Introduced flow variability and its propagation downstream of hydropower stations in Sweden

Abstract

Hydropower regulations may increase flow variability when compared with the natural hydrological regime, with detrimental impacts on river habitats. Attenuation of the variability improves ecological status at some distance downstream of the introduced variability, and being able to accurately estimate this distance is critical for the evaluation of ecological status. The attenuation has only been studied previously for specific rivers, and the dominant mechanisms have not been analyzed in detail. In this work, the attenuation and its important drivers are studied for regulated rivers in all of Sweden by comparing Fourier components and their attenuation based on hydrological and hydraulic models and observations, with comparisons also to lake attenuation. In many rivers, weekly flow variability is dominant among periods up to 1 month, and variability with periodicity days to months attenuates with an exponential rate that is largest for short periods. This is mainly driven by instream processes. Furthermore, regulated systems often resemble cascades with low-gradient river stretches between the dams. The associated attenuation can be described by hydrological models using a linear channel and linear reservoir. In contrast, the sometimes-used diffusion wave equation is often unable to replicate the observed attenuation here. Lakes may contribute significantly to attenuation.