Hydrology

The hydrology of the Vaud Alps is determined by: (1) their location within Europe, which subjects the region to strong Atlantic influences on the climate; (2) their altitudinal gradient, meaning a significant proportion of winter precipitation can fall as snow; and (3) their position to the northwest of the main Swiss Alps range, leading to precipitation caused by strong orographic effects. Due to the substantial amount of winter precipitation falling as snow, the runoff in the watersheds is typically snowmelt-rainfall driven. The balance between the winter snowpack, summer melt, and direct runoff depends primarily on the altitudinal gradient of the watersheds. This gradient determines both the proportion of precipitation falling as snow and the duration for which the snow cover persists. This balance is not solely controlled by total winter precipitation but also by temperature, which affects the snowpack in each watershed. More generally, the hydrological response can be summarized as follows: (1) for low-altitude watersheds (< approx. 800m), runoff typically follows precipitation, although some may be temporarily stored as snow; (2) for mid-altitude watersheds (approx. 800m to 1,500m), there is usually winter snow accumulation, and snowmelt typically occurs in early spring; and (3) for high-altitude watersheds, significant snow accumulation can lead to a peak in snowmelt, making runoff occur later in the spring. Note that these altitudes are indicative and vary annually depending on the prevailing conditions in each watershed, and they also vary according to the watershed's aspect.

In summer, in some watersheds, the accumulated snow may be enough to maintain high minimal flow levels. Without this, river flow would be very low. However, it is still influenced by occasional convective storm events. High-altitude watersheds can contain very small glaciers (generally less than 5% of the watershed's surface), which also help maintain low flow during the summer. The contribution of groundwater is also important for maintaining minimal flow.

Human impact on hydrology is significant in most regions. Many watersheds are used for hydroelectric purposes, though this is more related to water transfers than to dams. Water is extracted and transferred to higher altitudes via tunnels, then returned either to its original river or to a river in a neighboring valley through small hydroelectric plants. Although the water storage capacity is relatively small, this type of abstraction can significantly alter the water volume in a canalized river and lead to sediment management issues.

(Written in collaboration with Prof. Stuart Lane)