Dominant discharge periodicities and their attenuation in rivers of Sweden

Hydropower regulations may significantly increase the variability of flow when compared with the natural hydrological regime to which river ecosystems have evolved over long time periods. This can be detrimental for river habitats and for many organisms. Attenuation of the variability in rivers improves ecological status at some distance downstream of the introduced variability. Being able to accurately estimate this distance is critical for the evaluation of ecological status. The attenuation of introduced flow variability has only been studied previously for specific rivers, and the dominant mechanisms have not been analyzed in detail. In this work, the attenuation rate and its important drivers is studied for regulated rivers in all of Sweden by comparing the results of hydrological and hydrodynamic models with observations. We performed Fourier transformation of flow time series from the Hydrological Predictions for the Environment (HYPE) model, an extracted model representing only river processes, the diffusion wave equation, and from observed flow at several hundred stations. The reduction of the amplitudes along rivers was then analysed.

In many regulated rivers in Sweden, flow variability of periodicity 7 days is dominant among periods varying from a couple of days up to one month. The analysis further shows that variability with periodicity days to months typically attenuate with an exponential rate that is largest for short periods. Attenuation of these periods is mainly driven by processes within rivers, as opposed to catchment features such as the distribution of rain or soil properties. Further, rivers in regulated systems often resemble cascades with long stretches of rivers with low gradients in elevation between the dams. The associated attenuation in these “lake-alike” rivers can be well described by hydrological simulations with HYPE using a simple linear attenuation box. In contrast, the sometimes-used diffusion wave equation is often unable to replicate the observed attenuation here. Our work supports the assessment of ecological status and management decisions by improving the estimates of distances required for attenuation, and provides important insights on attenuation processes. Further, the analysis of dominant modes can be used to parameterize short-term regulations in hydrological simulations to improve their forecast skills.