Evaluation of water temperature under changing climate and its effect on river habitat in a regulated Alpine catchment

Habitat quality of alpine river is largely affected by human activity. The exploitation for hydropower, combined with anthropogenic climate change, can alter mountain riverine ecosystems, leading to less suitable hydrothermal regimes for the fish. Here, we present a new methodology to assess water temperature within a river featuring water exploitation for hydropower purposes, usable to assess future potential deterioration of riverine habit suitability in response to (increasing) water temperature. We then propose an application focusing upon the case study of the Serio River, in Northern Italy, largely exploited by hydropower productions and highly populated by a very sensitive species, brown trout (Salmo trutta). The methodology proposed involves a set of tools, i.e. i) the hydrological model Poli-Hydro, to evaluate natural hydrological regime, ii) a hydropower plants scheme to assess river water withdrawal, iii) fish density-environment curves to evaluate the hydraulic suitability in terms of trout potential density for adult, young, and fry as a function of hydraulic features, i.e. depth and velocity, and iv) a new, physically based model, Poli-Wat.Temp, to assess changes in river water temperature, and possible outbreaks of temperature dependant lethal conditions, such as proliferative kidney disease, and others. To provide an assessment of river suitability, possibly complementing (improving?) models based upon solely hydraulic indexes, we propose a new synthetic River Stress index, combining i) potential fish density as driven by hydraulic variables, and ii) thermal suitability. Given that utmost unsuitable conditions (thermally, and likely hydrologically) are expected under future climate conditions pending global warming, we then projected water temperature, and stream flows until the end of the century, in response to socio-economic scenarios of AR6 of the IPCC, to explore the potential for future decrease of river quality. Water temperature would be largely susceptible to climate change with increase up to + 6.5C degrees in the worst scenarios, while no clear trend is observed for fish density. Overall, potential density would decrease in winter for adults, and in summer for juvenile and fry in downstream sections. Therefore, by coupling hydraulic, and thermal suitability, one finds that i) Alpine rivers would likely face longer critical periods, with respect to those predicted based upon a solely hydraulic habitat based assessment, and ii) continuous temperature increase as projected until the end of the century would result into worse conditions in summer months, seriously endangering fish guilds.