Water and solutes dynamics during rainfall events in headwater catchments in the Central Swiss Alps under the influence of green alder shrubs and wetland soils

In the Swiss Alps, shrubs (e.g. Alnus viridis (Chaix) DC) are encroaching into formerly open habitats. The shrub encroachment might affect soil hydrological properties, which in turn influence runoff generation. Moreover, alder species (Alnus spp.) are known to affect chemical soil properties (e.g. increase of nitrate concentrations in the soil solution) and can therefore alter the water quality of stream water. In our study, we investigated four small alpine headwater catchments to assess the influence of shrub encroachment and wetland soils on stream water geochemistry during storm runoff. Stream water was sampled in the growing season of 2010 at hourly intervals during one single rainfall event. Stable isotope values (δ18O) of stream water (ranging from −13.8 to −8.5‰) and rainfall (bulk mean δ18O value of about −5.6‰) during the single event were used to estimate the fraction of event water in stream discharge. Continuously measured electrical conductivity in the growing seasons of 2010 and 2011 was used to infer information on runoff generation during 15 rainfall events. Riparian wetland soils were flushed by a high fraction of event water of up to 72% during peak discharge, which increased the dissolved organic carbon export during the single rainfall event. Besides the atmospheric input through nitrate in rainwater, the expected expansion of green alder shrubs in the region, associated with increasing number and intensity of summer rainfall events in the future, might increase the episodic export of nutrients such as dissolved organic carbon and NO3− from these catchments.