Stream geochemistry and water flow paths in alpine headwater catchments : the influence of shrub encroachment and soil cover

Various external and internal factors such as atmospheric inputs and catchment characteristics, e.g. soil and vegetation cover, influence stream water geochemistry. In the Swiss Alps, shrubs (e.g. Alnus viridis subsp. viridis, Sorbus aucuparia, Calluna vulgaris, Salix appendiculata, Rhododendron ferrugineum) are encroaching into formerly open habitats and the effects of the latter on the catchment and hillslope scale hydrology and stream water geochemistry have not been investigated so far. The shrub encroachment might affect soil hydrological properties, which in turn could influence runoff generation. Moreover, alder species (Alnus spp.) are known to affect chemical soil properties and can therefore alter the export of nutrients via stream water.
Therefore, the hydrological and geochemical behavior of four alpine headwater sub catchments, which differ in vegetation and soil cover characteristics, were investigated. The aim was to gain information on water flow paths and export of nutrients during base flow, rainfall and snowmelt conditions at the micro catchment scale. Subsurface water flow paths at the hillslope scale were also investigated.
First, the influence of vegetation cover on mean transit times of water (MTT) was assessed. The MTT of water in a catchment provides important information about storage, flow paths, sources of water and thus also about retention and release of solutes in a catchment. MTTs between 70 to 102 weeks were calculated via time series of water stable isotopes using a convolution integral method. The high temporal variation of the stable isotope signals in precipitation was strongly dampened in stream base flow samples. This pointed to deeper flow paths and mixing of waters of different ages at the catchments’ outlets, which was supported by additional geochemical stream water data (e.g. Ca and Si). The study with four sub catchments suggests that MTTs are neither related to topographic indices nor vegetation cover. Water balance calculations and the geochemical data suggest that the major part of the quickly infiltrating precipitation likely percolates through fractured and partially karstified deeper rock zones. This process increases the control of bedrock flow paths on MTT.
In a next step, the water pathways at two steep hillslopes were tracked, since they strongly affect runoff generation processes and therefore control water geochemistry on the short term scale. Soil water stable isotope profiles, which offer a time-integrating overview of subsurface hydrological processes, were used. Furthermore, an advection-dispersion model was applied to simulate the delta18O profiles. The variability of delta18O values with depth within each profile and a comparison of the simulated and measured profiles revealed that vertical downward subsurface flow plays an important role, even at high slope angles. Lateral subsurface flow was also observed in deeper soil layers and at sites near a small stream. Physical soil data further supported the fast percolation of water towards deeper soil layers, from where it can subsequently recharge to the fractured bedrock, which led to the aforementioned strong dampening of stable isotope signals in base flow stream water.
Finally, the study focused on the hydrological and geochemical processes on the short term scale, i.e. a rainfall event in the growing season and a spring snowmelt period. The hydrological and geochemical differences in the sub catchments were assessed. Stream water was sampled at hourly intervals during the rainfall event and on a daily basis during the snowmelt period. Stream geochemistry was strongly influenced by the land cover, i.e. soil and shrub cover. Riparian wetland soils were flushed by a high proportion of event water, which increased dissolved organic carbon export during the rainfall event and the snowmelt period. A slight increase in nitrate export during the rainfall event was likely due to the encroachment of green alder shrubs.
In conclusion, the bedrock geology and geochemistry was mainly controlling stream water geochemistry on the hydrological long term scale, i.e. during base flow conditions. The soil properties in the investigated valley allow vertical downward flow of water within soil profiles even at steep slopes, and facilitate recharge of water to deeper zones and subsequently to the bedrock. The differences in vegetation and soil cover characteristics were most notably observed on the hydrological short term scale, when stream water geochemistry was highly variable. The connection of the shallow soil layers, which act as a “reservoir” for biogeochemical reactions, with the streams is mostly activated during rainfall and snowmelt events. Since duration of snow cover will be shortened and rainfall events during the growing season will become more frequent and intense due to climatic changes, the importance of vegetation and soil characteristics for the export of nutrients might still increase in the future.