How do environmental flows impact on water availability under climate change scenarios in European basins?

Environmental flows (Qeco) facilitate a good ecological status of fluvial ecosystems, but they usually represent a constraint for water uses. Qeco flow regime should not only be based on the minimum flows, but it should also account their variability. It is expected that climate change impact on some hydrological systems diminishing the natural water resources and stressing the river ecosystems. In this context, the balance between ecosystems conservation and human water needs becomes even more difficult to manage. We performed a comprehensive analysis over European territory to assess the behaviour of basins regarding different criteria for environmental flow determination under climate change scenarios. We used a water allocation model, WAAPA, to estimate the water availability (WA). In this study, WA represents the maximum demand that can be supplied at a certain point of the river network with a given reliability criteria, considering drinking and irrigation water supply. We considered two methods for calculating Qeco, Qeco1 based on mean monthly flow (MMF) and Qeco2 based on mean annual runoff (MAF). We analyzed the current scenario (historical from 1960 to 2000) and 40 future projections, which combine short and long term (from 2020 to 2059, and from 2060 to 2099, respectively), four emission scenarios (RCP2.6 to RCP8.5) and five climate models. Expected changes on MAF due to climate change are not uniform through Europe and also vary regarding the specific climate scenario. >70 % of basins show a trend to reduce their MAF under severe emission scenarios. Conservative values of Qeco represent a heavy constraint for WA and stress the water systems similarly than climate change impacts. The study also highlights that regulation capacity helps on buffering the effects of both climate change and environmental requirements. This study provides a good insight for understanding basin response in terms of WA, regarding environmental criteria and climate change effects.