Climate Change and the European Water Dimension - Agri ...

Large, global scale climatic drivers cause droughts. As a result droughts affect large
areas and tend to continue for several seasons – causing “clusters” of drought years.
Ultimately these systems can produce exceptionally protracted rainfall deficiencies
such as occurred over much of western Europe during the late nineteenth and early
twentieth century (Figure V.B.1) (Thomsen, 1993). There have been no close recent
parallels to drought episodes of such duration but the 1975/76 drought was of
unprecedented intensity over parts of Europe (from western Germany to the English
Midlands) (Doorkamp et al., 1980) resulting in substantial agricultural and industrial
losses and environmental stress. However, the event was perceived, at the time, to
be extremely rare – with return periods exceeding 100 years in many regions. As a
consequence, it did not provide a sufficiently strong stimulus for the development of
cross-sectoral coping strategies appropriate for the potential increases in drought
magnitude as global warming intensifies.
These periods of drought (and indeed floods at the other end of the flow regime)
show clear patterns, alternating between drought-rich and drought-poor times. See
for example, in Figure V.B.1, the drought period of the 1880-1900’s replaced by
wetter conditions around 1915. These sequences occur due to natural climate
variability. Currently there is no statistically significant signal in this historical data to
suggest a trend towards an increased frequency or magnitude of extremes. Natural
variability (at all time scales) will also change with global warming over the next
century, but there is much uncertainty associated with aspects of current climate
change scenarios.
Figure V.B.1.. The derivation of winter rainfall (November through March) from the
Figure V.B.1. The derivation of winter rainfall (November through March)
V.B.4. Water and Drought Management
Reservoirs, rivers and aquifers are sustained not by rainfall directly but by that
proportion which remains after evaporative demands have been met. Evaporation
losses are concentrated in the summer half-year and impose a strong seasonality on
river flows and groundwater replenishment. On average around three-quarters of
Europe’s rainfall is lost to evaporation, the proportion increasing to >90% in the driest
regions. Thus, variability in river flows and groundwater replenishment is normally
much greater than for rainfall, with many parts of Europe being vulnerable to even
small decreases in rainfall. Evaporation losses mean that relatively minor rainfall
deficiencies can translate into large deficiencies in runoff and aquifer replenishment.
Adequate river flow and groundwater monitoring networks are therefore essential to
assess any climate-induced changes in drought episodes.
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