River flow (CLIM 016) - Assessment published Nov 2012

Key policy question: What is the trend in mean river flow in different seasons across Europe?

Trends in monthly stream flow for the period 1962-2004

Note: Trend given as standard deviations per year for monthly stream flow. Red colours mark decreases in stream flow whereas blue colours mark increases in stream flow.

Data source:

• Flow Regime for International Experiment and Network Data (FRIEND) based on Unesco's European Water Archive (EWA) provided by Flow Regimes from International Experimental and Network Data (FRIEND Network)

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Projected change in average annual and seasonal river flow

Note: Projected change in mean annual and seasonal river flow between the climate change scenario (SRES A1B, 2071-2100) and the control period (1961-1990). Simulations with LISFLOOD based on an ensemble of 11 RCMs.

Data source:

• Assessment of Future Flood Hazard in Europe Using a Large Ensemble of Bias Corrected Regional Climate Simulations provided by American Geophysical Union (AGU)

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Projected change in daily average river flow for four rivers

Note: Projected river flow 2071-2100 (green line) and the observed river flow 1961-1990 (orange line)

Data source:

• Climate change impact assessment provided by Joint Research Centre (JRC)

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Key assessment

Past trends

Human interventions in catchments including water abstractions, river regulation and land-use change have considerably altered river flow regimes in large parts of Europe, making it difficult to discern any climate-driven changes in river flow to date. However, a comprehensive recent study has investigated time series of river flows in more than 400 small catchments with near-natural flow regimes to overcome these limitations [i]. The study finds indicate that annual river flow has generally decreased over the period 1962 – 2004 in southern and eastern Europe, and it has increased elsewhere. These findings are broadly consistent results from earlier studies (e.g. [ii]. Seasonal changes are also apparent, with a decreased flow in summer months and an increase in winter months in most catchments (see Figure 1). Similar results were found in national and regional studies [iii].

The magnitude of the observed seasonal changes clearly raises concerns for water resource management both today and in future decades. To date, however, despite the evidence of monthly changes to flow, there is no conclusive evidence that low river flows have generally become more severe or frequent in Europe during recent decades [iv].

Projections

Annual river flow is projected to decrease in southern and south-eastern Europe and increase in northern and north-eastern Europe [v]. Strong changes are projected in the seasonality of river flows, with large differences across Europe. Winter and spring river flows are projected to further increase in most parts of Europe, except for the most southern and south-eastern regions, which would exacerbate the observed trend. In summer and autumn, river flows are projected to decrease in most of Europe, except for northern and north-eastern regions where they are projected to increase (see Figure 2) [vi]. Such a trend cannot be seen that clear in the observed monthly stream flow for the period 1962-2004 (Figure 1).

In snow dominated regions, such as the Alps, Scandinavia and parts of the Baltic, the fall in winter retention as snow, earlier snowmelt and reduced summer precipitation is projected to increase river flows in winter and reduce them in summer, when demand is typically highest [vii]. For most parts of Europe the peak of the average daily flow for 2071-2100 is projected to occur earlier in the year compared to observations. For Northern Europe a slight increase of the peak of average daily flow is projected compared to a decrease in the other stations evaluated (see Figure 3).

[i] K. Stahl et al., „Streamflow trends in Europe: evidence from a dataset of near-natural catchments“, Hydrology and Earth System Sciences 14, Nr. 12 (Dezember 1, 2010): 2367–2382, doi:10.5194/hess-14-2367-2010.

[ii] P. C. D. Milly, K. A. Dunne, and A. V. Vecchia, „Global Pattern of Trends in Streamflow and Water Availability in a Changing Climate“, Nature 438, Nr. 7066 (November 17, 2005): 347–350, doi:10.1038/nature04312.

[iii] Marius-Victor Birsan et al., „Streamflow trends in Switzerland“, Journal of Hydrology 314, Nr. 1–4 (November 2005): 312–329, doi:10.1016/j.jhydrol.2005.06.008; Donna Wilson, Hege Hisdal, and Deborah Lawrence, „Has streamflow changed in the Nordic countries? – Recent trends and comparisons to hydrological projections“, Journal of Hydrology 394, Nr. 3–4 (November 26, 2010): 334–346, doi:10.1016/j.jhydrol.2010.09.010.

[iv] K. Stahl et al., Trends in Low Flows and Streamflow Droughts Across Europe (Paris: UNESCO, 2008), http://library.wur.nl/WebQuery/wurpubs/374150.

[v] Milly, Dunne, and Vecchia, „Global Pattern of Trends in Streamflow and Water Availability in a Changing Climate“; Joseph Alcamo, Martina Flörke, and Michael Märker, „Future long-term changes in global water resources driven by socio-economic and climatic changes“, Hydrological Sciences Journal 52, Nr. 2 (April 2007): 247–275, doi:10.1623/hysj.52.2.247; Rutger Dankers and Luc Feyen, „Flood hazard in Europe in an ensemble of regional climate scenarios“, Journal of Geophysical Research 114, Nr. D16 (August 27, 2009), doi:10.1029/2008JD011523.

[vi] R. Rojas et al., „Assessment of Future Flood Hazard in Europe Using a Large Ensemble of Bias Corrected Regional Climate Simulations“, Journal of Geophysical Research Nr. in press (2012), doi:10.1029/2012JD017461.

[vii] Martin Beniston, Markus Stoffel, and Margot Hill, „Impacts of climatic change on water and natural hazardsin the Alps: Can current water governance cope with future challenges? Examples from the European ‘‘ACQWA’’ project“, Environmental Science and Policy 14, Nr. 7 (2011): 734–743, doi:10.1016/j.envsci.2010.12.009; BAFU, Auswirkungen der Klimaänderung auf Wasserressourcen and Gewässer: Synthesebericht zum Projekt «Klimaänderung and Hydrologie in der Schweiz» (CCHydro) (Bern: Bundesamt für Umwelt, 2012).