Influences of vegetation, soil properties, topography and microclimate on alpine catchment hydrology

Climate change is causing severe impacts in mountainous regions, leading to "greening" as vegetation densifies and shifts upslope as a response to rising temperatures. This vegetation change affects the hydrological cycle, influencing aspects like infiltration, retention, and evapotranspiration, which in turn alters water availability in both mountain catchments and downstream areas. As snow and ice storage decrease, understanding these hydrological effects becomes increasingly important for human water security.

To investigate how mountain vegetation changes could affect hydrology, we established 40 vegetation plots in the alpine Meretschi Catchment (6.2 km²) in Switzerland in five vegetation classes: bare, pioneer, grass, dwarf shrubs and larger shrubs/forest. At each plot we measured soil temperature and soil moisture with TOMST-TMS4 loggers at 15-minute intervals over the period 2023-2024. In addition, we collected and derived data on plot species composition, soil characteristics and topography. Using uni- and multivariate statistical analyses (Spearman, non-metric multidimensional scaling (NMDS) with post-hoc and Kruskal-Wallis with Dunn test) , we investigated interactions between vegetation, soil properties, topography, microclimate and hydrology (soil moisture, saturated conductivity (Ksat) and snowmelt driven moisture increase).

Our results show that:

• Soil moisture responds differently under different vegetation classes. Bare and pioneer classes have relatively low soil moisture values, while grass, dwarf shrub and larger shrubs/forest have higher values. Dwarf shrubs distinguish themselves from the others by having low soil moisture values during winter. This means that shifting vegetation due to greening is likely to affect the hydrology in a mountain catchment.
• Soil characteristics seem to be closely linked to soil moisture and vegetation following the same division. This is likely due to soil developmental properties of the vegetation.
• Topography has weak links to hydrology, with only elevation negatively correlating with soil moisture. This indicates that the differences hydrological response found between the vegetation classes cannot be attributed to differences in slope or aspect.
• Temperature showed to be variable between plots and vegetation classes resulting in differences in snow cover durations. For dwarf shrubs it was very noticeable that the melting period set in earlier and was shorter. This did not reflect in the snowmelt driven moisture increase.

Our research shows that in the Meretschi Catchment hydrological factors such as soil moisture and Ksat are influenced by vegetation. This indicates that changing vegetation might considerably alter the  present-day hydrology of mountain catchments. While other contributing factors, such as soil properties, topography, and climate are important, vegetation is key in understanding the complexities of the hydrological system.