INSIGHTS | PERSPECTIVES
By Louise J. Slater and Robert L. Wilby
River flood risks are expected to rise as climate change intensifies the global hydrological cycle and more people live in floodplains (1). Changing risk may be revealed by trends in flood frequency, magnitude, or seasonality,
as well as by shifts in the mechanisms that
generate inundations (2). However, detection
and attribution of climate signals in flood records is often hampered by brief, incomplete,
or poor-quality flood data (3). Additionally, it
can be difficult to disentangle the effects of
changing climate, land cover, channel morphology, and human activities (2, 4). On page
588 of this issue, Blöschl et al. (5) overcome
these problems through a consistent pan-European assessment of observed flood seasonality trends between 1960 and 2010. They
thus provide the first evaluation of how climatic changes are influencing flood regimes
at the continental scale.
Rather than applying a conventional analysis of flood magnitudes or frequency, the
authors use flood seasonality, a more sensitive metric for detecting climate signals in
flood regimes (6). They track changes in the
timing of the largest annual floods over five
decades, using a database that pools flood
records from more than 4000 riverine gauging stations across 38 European countries (7).
The results show substantial changes in flood
timing across Europe. Floods now occur up
to several weeks earlier than in the 1960s in
areas of Northeastern and Western Europe
due to earlier snowmelt and soil moisture
maxima, respectively. Possible reasons are
higher air temperatures, reduced influence of
arctic air masses, and changing time scales of
subsurface water storage. In contrast, floods
now occur several weeks later in parts of the
North Sea and Mediterranean Europe, likely
due to changes in the North Atlantic Oscillation and increasing atmospheric flow from
the Atlantic in winter, respectively.
Without adaptations, such changes in
flood seasonality can profoundly affect agricultural yields, infrastructure safety and
operation, hydropower production, water
supplies, and water management. The study
thus prompts important questions about
flood predictability. For example, how have
human activities affected the climate variations that contribute to the observed flood
seasonality changes? How might changing
seasonality manifest in multibasin flood
signatures? And how might changes in
flood-generating mechanisms affect future
patterns of severe flooding (8)?
Over decadal and centennial time scales,
variability in hydrologic, ecological, and
even geomorphic systems will reflect large-
scale climate conditions (9, 10). However,
the interconnectedness of climate and hy-
drologic regimes is complicated by lag times
in fluvial systems and the dynamic behavior
of river networks. Following sequences
of wet or dry months, years, or decades,
groundwater levels fluctuate, increasing or
decreasing the spatial extent of river net-
works and thus the likelihood of flooding
(11). Climate may also affect flood season-
ality indirectly through changes to wildfire
regimes and land cover or by dust loads in
snow that decrease snow albedo and alter
runoff timing (12).
These natural flood-generating mechanisms do not act in isolation of human
land-surface modifications and river regulation. Further research is needed to develop
more robust attribution methods (13) that
can discern the relative contribution of different flood-generating mechanisms (2, 4).
This includes going beyond existing trend
detection procedures (4, 6) and developing more holistic analytical frameworks for
linking flood-rich and flood-poor episodes
to climate, land cover, river networks, and
Whether global river monitoring networks are up to this task is questionable, as
hydrometric records and associated metadata are not always quality controlled or
publicly accessible. But given that flood risk
could rise 20-fold by the end of the 21st century (14), better metrics to describe the vital
signs of shifting flood regimes (5) are a step
in the right direction. j
1. C. B. Field et al ., Eds., Managing the Risks of Extreme
Events and Disasters to Advance Climate Change
Adaptation: Special Report of the Intergovernmental Panel
on Climate Change (Cambridge Univ. Press, Cambridge,
2. W. R. Berghuijs, R. A. Woods, C. J. Hutton, M. Sivapalan,
Geophys. Res. Lett. 43, 4382 (2016).
3. R. L. Wilby etal. , WileyInterdiscip.Rev.Water4, e1209
4. L. J. Slater, M. B. Singer, J. W. Kirchner, Geophys. Res. Lett.
42, 370 (2015).
5. G. Blöschl et al., Science 357, 588 (2017).
6. R. Merz, G. Blöschl, Water Resour. Res .39, 1340 (2003).
7. J. Hall et al. , Proc. Int. Assoc. Hydrol. Sci .370, 89 (2015).
8. R. Rojas, L. Feyen, A. Bianchi, A. Dosio, J. Geophys. Res.
Atmos. 117, D17109 (2012).
9. A. Sankarasubramanian, U. Lall, Water Resour. Res. 39, 1
10. I.Mallakpour, G.Villarini, Adv. Water Resour. 92,159
11. S.E.Godsey,J. W.Kirchner, Hydrol. Process. 28,5791
12. T.H.Painter et al., Proc. Natl. Acad. Sci. U.S.A. 107,17125
13. S.Harrigan, C.Murphy,J.Hall,R.L. Wilby,J.Sweeney,
Hydrol. Earth Syst. Sci. 18, 1935 (2014).
14. H. C. Winsemius et al. , Nat. Clim. Chang. 6, 381 (2015).
Measuring the changing pulse of rivers
A 50-year data set shows changes in the seasonal timing of river floods in Europe
Department of Geography, Loughborough University,
Loughborough, UK. Email: email@example.com
In August 2002, Meissen, Germany, was flooded as the Elbe River rose to its highest point in a millennium.