from pre-event to event water dominated. Antecedent wetness was not found to control the pre-event water contribution. The fact that catchment properties and antecedent conditions were only secondary factors in runoff processes, was mainly due to the dominant and frequent rainfall, which obscured a potential signal indicating differences in catchment properties. At the eight locations sequentially sampled rainfall revealed a spatial variability in total rainfall, rainfall intensity and its isotopic composition. The spatial variability in the isotopic composition varied from event to event. No clear relation between the isotopic composition and rainfall or altitude was observed. The isotope hydrograph separation results varied considerably depending on which temporal weighing technique or rain sampler was used. These results demonstrated that even in small catchments the spatial variability in the isotope composition of event rainwater has to be considered in hydrograph separation studies. When data from only one rain gauge are available, the location of the gauge might largely affect results and this source of uncertainty must be considered. The combination of long-term and spatially short-term hydrometeorological measurements, together with three baseflow sampling campaigns and event water sampling in different neighboring streams and multiple events, complemented each other and helped to overcome individual limitations. The results show the necessity and benefits of this spatially distributed dataset to derive and better understand controlling factors in runoff generation in a headwater catchment with high precipitation amounts and heterogeneous catchment properties.