Impact of reservoirs on river flow from a large sample of catchments in France

Reservoirs are present in many catchments worldwide. They allow to regulate river flow in support of human activities, and they help to reduce flood risk and sustain low flows. They can significantly alter the natural flow of rivers, depending on how they are used and managed, and thus affect ecosystem functioning. However, the large-scale impact of reservoirs on streamflow regimes is not well understood because data on their operating rules are rarely publicly available. Hydrological studies often neglect reservoirs and are often limited to “natural” catchments that are not influenced by human activities.

Here we aim to assess the impact of reservoir regulation on river flow at the national scale in France over the 1970–2020 period, and to determine to what extent the impact varies depending on the reservoir purpose and the physio-climatic conditions. We focus on France, which presents a large variety of landscapes and reservoirs, and where the construction of new reservoirs is envisaged in the face of rising irrigation water demand. We compile a national reservoir dataset by combining data from different sources. We assess their impact by comparing the observed streamflow between 227 regulated catchments and 908 benchmark catchments, that are assumed to be representative of natural flow conditions. We adopt different hydrological signatures that capture different aspects of the streamflow, namely its average value, its inter- and intra-annual variability, and hydrological extremes.

The results show some similarities with those of previous studies in the United Kingdom and the United States, for instance reduced seasonality and flood peaks in regulated compared to benchmark catchments. Notably, we also observe specificities for the French reservoirs that can, among others, increase drought severity. Ultimately, the study allows us to better understand how reservoirs can affect regulated rivers, thus informing their management and their integration into large-scale hydrological models.