The historical and future effects of Environmental Flow Requirements on hydrologic indicators and water scarcity in the Danube River Basin

Abstract

Extreme low-flow conditions have a detrimental effect on aquatic life in freshwater systems. The chance at low-flow conditions is expected to grow as a result of climate change and increased human demand. The European Union is advising its member states to implement Environmental Flow Requirements (EFRs). The Danube, with its great importance and size, is a fitting case study for the implementation of EFRs. This study used the PCR-GLOBWB 2 model, at 5” arcmin scale, to analyse the effectiveness of EFRs based on changes of hydrologic parameters and the impact on water scarcity. This was done for a monthly timeframe to determine the influence of seasonality. Two different EFRs have been studied: the Variable Monthly Flow and the 90th exceedance percentile. The WATCH dataset supplied validated data to study the changes in water scarcity levels and hydrologic parameters as a result of EFR inclusion for the period from 1960 till 2000. Subsequently, the model was forced precipitation and evapotranspiration data from 5 bias-corrected Global Circulation Models, for the RCP 6.0 scenario, in order to study EFR effects on the near (2010-2050) and far (2060-2100) future. The results show an improvement in drought related indicators, namely minimal annual flow and extreme low-flow (Q95), with the implementation of EFRs with the WATCH Forcing Data. This improvement is most distinct in the Hungarian low-lands where without regulations abstractions are responsible for a significant decrease in IHA performance. As water is reserved for the environment, competition over available water resources is enhanced. The EFR definition can have a significant impact on the temporal and spatial distribution of periods of water scarcity. Model runs based on GCM forcing did not show significant IHA improvement related to EFR inclusion, but water scarcity was is expected to exacerbate with time.