Climate change, impacts and vulnerability in Europe ... - MemoFin.fr

Climate impacts on environmental systems3.3.2 River flowRelevanceRiver flow is a measure of overall fresh wateravailability in a river basin. Variations in riverflow are determined mainly by the seasonalityof precipitation and temperature, as well as bycatchment characteristics such as geology, soiland land cover. Changes in temperature andprecipitation patterns due to climate changemodify the annual water budget of river basins aswell as the timing and seasonality of river flows.The consequent changes in water availabilitymay adversely affect ecosystems and severalsocio‐economic sectors including abstractionfor drinking water, agriculture, industry, energyproduction and navigation. Extreme dry periodswith low river flow events can have considerableeconomic, societal and environmental impacts(see Section 3.3.4).Past trendsHuman interventions in catchments includingwater abstractions, river regulation and land-usechange have considerably altered river flow regimesin large parts of Europe, making it difficult todiscern any climate-driven changes in river flow todate. However, a comprehensive recent study hasinvestigated time series of river flows in more than400 small catchments with near-natural flow regimesto overcome these limitations (Stahl et al., 2010).The study finds indicate that annual river flow hasgenerally decreased over the period 1962–2004 insouthern and eastern Europe, and it has increasedelsewhere. These findings are broadly consistentwith results from earlier studies (e.g. (Milly et al.,2005). Seasonal changes are also apparent, with adecreased flow in summer months and an increasein winter months in most catchments (see Map 3.5).Similar results were found in national and regionalstudies (Birsan et al., 2005; Wilson et al., 2010).The magnitude of the observed seasonal changesclearly raises concerns for water resourcemanagement both today and in future decades.To date, however, despite the evidence of monthlychanges to flow, there is no conclusive evidence thatlow river flows have generally become more severeor frequent in Europe during recent decades (Stahlet al., 2008).ProjectionsAnnual river flow is projected to decrease insouthern and south-eastern Europe and increasein northern and north-eastern Europe (Milly et al.,2005; Alcamo et al., 2007; Dankers and Feyen, 2009).Strong changes are projected in the seasonality ofriver flows, with large differences across Europe.Winter and spring river flows are projected tofurther increase in most parts of Europe, exceptfor the most southern and south-eastern regions,which would exacerbate the observed trend. Insummer and autumn, river flows are projected todecrease in most of Europe, except for northern andnorth‐eastern regions where they are projected toincrease (Map 3.6) (Rojas et al., 2012). Such a trendcannot be seen that clear in the observed monthlystream flow for the period 1962–2004 (Map 3.5).In snow-dominated regions, such as the Alps,Scandinavia and parts of the Baltic, the fall in winterretention as snow, earlier snowmelt and reducedsummer precipitation is projected to increase riverflows in winter and reduce them in summer, whendemand is typically highest (Beniston et al., 2011;BAFU, 2012). For most parts of Europe the peak ofthe average daily flow for 2071–2100 is projected tooccur earlier in the year compared to observations.For northern Europe a slight increase of the peakof average daily flow is projected comparedto a decrease in the other stations evaluated(see Figure 3.9).Key messages: 3.3.2 River flow• Long-term trends in river flows due to climate change are difficult to detect due to substantialinterannual and decadal variability as well as modifications to natural water flows arising from waterabstractions, man‐made reservoirs and land-use changes. Nevertheless, increased river flows duringwinter and lower river flows during summer have been recorded since the 1960s in large parts ofEurope.• Climate change is projected to result in strong changes in the seasonality of river flows across Europe.Summer flows are projected to decrease in most of Europe, including in regions where annual flows areprojected to increase.114 Climate change, impacts and vulnerability in Europe 2012