Climate change impacts and vulnerability in Europe 2016

Climate change impacts on environmental systems
4.3.2 River flows
Key messages
• Available studies suggest that run-off in near-natural rivers during the period 1963–2000 increased in western and
northern Europe, in particular in winter, and decreased in southern and parts of eastern Europe, in particular in summer.
However, comprehensive observation data on river flows are not available across Europe.
• Long-term trends in river flows due to climate change are difficult to detect because of substantial interannual and
decadal variability, as well as modifications to natural water flows arising from water abstractions, morphological changes
(such as man-made reservoirs) and land-use changes.
• Climate change is projected to result in significant 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 are projected to increase. Where
precipitation shifts from snow to rain, spring and summer peak flow will shift to earlier in the season.
Relevance
Annual average river flows are one of the elements that
affect freshwater availability in a river basin, in addition
to groundwater sources, lakes or artificial water storage
facilities. Variations in river flows are determined
mainly by the seasonality of precipitation and
temperature, as well as by catchment characteristics
such as geology, soil and land cover. Changes in
temperature and precipitation patterns due to climate
change modify the annual water budget of river basins
and the timing and seasonality of river flows. The
consequent changes in water availability may adversely
affect ecosystems and several socio-economic sectors,
including abstraction for drinking water, agriculture,
industry, energy production and navigation. Extreme
dry periods with low river flow events can have
considerable economic, societal and environmental
impacts (see Section 4.3.4).
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 data. An
inventory of river flows in Europe was produced by
combining over 400 time series (from 1962 to 2004)
of river catchments with near-natural flow conditions
for Europe and an ensemble of eight large-scale
hydrological models (for 1963–2000) (Stahl et al.,
2012). According to this inventory, run-off showed
positive trends in western and northern Europe and
negative trends in southern and parts of eastern
Europe (Map 4.7). The European pattern of annual
run-off trends modelled by the ensemble mean shows
a regionally coherent picture. The areas where models
disagreed on the trend direction were largely located
in areas of weak trends, notably in the transition
areas between regions with consistent negative and
positive trends. The pattern of changes in regional
high flows is very similar to the pattern of changes in
annual flows, whereas summer low flows have also
decreased in various regions where annual flows have
increased. Overall, positive trends in annual stream
flow appear to reflect the marked wetting trends of
the winter months, whereas negative annual trends
result primarily from a widespread decrease in
stream flow in spring and summer months, consistent
with a decrease in summer low flow in large parts
of Europe. The model uncertainties were largest in
complex terrain with high spatial variability and in
snow‐dominated regimes.
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 changes in the seasonality
of flows, there is no conclusive evidence that low
river flows have generally become more severe or
frequent in Europe during recent decades (Stahl et al.,
2010, 2012). Whereas many studies detect significant
hydrological changes in observed datasets, more
scientific rigour is needed in the attribution of river flow
changes, as these studies often fall short in proving and
quantifying the relationship between these changes
and potential drivers (Merz et al., 2012).
Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report
137