Disentangling reservoir regulation patterns from natural streamflow in the Alps and their downstream regions

Streamflow seasonality in mountain regions is besides climate often shaped by reservoir regulation. Such regulation is particularly important in the Alps where meltwater from glaciers and the snowpack are captured in reservoirs to generate hydropower during the winter season. While reservoirs affect streamflow seasonality, information on past seasonal reservoir operation patterns is rarely publicly available. Consequently, little is known about spatial variations in reservoir storage and release signals in dependence of climate and catchment characteristics. Here, we develop a generalized additive modelling approach to reconstruct daily and seasonal reservoir patterns from observed streamflow time series that encompass a period before and a period after a known year of reservoir construction.

We apply this approach to reconstruct the seasonality of reservoir regulation, i.e. information on when water is stored in and released from a reservoir, for a dataset of 74 regulated catchments in the Central Alps. Using these reconstructed seasonal regulation patterns, we identify groups of catchments with similar reservoir operation strategies using functional clustering. We find that reservoir management varies by catchment elevation. Seasonal redistribution from summer to winter is strongest in high-elevation catchments, where reservoirs are mostly used for hydropower production, while seasonal redistribution is much weaker in the downstream regions, where reservoirs are used for a range of different purposes. The clear relationship between reservoir operation and elevation has practical implications. First, these elevational differences in reservoir regulation can and should be considered in hydrological model calibration. Furthermore, the reconstructed reservoir operation signals can be used to study the joint impact of climate change and reservoir operation on different streamflow signatures, including extreme events. Last, the potential of regulation as a climate adaptation measure may vary for the high-elevation and downstream regions.