More water in rivers or more water entering turbines for hydropower generation? Since 1975, this question has raised two others. For that year, the new water resource management article (24bis, now Art. 76) was included in the Federal Constitution. Here, it is specified that the Confederation shall legislate to ensure appropriate residual flows, also called “environmental flows”. But what does “appropriate” mean, and how many more kilowatt-hours of electricity could theoretically be produced if “appropriate” meant almost no water or none at all?

With aquatic ecosystems under growing pressure [1,2], increasingly precise answers have since been provided to the first question (see, e.g. [3] and [4]); however, previous estimates concerning “production losses” resulting from the current residual flow requirements of the Waters Protection Act (WPA) have varied widely. These estimates are not a trivial matter, for various reasons, including the fact that, for existing hydropower plants, two different legal procedures exist with regard to residual flows – one for a current concession and another when a concession is to be renewed. To complicate matters, minimum residual flows are to be increased in the light of a weighing of interests, but exceptions involving reductions are also possible. Crucially, however, details of the decisions made have not so far been available to the public for inspection.

Modelling involving discharge time series for the first time

Now, a group of researchers from WSL, Bern University and Eawag has systematically collected data on legal, hydrological and technical attributes for 252 large-scale hydropower plants (installed capacity ≥ 3 MW). Included in the database [5] are 160 run-of-river plants, 75 storage plants, 16 pumped-storage plants and 1 closed-loop pumped-storage plant. Together, these plants have an expected annual output of 31,540 GWh – over 84% of Switzerland’s total domestic hydropower generation. The database can be easily linked to the national hydropower statistics (WASTA); in addition, to determine the proportions of process water, residual flows and spills, hydrological discharge time series for the respective catchments were taken into account for the first time.

An initial analysis was conducted by the database authors for 217 hydropower plants for which residual flow levels are currently specified and adequate data is available: the simulation shows an annual decrease in production resulting from the residual flow requirements of 1113 GWh. This is equivalent to around 3% of total expected annual domestic hydropower production. The winter months (October to March) account for almost 70% (771 GWh) of the decrease in production. [6]