On quantifying ecologically sustainable flow releases in a diverted river reach

Water demand for hydropower production has been increasing along with a growing awareness of the importance of preserving riparian ecosystems and their biodiversity. Some Cantons in Switzerland have begun replacing the inadequate concept of Minimum Flow Requirement (MFR) with a dynamic one, by continuously releasing a percentage of the total inflow. In this work, we evaluate both ecological and economical benefits of dynamic release policies within a diverted river reach in the Swiss Canton of Graubünden. We compare such policies to another one that generates inflow-dependent variable releases as a result of an economic competition between traditional (e.g., hydropower) and non-traditional (e.g., environment) water uses (). We propose to compute flow statistics from different release polices by using Indicators of Hydrologic Alteration (). Then, we aggregate the hydrological differences from the natural flow regime as a proxy for assessing environmental benefits and we look at the mean of the ratio of the allocated net flows between environment and hydropower as a suitable engineering parameter to represent their relative value. Eventually all the simulated release policies find an economical significance explained by marginal utility functions. We show that dynamic redistribution policies can perform better than MFR-like ones. Moreover, by introducing the concept of inflow-dependent water allocation, both economic and ecological indicators can be further improved, without necessarily implying higher installation costs. This method aims to provide a simple but effective step towards eco-sustainability in the growing market of mini hydropower plants, where MFR-rules are still widespread.