How does reservoir-regulation impact hydrological extremes in the Alps?

Low-flows and floods cannot be viewed as purely natural phenomena since their
occurrence and characteristics are influenced by water storage and regulation.
Reservoir-regulation has strong impacts on flow seasonality and can intensify
or attenuate hydrological extremes and change their duration. It is yet hardly
quantified how reservoir regulation affects low- and high flows in the Alps, where
most reservoirs are operated for hydropower production. We need a better un-
derstanding of the effect of reservoir-regulation on hydrological extreme events
in order to assess the readiness of current regulation schemes for the future.
However, the analysis of river flow and estimation of hydrological extremes is
challenging in regulated catchments, particularly in large-samples studies, where
detailed information about reservoir-regulation is missing.
In this study, we analyse how reservoir-regulation has changed the magni-
tude and frequency of hydrological extreme events in the European Alps. To do
so, we have compiled a dataset of discharge stations and reservoirs which in-
cludes reservoir characteristics such as the first year of operation or the storage
capacity. With this information, we distinguish between discharge time series
before and after reservoir construction for about 70 catchments in the European
Alps and calculate a normalized reservoir storage capacity for each catchment.
Then, we calculate flood return periods based on annual maxima discharges
and a generalized extreme value distribution and the minimum 7 day moving
average runoff (MAM7) for each time series. We compare flood and low-flow
characteristics before and after reservoir construction for each catchment to as-
sess the influence of reservoir-regulation on hydrologic extremes. Furthermore,
we analyse changes in the seasonality of hydrological extremes and evaluate how
it is affected by seasonal reservoir-regulation schemes.
Our preliminary results show that reservoirs affect both, low-flows and floods.
Annual low-flows have mostly increased since reservoir-construction, while their
variability has decreased. Annual maximum flows with low return periods (be-
low 10-years) have mostly decreased after reservoir-construction with catch-
ments with a larger normalized storage capacity showing a stronger effect of
reducing extreme flows. Consequently, we conclude that reservoirs operated for
hydropower production mostly have an alleviating effect on both low-flows and
floods.