Grasping Hydrological Droughts in Highly Hydropower Regulated Alpine Watersheds

Abstract

Hydrological droughts in Alpine regions are increasingly shaped by human regulation, with hydropower operations playing a central role in modifying their statistical characteristics. This study examines how the representation of hydropower systems in hydrological models influences the identification and statistical characterization of drought. Using the HYPERstreamHS hydrological model, we simulate streamflow in the Adige River basin under three configurations of increasing complexity, ranging from natural flow conditions, to simplified rule‐based operations, to detailed representations of reservoir management based on historical operation patterns. Drought events are analyzed from both univariate and bivariate perspectives, with the latter relying on copula‐based frequency analysis to capture the joint statistical behavior of drought duration and severity. Our findings show that conventional performance metrics, such as the Nash–Sutcliffe Efficiency and its logarithmic variant, can mask substantial differences in drought statistics across model configurations. In particular, simplified or naturalized representations systematically underestimate the frequency of hydrological droughts, failing thus to reproduce the observed dependence structure between hydrological drought attributes. Only the configuration that explicitly incorporates reservoir infrastructure and realistic operational rules is able to replicate both the marginal distributions and the joint behavior of observed droughts. These results demonstrate that hydropower regulation not only alters streamflow regimes but also reshapes the statistical properties of droughts. Capturing this influence is essential for drought risk assessment in regulated basins. More broadly, the study highlights the limitations of simplified modeling approaches and underscores the need to integrate realistic storage reservoir operations into hydrological models to support decision‐making under drought conditions.