The role of rivers and lakes in damping flow variability introduced by hydropower

Hydropower regulations may significantly increase the variability of flow at especially short time scales 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 and lakes 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 lakes, and the dominant mechanisms have not been analyzed in detail. In this work, the attenuation rate and its important drivers is studied for lakes and 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 obtained with a) the Hydrological Predictions for the Environment (HYPE) model, b) an extracted model representing only river processes, c) the diffusion wave equation, and d) from observed flow at several hundred stations. The reduction of the amplitudes along rivers and in lakes was then analysed. This damping in rivers and lakes was further compared.

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 in rivers 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. Lakes have larger attenuation potential than rivers, especially at low flows.

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.

Elenius, M.T. and G. Lindström (2022) Introduced flow variability and its propagation downstream of hydropower stations in Sweden. Hydrology Research 53(11), 1321-1339. doi: 10.2166/nh.2022.138