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Climate models project a much more substantial warming than the 2 °C target under the more probable emission scenarios, making higher-end scenarios increasingly plausible. Freshwater availability under such conditions is a key issue of concern. In this study, an ensemble of Euro-CORDEX projections under RCP8.5 is used to assess the mean and low hydrological states under C4 °C of global warming for the European region. Five major European catchments were analysed in terms of future drought climatology and the impact of C2 °C versus C4 °C global warming was investigated. The effect of bias correction of the climate model outputs and the observations used for this adjustment was also quantified. Projections indicate an intensification of the water cycle at higher levels of warming. Even for areas where the average state may not considerably be affected, low flows are expected to reduce, leading to changes in the number of dry days and thus drought climatology. The identified increasing or decreasing runoff trends are substantially intensified when moving from the C2 to the C4° of global warming. Bias correction resulted in an improved representation of the historical hydrology. It is also found that the selection of the observational data set for the application of the bias correction has an impact on the projected signal that could be of the same order of magnitude to the selection of the Global Climate Model (GCM). [ABSTRACT FROM AUTHOR]

European agriculture and water policies require accurate information on climate change impacts on available water resources. Water accounting, that is a standardized documentation of data on water resources, is a useful tool to provide this information. Pan-European data on climate impacts do not recognize local anthropogenic interventions in the water cycle. Most European river basins have a specific toolset that is understood and used by local experts and stakeholders. However, these local tools are not versatile. Thus, there is a need for a common approach that can be understood by multi-fold users to quantify impact indicators based on local data and that can be used to synthesize information at the European level. Then, policies can be designed with the confidence that underlying data are backed-up by local context and expert knowledge. This work presents a simplified water accounting framework that allows for a standardized examination of climate impacts on water resource availability and use across multiple basins. The framework is applied to five different river basins across Europe. Several indicators are extracted that explicitly describe green water fluxes versus blue water fluxes and impacts on agriculture. The examples show that a simplified water accounting framework can be used to synthesize basin-level information on climate change impacts which can support policymaking on climate adaptation, water resources and agriculture. [ABSTRACT FROM AUTHOR]

Anthropogenic climate change as well as human water and land management are expected to impact the world's freshwater resources and hydrological extremes. However, model estimates are imperfect, leaving many open questions regarding the sign and the magnitude of the associated impacts. Likewise, relevant in-situ observations have incomplete spatio-temporal coverage and are often not sufficient to disentangle the effects of anthropogenic climate change and land management from natural variability and observational errors. The recently established Global Streamflow Indices and Metadata archive (GSIM) includes observations from more than 30000 stations and hence provides an unprecedented opportunity to investigate observed trends in indicators of low, mean and high streamflow at the global scale. Analysing the GSIM reveals complex spatial patterns of global hydrological change, where some regions (e.g. the Mediterranean) are getting significantly dryer, while others (e.g. North Europe) have been subject to wetting. Notably, these changes are often consistent throughout the entire flow distribution, implying that increasing drought risk might be compensated by decreasing flood magnitudes. To investigate whether human water management or anthropogenic climate change are driving this change pattern, detection and attribution methods are used. These methods exploit ensembles of factorial global model experiments in which the hypothesised agents of change (e.g. greenhouse gas emissions, human water management) are systematically switched on and off. Simulations from the ISIMIP2b ensembles are considered which combines the versatility of global hydrology models (GHM) with state of the art global climate model (GCM) runs. The results highlight that the observed global-scale trend pattern cannot be explained by natural variability alone, as estimated by driving GHMs with GCM runs forced with pre-industrial greenhouse gas levels. Adding water management (e.g. irrigation, flood regulation) to the GHMs is also not sufficient for capturing the observed trend patterns. However, trend patterns derived from simulations that account for both water management (through GHMs) and historical greenhouse gas emissions (through GCMs) do qualitatively capture the observed streamflow trend patterns at the global scale. Repeated analysis of the data using correlation based methods and optimal fingerprinting suggests that this outcome is most stable for low-flow indicators (p<0.1) while the results are less stable for mean and high flows. The results highlight that the combined effect of human water management and ongoing anthropogenic climate change is already detectable in freshwater resources at continental to global scale, thereby increasing confidence in future projections of the world's freshwater resources. [ABSTRACT FROM AUTHOR]