Dynamics of streamflow and stream chemistry in a Swiss pre-Alpine headwater catchment : A fine scale investigation of flow occurrence and electrical conductivity in the temporary streams in the lower Studibach catchment

Sammanfattning: Temporary streams and their dynamics have often been largely overseen in hydrological research and there is relatively little knowledge about how the occurrence of flow in these streams varies temporally and spatially. Temporary streams are important from a hydro- logical perspective because they affect water quantity and quality in downstream peren- nial reaches, and from an ecological perspective because they provide habitat to unique species. In order to gain knowledge about these important streams, this maser thesis was conducted, within the Msc program in Water and Environmental Engineering at Uppsala University and the Swedish University of Agricultural Sciences, in collaboration with the Hydrology and Climate group at the University of Zurich. In this study, the temporal and spatial variation of the temporary streams in a small pre-Alpine catchment in Switzerland were investigated, both in terms of the presence of flowing water and stream chemistry. The 20 ha Studibach catchment is typical for the pre-Alpine area, with frequent precipi- tation. The streams in the lower part of the Studibach catchment were mapped in the field during September 2020. The temporal and spatial variations of the presence of flow and stream chemistry within the stream network was investigated in September and October 2020 during varying weather conditions. During ten field campaigns the flow state of the streams was classified and the Electrical Conductivity (EC) of the streams was mea- sured approximately every 20 meter. The findings from the field campaigns were related to topographic indices, in particular the Topographic Wetness Index (TWI) and Upslope Accumulated Area (A), in order to see how topography influenced the presence of stream- flow and stream EC. The results show a high temporal and spatial variation in both stream chemistry and streamflow. The active network length expanded by a factor of two in re- sponse to precipitation events. The stream EC also had a large spatial variation, and the streams in the southeast part of the catchment had a higher EC than the other streams. This spatial variation is expected to reflect the large variability in groundwater EC within the catchment. The spatial variation of the streamflow demonstrated a difference between the north-middle and the south part of the catchment, where the south part responded quicker to events and drained and retracted faster after the event. The findings also indicate that topographic indices can predict the occurrence of flow in the stream network, with sites with higher topographic index values having a higher probability of flowing water in the stream. Topography also influences the stream chemistry. The variation in stream chem- istry was smaller for sites with higher values for the topographic indices, something that can be explained by the Representative Elementary Area (REA) concept, because sites with higher topographic index values are located further downstream and water at these locations is a mixture of the smaller streams that feed these streams.