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Changing climate shifts timing of European floods

Authors:
  • Meteoceanics Institute for Complex System Science

Abstract

A warming climate is expected to have an impact on the magnitude and timing of river floods; however, no consistent large-scale climate change signal in observed flood magnitudes has been identified so far. We analyzed the timing of river floods in Europe over the past five decades, using a pan-European database from 4262 observational hydrometric stations, and found clear patterns of change in flood timing. Warmer temperatures have led to earlier spring snowmelt floods throughout northeastern Europe; delayed winter storms associated with polar warming have led to later winter floods around the North Sea and some sectors of the Mediterranean coast; and earlier soil moisture maxima have led to earlier winter floods in western Europe. Our results highlight the existence of a clear climate signal in flood observations at the continental scale. http://science.sciencemag.org/content/357/6351/588
FLOODING
Changing climate shifts timing of
European floods
Günter Blöschl,
1
*Julia Hall,
1
Juraj Parajka,
1
Rui A. P. Perdigão,
1
Bruno Merz,
2
Berit Arheimer,
3
Giuseppe T. Aronica,
4
Ardian Bilibashi,
5
Ognjen Bonacci,
6
Marco Borga,
7
Ivan Čanjevac,
8
Attilio Castellarin,
9
Giovanni B. Chirico,
10
Pierluigi Claps,
11
Károly Fiala,
12
Natalia Frolova,
13
Liudmyla Gorbachova,
14
Ali Gül,
15
Jamie Hannaford,
16
Shaun Harrigan,
16
Maria Kireeva,
13
Andrea Kiss,
1
Thomas R. Kjeldsen,
17
Silvia Kohnová,
18
Jarkko J. Koskela,
19
Ondrej Ledvinka,
20
Neil Macdonald,
21
Maria Mavrova-Guirguinova,
22
Luis Mediero,
23
Ralf Merz,
24
Peter Molnar,
25
Alberto Montanari,
9
Conor Murphy,
26
Marzena Osuch,
27
Valeryia Ovcharuk,
28
Ivan Radevski,
29
Magdalena Rogger,
1
José L. Salinas,
1
Eric Sauquet,
30
Mojca Šraj,
31
Jan Szolgay,
18
Alberto Viglione,
1
Elena Volpi,
32
Donna Wilson,
33
Klodian Zaimi,
34
Nenad Živković
35
A warming climate is expected to have an impact on the magnitude and timing of river
floods; however, no consistent large-scale climate change signal in observed flood
magnitudes has been identified so far. We analyzed the timing of river floods in Europe
over the past five decades, using a pan-European database from 4262 observational
hydrometric stations, and found clear patterns of change in flood timing. Warmer
temperatures have led to earlier spring snowmelt floods throughout northeastern
Europe; delayed winter storms associated with polar warming have led to later winter
floods around the North Sea and some sectors of the Mediterranean coast; and
earlier soil moisture maxima have led to earlier winter floods in western Europe. Our
results highlight the existence of a clear climate signal in flood observations at the
continental scale.
River flooding affects more people world-
wide than any other natural hazard, with
an estimated global annual average loss of
US $104 billion (1). Such damages are ex-
pected to increase as a result of continued
economic growth and climate change (2,3). The
intensification of the water cycle due to a warm-
ing climate is projected to change the magnitude,
frequency, and timing of river floods (3). How-
ever, existing studies have been unable to iden-
tify a consistent climate change signal in flood
magnitudes (4). Identification of a large-scale
climate change signal in flood observations has
been hampered by the existence of many pro-
cesses controlling floods, including precipitation,
soil moisture, and snow; by nonclimatic drivers
of flood change, such as land use change and
river training; and by the inconsistency of data
sets and their limited spatial extents (4,5). Use of
the seasonal timing of floods as a fingerprint of
climate effects on floods may be a way to avoid
some of those complications (6,7). For example,
in cold regions, earlier snowmelt due to warmer
temperatures leads to earlier spring floods (6), and
this climate-related signal may be less confounded
by nonclimatic drivers than flood magnitudes
themselves because of the strong seasonality of
climate. The changing timing of floods has been
studied at local scale in Nordic and Baltic countries
(810), but no consistent analysis exists at the
European scale.
Here, we analyzed a large data set of flood ob-
servations in Europe to assess whether a changing
climate has shifted the timing of river floods
during the past five decades. Our analysis is based
on river discharge or water-level observations
from 4262 hydrometric stations in 38 European
countries for the period 19602010 (table S1). For
each station, we use a series consisting of the
dates of occurrence of the highest peak in any
calendar year. We define the average timing of
the floods by the average date on which floods
have occurred during the observation period.
We then use the Theil-Sen slope estimator (11)
to estimate the trend in the timing of the floods
for stations with at least 35 years of data, and
apply a 10-year moving-average filter to estimate
the long-term evolution. Finally, we analyze the
change signal of three potential drivers of flood
changesinasimilarfashion:(i)themiddledate
of the maximum 7-day precipitation; (ii) the middle
day of the month with the highest soil moisture;
and (iii) the middle day of the first 7 days in a year
with air temperature above 0°C as a proxy for
spring snowmelt and snowfall-to-rain transition
(see supplementary materials).
Our data show a clear shift in the timing of
floods in Europe during the past 50 years (Fig. 1).
The regionally interpolated trend patterns shown
in Fig. 1 range from 13 days per decade toward
earlier floods to +9 days toward later floods, which
translates into total shifts of 65 and +45 days,
respectively, of linear trends over the entire 50-year
period. The local, station-specific, trends (fig. S2)
are larger, but these trend sizes reflect smaller-
scale rather than regional-scale processes. The
changes are most consistent in northeastern Europe
(Fig. 1, region 1), where 81% of the stations show a
shift toward earlier floods (50% of the stations
by more than 8 days per 50 years) (fig. S2). The
changes are largest in western Europe along the
North Atlantic coast from Portugal to England
(region 3), where 50% of the stations show a shift
toward earlier floods by at least 15 days per
50 years (25% of the stations by more than 36 days
per 50 years). Around the North Sea (region 2;
southwestern Norway, the Netherlands, Denmark,
and Scotland), 50% of the stations show a shift
towardlaterfloodsbymorethan+8daysper
50 years. In some parts of the Mediterranean coast
(region 4; northeastern Adriatic coast, northeast-
ern Spain), there is a shift toward later floods (50%
of the stations by more than +5 days per 50 yea rs).
Apart from the large-scale change patterns described
RESEARCH
Blöschl et al., Science 357, 588590 (2017) 11 August 2017 1of3
1
Institute of Hydraulic Engineering and Water Resources Management, Technische Universität Wien, Vienna, Austria.
2
Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences,
Potsdam, Germany.
3
Swedish Meteorological and Hydrological Institute, Norrköping, Sweden.
4
Department of Engineering, University of Messina, Messina, Italy.
5
Control Systems Engineering,
Renewable Energy Systems & Technology, Tirana, Albania.
6
Faculty of Civil Engineering, Architecture and Geodesy, Split University, Split, Croatia.
7
Department of Land, Environment, Agriculture
and Forestry, University of Padova, Padua, Italy.
8
Department of Geography, Faculty of Science, University of Zagreb, Zagreb, Croatia.
9
Department of Civil, Chemical, Environmental and
Materials Engineering (DICAM), Università di Bologna, Bologna, Italy.
10
Department of Agricultural Sciences, University of Naples Federico II, Naples, Italy.
11
Department of Environment, Land and
Infrastructure Engineering (DIATI), Politecnico di Torino, Turin, Italy.
12
Lower Tisza District Water Directorate, Szeged, Hungary.
13
Department of Land Hydrology, Lomonosov Moscow State
University, Moscow, Russia.
14
Department of Hydrological Research, Ukrainian Hydrometeorological Institute, Kiev, Ukraine.
15
Department of Civil Engineering, Dokuz Eylul University, Izmir,
Turkey.
16
Centre for Ecology & Hydrology, Wallingford, Oxfordshire, UK.
17
Department of Architecture and Civil Engineering, University of Bath, Bath, UK.
18
Department of Land and Water
Resources Management, Faculty of Civil Engineering, Slovak University of Technology, 810 05 Bratislava, Slovakia.
19
Finnish Environment Institute, Helsinki, Finland.
20
Czech Hydrometeorological
Institute, Prague, Czechia.
21
Department of Geography and Planning & Institute of Risk and Uncertainty, University of Liverpool, Liverpool, UK.
22
University of Architecture, Civil Engineering and
Geodesy, Sofia, Bulgaria.
23
Department of Civil Engineering: Hydraulic, Energy and Environment, Technical University of Madrid, Madrid, Spain.
24
Department for Catchment Hydrology, Helmholtz
Centre for Environmental ResearchUFZ, Halle, Germany.
25
Institute of Environmental Engineering, ETH Zürich, Zürich, Switzerland.
26
Irish Climate Analysis and Research Units (ICARUS),
Department of Geography, Maynooth University, Ireland.
27
Department of Hydrology and Hydrodynamics, Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland.
28
Hydrometeorological Institute, Odessa State Environmental University, Odessa, Ukraine.
29
Institute of Geography, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius
University, Skopje, Republic of Macedonia.
30
Irstea, UR HHLY, Hydrology-Hydraulics Research Unit, Lyon, France.
31
Faculty of Civil and Geodetic Engineering, University of Ljubljana, Ljubljana,
Slovenia.
32
Department of Engineering, University Roma Tre, Rome, Italy.
33
Norwegian Water Resources and Energy Directorate, Oslo, Norway.
34
Institute of Geo-Sciences, Energy, Water and
Environment, Polytechnic University of Tirana, Tirana, Albania.
35
Faculty of Geography, University of Belgrade, Belgrade, Serbia.
*Corresponding author. Email: bloeschl@hydro.tuwien.ac.at These authors contributed equally to this work.
on August 11, 2017 http://science.sciencemag.org/Downloaded from
for the four regions above, smaller-scale patterns
of changes in flood timing can also be identified.
To infer the causes of these changes in timing,
we focused on six subregions or hotspots where
changes in flood timing are particularly clear
(fig.S2andtableS2).Becausefloodsaretheresult
of the seasonal interplay of precipitation, soil mois-
ture, and snow processes (12), we analyzed the
temporal evolutions of these variables and com-
pared them to those of the floods (Fig. 2). In
southern Sweden (Fig. 2A) and the Baltics (Fig.
2B), floods are mainly due to spring snowmelt
(9,10). The temporal evolution of flood timing
therefore closely follows that of snowmelt, shift-
ing from late March to February (green and orange
linesinFig.2,AandB).Earliersnowmeltisknown
to be driven by both local temperature increases
and a decreasing frequency of advection of arctic
air masses (13). The Baltics are topographically
less shielded than southern Sweden from these
air masses; this is reflected by larger variations in
the timing of snowmelt in the 1990s. In south-
western Norway (Fig. 2C), precipitation maxima
at the end of the year generate floods around
the same time, because the prevalent shallow soils
have only limited subsurface water storage capacity.
Changes in the North Atlantic Oscillation (NAO)
since 1980 (14) may have resulted in a delayed
arrival of heavy winter precipitation, with max-
ima shifting from October to December. These
NAO anomalies have been less pronounced since
the early 2000s. The floods follow closely the
timing of extreme precipitation (Fig. 2C), which
strongly suggests a causal link. The changes in
theNAOmayberelatedtopolarwarming,among
many other factors, although the role of anthro-
pogenic effects is still uncertain (15,16). In south-
ern England (Fig. 2D), subsurface water storage
capacity tends to be much larger than in coastal
Norway. The maximum rainfall, which occurs in
autumn, therefore tends to get stored, and soil
moisture and groundwater tables continuously
increase until they reach a maximum in winter.
Sustained winter rainfall on saturated soils then
produces the largest floods in winter. As a result,
the flood timing in southern England is more
closely associated with the timing of maximum
soil moisture than with the timing of extreme
precipitation (17). The variations in flood timing
in northwestern Iberia (Fig. 2E) are similar to
those of southern England, although precipita-
tion in Iberia occurs more in the winter, so extreme
precipitation and maximum soil moisture (driven
by sustained precipitation) are more closely aligned.
Along the northern Adriatic coast (Fig. 2F), large-
scale effects of the Atlantic Ocean influence
Adriatic mesoscale cyclonic activity, which pro-
duces heavy precipitation toward the end of the
year (18). Meridional shifts in storm tracks have
increased atmospheric flow from the Atlantic
to the Mediterranean in winter (19), leading to
later extreme precipitation and floods in the sea-
son (Fig. 2F).
The spatial pattern of the average within-year
flood timing between 1960 and 2010 (Fig. 3) pro-
vides further support for the interpretation of
trends in flood timing across Europe. The average
Blöschl et al., Science 357, 588590 (2017) 11 August 2017 2of3
Days per decade
earlierlater
1
2
3
44
14
8
4
0
−4
−8
−14
Fig. 1. Observed trends of river flood timing in Europe, 19602010. The color scale indicates earlier or
later floods (days per decade). Regions with distinct drivers: Region 1, northeastern Europe (earlier snow-
melt); region 2, North Sea (later winter storms); region 3, western Europe along the Atlantic coast (earlier soil
moisture maximum); region 4, parts of the Mediterranean coast (stronger Atlantic influence in winter).
S Sweden
1960 1985 2010
J
F
M
A
M
J
J
A
S
O
N
D
Baltics
1960 1985 2010
J
F
M
A
M
J
J
A
S
O
N
D
SW Norway
1960 1985 2010
J
F
M
A
M
J
J
A
S
O
N
D
J
F
M
A
M
J
J
A
S
O
N
D
S England
1960 1985 2010
J
F
M
A
M
J
J
A
S
O
N
D
NW Iberia
1960 1985 2010
J
F
M
A
M
J
J
A
S
O
N
D
Flood
Precipitation
Snow Melt
Soil Moisture
Adriatic Coast
1960 1985 2010
J
F
M
A
M
J
J
A
S
O
N
D
J
F
M
A
M
J
J
A
S
O
N
D
Median Timing
± 0.5 circular σ
Fig. 2. Long-term temporal evolution of timing of floods and their drivers for six hotspots
in Europe. (A) Southern Sweden; (B) Baltics; (C) southwestern Norway; (D) southern England;
(E) northwestern Iberia; (F) Adriatic coast. Solid lines show median timing over the entire hotspot;
shaded bands indicate variability of timing within the year (±0.5 circular standard deviations). Green,
timing of observed floods; purple, 7-day maximum precipitation; orange, snowmelt indicator; blue,
timing of modeled maximum soil moisture. All data were subjected to a 10-year moving average
filter. Vertical axes show month of the year (June to May).
RESEARCH |REPORT
on August 11, 2017 http://science.sciencemag.org/Downloaded from
timing of the floods varies gradually both from
the west to the east because of increasing con-
tinentality (distance from the Atlantic) and from
the south to the north because of the increasing
influence of snow-related processes. The effect
of snow storage and melt at high altitudesfor
example, in the Alps and the Carpathians (orange
to red arrows in Fig. 3)is superimposed on this
spatial pattern. The spatial patterns of the average
timing of potential drivers, and their trends, are
showninfigs.S3toS5.
Throughout northeastern Europe (Fig. 1, re-
gion 1), spring occurrence of snowmelt and floods
(yellow and green arrows in figs. S3 and S4A)
combined with a warmer climate (fig. S4A) has
led to earlier floods. In the region around the
NorthSea(Fig.1,region2),extremeprecipitation
and floods in the winter (blue arrows in Fig. 3
and fig. S3A) combined with a shift in the timing
of extreme winter precipitation (fig. S3B) has
led to later floods. In western Europe (Fig. 1,
region 3), winter occurrence of soil moisture
maxima and floods (blue arrows in Fig. 3 and
fig. S5A) combined with a shift in the timing of
soil moisture maxima (fig. S5B) has led to earlier
floods. Although region 3 shows a consistent be-
havior in flood timing changes, closely aligned
with those of soil moisture, the effects of chang-
ing storm tracks on precipitation are different
in southern England and northwestern Iberia
because of the opposite effects of the NAO.
If the trends in flood timing continue, consid-
erable economic and environmental consequences
may arise, because societies and ecosystems have
adapted to the average within-year timing of
floods. Later winter floods in catchments around
the North Sea, for example, would lead to softer
ground for spring farming operations, higher soil
compaction, enhanced erosion, and direct crop
damage, thereby reducing agricultural produc-
tivity (20). Spring floods occurring earlier in the
season in northeastern Europe may limit the re-
plenishment of reservoirs if managers expect
later floods that never arrive, with substantial
reductions in water supply, irrigation, and hy-
dropower generation (21). Our flood timing ob-
servations at the continental scale also enable the
identification of a clear climatechangesignalthat
could not be obtained by earlier studies based on
flood magnitude data (4,5,22).
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ACKNO WLEDGME NTS
Supported by ERC Advanced Grant FloodChange,project
no. 291152; the Austrian Science Funds (FWF) as part of the
Doctoral Programme on Water Resource Systems (W1219-N22);
the EU FP7 project SWITCH-ON (grant 603587); and Russian
Science Foundation project no. 14-17-00155. We acknowledge
the involvement in the data screening process of C. Álvaro Díaz,
I. Borzì, E. Diamantini, K. Jeneiová, M. Kupfersberger, and
S. Mallucci during their stays at the Vienna University of
Technology. We thank L. Gaál and D. Rosbjerg for contacting
Finish and Danish data holders, respectively; A. Christofides
for pointing us to the Greek data source; B. Renard (France),
T. Kiss (Hungary), W. Rigott (South Tyrol, Italy), G. Lindström
(Sweden), and P. Burlando (Switzerland) for assistance in
preparing and/or providing data or metadata from their
respective regions; and B. Lüthi and Y. Hundecha for
preparing supporting data that are not part of the paper, to
cross-check the results. The flood date data used in this paper
can be downloaded from www.hydro.tuwien.ac.at/fileadmin/
mediapool-hydro/Downloads/Data.zip. The precipitation and
temperature data can be downloaded from www.ecad.eu/
download/ensembles/ensembles.php. The soil moisture data
can be downloaded from www.esrl.noaa.gov/psd.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/357/6351/588/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S5
Tables S1 and S2
References (2341)
17 March 2017; accepted 30 June 2017
10.1126/science.aan2506
Blöschl et al., Science 357, 588590 (2017) 11 August 2017 3of3
Jan
Mar
May
Jul
Sep
Nov
0.2
0.4
0.6
0.8
1.0
Fig. 3. Observed average timing of river floods in Europe, 19602010. Each arrow represents
one hydrometric station (n= 4062). Color and arrow direction indicate the average timing of
floods, as indicated by the circular color scale (light blue, winter floods; green to yellow, spring
floods; orange to red, summer floods; purple to dark blue, autumn floods). Lengths of the
arrows indicate the concentration of floods within a year (0, evenly distributed; 1, all floods occur
on the same date).
RESEARCH |REPORT
on August 11, 2017 http://science.sciencemag.org/Downloaded from
Changing climate shifts timing of European floods
and Nenad Zivkovic
Rogger, José L. Salinas, Eric Sauquet, Mojca Sraj, Jan Szolgay, Alberto Viglione, Elena Volpi, Donna Wilson, Klodian Zaimi
Ralf Merz, Peter Molnar, Alberto Montanari, Conor Murphy, Marzena Osuch, Valeryia Ovcharuk, Ivan Radevski, Magdalena
Kjeldsen, Silvia Kohnová, Jarkko J. Koskela, Ondrej Ledvinka, Neil Macdonald, Maria Mavrova-Guirguinova, Luis Mediero,
Natalia Frolova, Liudmyla Gorbachova, Ali Gül, Jamie Hannaford, Shaun Harrigan, Maria Kireeva, Andrea Kiss, Thomas R.
Bilibashi, Ognjen Bonacci, Marco Borga, Ivan Canjevac, Attilio Castellarin, Giovanni B. Chirico, Pierluigi Claps, Károly Fiala,
Günter Blöschl, Julia Hall, Juraj Parajka, Rui A. P. Perdigão, Bruno Merz, Berit Arheimer, Giuseppe T. Aronica, Ardian
DOI: 10.1126/science.aan2506
(6351), 588-590.357Science
, this issue p. 588 see also p. 552Science
Europe caused by earlier soil moisture maxima.
North Sea and parts of the Mediterranean coast owing to delayed winter storms, and earlier winter floods in western
Wilby). These variations include earlier spring snowmelt floods in northeastern Europe, later winter floods around the
found clear patterns of changes in flood timing that can be ascribed to climate effects (see the Perspective by Slater and
analyzed the timing of river floods in Europe over the past 50 years andet al.magnitudes has not been found. Blöschl
Will a warming climate affect river floods? The prevailing sentiment is yes, but a consistent signal in flood
Flooding along the river
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... In a changing climate not only the dynamic processes of the climate system but also the flood generation processes on the land surface are changing [17][18][19][20] . Observed shifts in the timing of floods within the year and process analyses suggest that, in cold regions, floods caused by rain-on-snow are becoming more frequent at the expense of snowmelt floods, while in other regions, convective events become more frequent at the expense of synoptic events [21][22][23] . These shifts in flood generation processes affect the magnitude of individual flood events, their spatial extent and synchronicity [24][25][26] . ...
... We base our analysis on the annual maxima of the observed streamflow series of 1353 European catchments for the period from 1960 to 2010 taken from the European Flood Database 21,32 .We identify flood anomalies as unusually frequent (i.e., flood-rich periods) or infrequent (i.e., flood-poor periods) than expected exceedances by annual maxima of three thresholds corresponding to 2-, 5-and 10-year return periods using a method based on scan statistics 3 . ...
... Flood event database and causative classification of flood events. In this study, we use the European flood database 21,32 , which contains information on the date and the maximum peak discharge of observed annual maximum floods in 2370 European catchments for the period from 1960 to 2010. Corresponding beginning and end points of each reported flood event are extracted from daily streamflow time series simulated by the well-established mHM model 56,57 using an automated event identification method 58 (see Supplementary Note 2). ...
Article
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Anomalies in the frequency of river floods, i.e., flood-rich or -poor periods, cause biases in flood risk estimates and thus make climate adaptation measures less efficient. While observations have recently confirmed the presence of flood anomalies in Europe, their exact causes are not clear. Here we analyse streamflow and climate observations during 1960-2010 to show that shifts in flood generation processes contribute more to the occurrence of regional flood anomalies than changes in extreme rainfall. A shift from rain on dry soil to rain on wet soil events by 5% increased the frequency of flood-rich periods in the Atlantic region, and an opposite shift in the Mediterranean region increased the frequency of flood-poor periods, but will likely make singular extreme floods occur more often. Flood anomalies driven by changing flood generation processes in Europe may further intensify in a warming climate and should be considered in flood estimation and management.
... It is estimated that the losses are likely to increase in the future, with climate change, economic acceleration, and urbanization development [2,3]. The IPCC [4] has shown that climate change is already an undeniable phenomenon and that the occurrence of extreme flood events is also associated with rising temperatures, heavy precipitation, and an accelerated hydrological cycle at global and regional scales [5][6][7]. Catastrophic floods, primarily associated with climate change, have also attracted public attention, and have been the focus of much research [8][9][10][11]. Many recent flood events around the world have led to growing concern that flood disasters are becoming more frequent and severe [5,[12][13][14]. ...
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Analyzing trends in flood magnitude changes, and their underlying causes, under climate change, is a key challenge for the effective management of water resources in arid and semi-arid regions, particularly for inland rivers originating in the Qilian Mountains (QMs). Sen’s slope estimator and the Mann–Kendall test were used to investigate the spatial and temporal trends in flood magnitude, based on the annual maximum peak discharge (AMPD) and Peaks Over Threshold magnitude (POT3M) flood series, of twelve typical rivers, from 1970 to 2021. The results showed that, in the AMPD series, 42% of the rivers had significantly decreasing trends, while 8% had significantly increasing trends; in the POT3M series, 25% of the rivers had significantly decreasing trends, while 8% had significantly increasing trends. The regional differences in the QMs from east to west were that, rivers in the eastern region (e.g., Gulang, Zamu, and Xiying rivers) showed significantly decreasing trends in the AMPD and POT3M series; most rivers in the central region had non-significant trends, while the Shule river in the western region showed a significantly increasing trend. Temperatures and precipitation showed a fluctuating increasing trend after 1987, which were the main factors contributing to the change in flood magnitude trends of the AMPD and POT3M flood series in the QMs. Regional differences in precipitation, precipitation intensity, and the ratio of glacial meltwater in the eastern, central and western regions, resulted in the differences in flood magnitude trends between the east and west.
... Hydrological disasters, such as floods, are the natural hazard that most affect people in the world, causing major social and economic losses (CRED/UNDRR 2020). Climate change and the impact of human activities on land use may be changing the pattern of intense rainfall and consequent flooding, creating more variability in these phenomena (Blöschl et al. 2017). Flash floods, i.e., sudden local floods typically due to heavy and local rain, often occur in small mountainous rivers, where there may be low rainfall and river level monitoring. ...
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Flash floods are natural hazards and often occur in small and mountainous river basins with low monitoring. The hydrological and hydrodynamic reconstruction of past rainfall events is useful for understanding the phenomena that led to a flood. This study aims to reconstruct a rainfall event that triggered landslides and floods in 2017 in the Rolante basin (771 km²), southern Brazil, a region with low monitoring. Due to the large magnitude of the flood event, a question was raised whether only the basin response to intense rainfall could have caused that flood. Therefore, different rainfall scenarios were tested with the use of official rain gauges and unofficial rainfall information from local farmers to determine the spatial and temporal distribution of rainfall. The reconstruction of the rainfall event was performed with the use of a hydrologic model (HEC-HMS) to define hydrographs and a hydrodynamic model (Nays2D Flood) to simulate flood propagation, with adjusted methods for the poorly monitored basin. The maximum flood depth and extent were analysed for three rainfall scenarios. The results showed that, with the information provided by the residents, it was possible to determine that extreme and concentrated rainfall occurred in the mountainous area and the basin ordinary response to that rainfall may have caused a flood of that great magnitude. The analysis of past extreme events can contribute to verifying if there are changes in the rainfall patterns and can assist in risk mitigation and disaster management, primarily in ungauged basins.
... One of the most important geohydrological and water resources-related disasters that also threatens the economic and social systems of watersheds is floods (Jothibasu and Anbazhagan 2016;Abdo 2020;Arora et al. 2021;Costache et al. 2021). The integration of several spatial variables such as lithology, faults, terrestrial features, climatic events and land use change causes floods (Blöschl et al. 2017;Hammad et al. 2018;Rahmoun et al. 2018;Nasiri Khiavi et al. 2019b). On the other hand, in recent years, rapid population growth and unplanned urban development have increased floods (Prinos 2009;Merz et al. 2014;El-Zein et al. 2021;Wu 2021). ...
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The effects of Sub-Watersheds (SWs) on each other can be more important in Flood Generation Potential (FGP). Therefore, the present study aims for prioritizing SWs based on FGP using Multi-Criteria Decision Making (MCDM) Methods including Game Theory (GT), Best-Worst Method (BWM), Analytic Hierarchy Process (AHP), Analytical Network Process (ANP), Fuzzy Analytic Hierarchy Process (FAHP), Fuzzy Analytical Network Process (FANP) and comparing its results with Hydrological Modeling Approach (HMA) in the Cheshmeh-Kileh Watershed, Iran. In GT, Condorcet algorithm were used. The best and worst criteria were identified using the BWM and compared with other criteria. In AHP and ANP, expert opinions were used and the final weight of criteria and alternative was calculated using Expert Choice and Super Decision softwares. In HMA, HEC-HMS software was used to calculate the discharge with return periods of 10- and 100-year, and finally, in all methods, FGP maps were prepared in three classes and SWs were prioritized. Based on the results of different methods, SWs 9, 2, 7, 10 and 11 were given high FGP priority. There are two possible explanations for this result. The first explanation is the difference between the values of geo-environmental criteria in each SW, and the ratio of these values and the effect of each of these criteria on the FGP. The next explanation is due to the different structural nature of each of the MCDM, which caused different prioritization of SWs based on FGP. Downstream SWs were also in a non-critical state due to dense forest cover and low slope. A comparative evaluation between the methods showed that BWM had the same result as the field evidence and HMA results and this method provided the best result. Based on SWs prioritization in BWM, high and low FGP were 33.33 and 46.67% of the study area, respectively. After BWM, GT gave a relatively good result. AHP, ANP, FAHP and FANP presented different results, but had poor performance in identifying critical areas. This study showed that optimal MCDM approaches can be used for flood management.
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The importance of soil moisture in triggering river floods is increasingly recognized. However, soil moisture represents only a fraction of the water stored in the unsaturated zone. In contrast, groundwater from the deeper, saturated zone, may contribute a significant proportion of river flow, but its effects on flooding are poorly understood. Here we analyze hydroclimatic records of thousands of North American watersheds spanning 1981-2018 to show that baseflow (i.e., groundwater-sustained river flows) affects the magnitude of annual flooding at time scales from days to decades. Annual floods almost always arise through the co-occurrence of high precipitation (rainfall + snowmelt) and baseflow. Flood magnitudes are often more strongly related to variations in antecedent baseflow than antecedent soil moisture and short-term (≤3-day) extreme precipitation. In addition, multi-decadal trends in flood magnitude and decadal flood variations tend to better align with groundwater storage and baseflow trends than with changing precipitation extremes and soil moisture. This reveals the importance of groundwater in shaping North American river floods and often decouples the spatial patterns of flood trends from those of shifting precipitation extremes and soil moisture.
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A warming climate may increase flood hazard through boosting the global hydrological cycle. However, human impact through modifications to the river and its catchment is not well quantified. Here, we show a 12,000-year-long record of Yellow River flood events by synthesizing sedimentary and documentary data of levee overtops and breaches. Our result reveals that flood events in the Yellow River basin became almost an order of magnitude more frequent during the last millennium than the middle Holocene and 81 ± 6% of the increased flood frequency can be ascribed to anthropogenic disturbances. Our findings not only shed light on the long-term dynamics of flood hazards in this world's most sediment-laden river but also inform policy of sustainable management of large rivers under anthropogenic stress elsewhere.
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Floods are a major natural hazard in the Mediterranean region, causing deaths and extensive damages. Recent studies have shown that intense rainfall events are becoming more extreme in this region, but paradoxically without leading to an increase in the severity of flood events. Consequently, it is important to understand how flood events are changing to explain this absence of trends in flood magnitude despite increased rainfall extremes. A database of 98 stations in Southern France with an average record of 50 years of daily river discharge data between 1958 and 2021 was considered, together with a high-resolution reanalysis product providing precipitation and simulated soil moisture. Flood events, corresponding to an average occurrence of one event per year (5317 events in total), were extracted and classified into excess rainfall, short rainfall and long rainfall event types. The evolution through time of the flood event characteristics and seasonality were analyzed. Results indicated that, in most basins, floods tend to occur earlier during the year, the mean flood date being on average advanced by one month. This seasonal shift can be attributed to the increased frequency of southern-circulation weather types during spring and summer. An increase in total and extreme event precipitation has been observed, associated with a decrease of antecedent soil moisture before rainfall events, linked to a smaller contribution of base flow during floods. The majority of flood events are associated with excess rainfall on saturated soils, but their relative proportion is decreasing over time with a concurrent increased frequency of short rain floods. Therefore, this study shows that even in the absence of trends, flood properties may change over time and these changes need to be accounted for when analyzing the long-term evolution of flood hazards.
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A record‐breaking flooding event occurred in Zhengzhou, Henan Province of China during 17–23 July 2021, causing hundreds of deaths and vast economic losses. Here, we evaluated the predictability of this extreme rainfall event and the impacts of tropical cyclones (TCs) using subseasonal‐to‐seasonal (S2S) operational models. On the monthly timescale, most models initialized in late June reasonably predicted the wet‐in‐north and dry‐in‐south patterns of anomalous rainfall over China in July, accompanied by the well‐predicted westward extension of the western North Pacific subtropical high (WNPSH) and eastward stretching of the South Asian High. On the weekly timescale, only four models captured the location, probability, and sudden intensification of the rainfall extremes in advance of 1 week, largely due to their reasonable prediction of WNPSH variability in mid‐latitudes. However, the S2S models still underestimated the super extremeness of this event. The prediction discrepancies came from the poor predictability of Typhoon IN‐FA and its impact on the daily evolution of the extreme rainfall event, even within a few days forecast lead. Compared with the observation, the prediction bias of tropical disturbance changed the environmental monsoon airflow to induce the earlier warning of rainfall extremes prior to the formation of IN‐FA. After the formation of IN‐FA, the prediction bias of the typhoon's moving speed distorted the typhoon location, which incorrectly predicted the moisture convergence center and underestimated their remote impacts on this heavy rainfall event.
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In recent years, large-scale flood risk analysis and mapping has gained more attention. Regional to national risk assessments are needed, for example, for national risk policy developments, for large-scale disaster management planning and in the (re-)insurance industry. Despite increasing requests for comprehensive risk assessments, some sectors have not received much scientific attention and one of these is the agricultural sector. In contrast to other sectors, agricultural crop losses depend strongly on the season. Also flood probability shows seasonal variation. Thus, the temporal superposition of high flood susceptibility of crops and high flood probability plays an important role for agricultural flood risk. To investigate this interrelation and provide a large-scale overview of agricultural flood risk in Germany, an agricultural crop loss model is used for crop susceptibility analyses and Germany-wide seasonal flood frequency analyses are undertaken to derive seasonal flood patterns. As a result, a Germany-wide map of agricultural flood risk is shown as well as the crop type most at risk in a specific region. The risk maps may provide guidance for federal statewide coordinated designation of retention areas.
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The current work addresses one of the key building blocks towards an improved understanding of flood processes and associated changes in flood characteristics and regimes in Europe: the development of a comprehensive, extensive European flood database. The presented work results from ongoing cross-border research collaborations initiated with data collection and joint interpretation in mind. A detailed account of the current state, characteristics and spatial and temporal coverage of the European Flood Database, is presented. At this stage, the hydrological data collection is still growing and consists at this time of annual maximum and daily mean discharge series, from over 7000 hydrometric stations of various data series lengths. Moreover, the database currently comprises data from over 50 different data sources. The time series have been obtained from different national and regional data sources in a collaborative effort of a joint European flood research agreement based on the exchange of data, models and expertise, and from existing international data collections and open source websites. These ongoing efforts are contributing to advancing the understanding of regional flood processes beyond individual country boundaries and to a more coherent flood research in Europe.
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There is an ongoing discussion whether floods occur more frequently today than in the past, and whether they will increase in number and magnitude in the future. To explore this issue in Sweden, we merged observed time series for the past century from 69 gauging sites throughout the country (450 000 km(2)) with high-resolution dynamic model projections of the upcoming century. The results show that the changes in annual maximum daily flows in Sweden oscillate between dry and wet periods but exhibit no significant trend over the past 100 years. Temperature was found to be the strongest climate driver of changes in river high flows, which are related primarily to snowmelt in Sweden. Annual daily high flows may decrease by on average -1% per decade in the future, mainly due to lower peaks from snowmelt in the spring (-2% per decade) as a result of higher temperatures and a shorter snow season. In contrast, autumn flows may increase by + 3% per decade due to more intense rainfall. This indicates a shift in floodgenerating processes in the future, with greater influence of rain-fed floods. Changes in climate may have a more significant impact on some specific rivers than on the average for the whole country. Our results suggest that the temporal pattern in future daily high flow in some catchments will shift in time, with spring floods in the northern-central part of Sweden occurring about 1 month earlier than today. High flows in the southern part of the country may become more frequent. Moreover, the current boundary between snow-driven floods in northern-central Sweden and rain-driven floods in the south may move toward higher latitudes due to less snow accumulation in the south and at low altitudes. The findings also indicate a tendency in observations toward the modeled projections for timing of daily high flows over the last 25 years. Uncertainties related to both the observed data and the complex model chain of climate impact assessments in hydrology are discussed.
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Estimation of both the frequency and variation of spring floods is a key issue for the assessment and management of flood risks. Changes in river floods in Estonia, Latvia and Lithuania have been investigated in few national studies. However, there are no studies of the changes of flood patterns by using a common methodology for the rivers of this region. In this study flood pattern changes in the rivers of the Baltic countries were estimated applying trend and frequency analysis for the periods of 1922–2010, 1922–1960, 1961–2010 and 1991–2010, i.e. for the whole spring flood data sets, periods before and after 1960 (this year was considered as the beginning of the remarkable climate change), as well as for the two past decades. A comparative study of five probability distributions was performed in order to estimate which distribution at best represents statistical characteristics of the flood data. The results showed that maximum discharges of spring floods decreased over the whole studied period. Only some insignificant positive trends of maximum discharges were found in the last time period in continental and transitional rivers. Generalized extreme value distribution provided the best approximation to the maximum discharge data series of the rivers of Baltic countries for the whole observation period. First published online: 08 Jul 2014
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Empirical climate variability and change studies may be particularly beneficial when they combine a search for dynamic causes with the examination of trends in meteorological var-iables. By using an air-mass-based methodology, arctic air masses over Latvia were identi-fied for the 1950–2005 period according to the classification used by Berliner Wetterkarte, which defines air mass types by their origin and the extent of continental or maritime influence. The frequencies of maritime, transformed and continental arctic air masses and the class of arctic air masses were examined to evaluate whether and to what extent these account for changes and variations in the surface temperature. Trends in the frequency of arctic air, monthly-average temperature and monthly lowest minimum temperature series at seven observation sites were determined by using the non-parametric Mann-Kendall test. The results indicate that the frequency of arctic air masses during winter seasons decreased significantly during this period, with the majority of the decrease being associated with maritime arctic air, and that the frequency of the bitterly cold continental arctic air has also demonstrated a decrease. This trend in the monthly frequency of arctic air was the greatest in February. The increase in the winter, near-surface air temperature was partially attrib-utable to a decrease in maritime arctic air mass frequency and, at a seasonal scale, these changes tended to smooth the peaks in the monthly temperature time series.
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Floods are the most prevalent natural hazard in Europe. But, has flood risk increased in the continent? How, where, and why? Are climate change impacts apparent? How do socio-economic trends and associated land-use change impact flood risk? This interdisciplinary book, authored by an international team, offers: • A comprehensive overview of flood risk in Europe, past and present, and future • National/regional chapters covering Central Europe, Western Europe, Southern Europe and Northern Europe, the Alpine region and the Iberian Peninsula. • A focus on detection and attribution of change with respect to climate change and its impacts, water resources and flood risk, the re-insurer's view point, and future projections of flood risk • Rectification of common-place judgements, e.g.: "climate is warming so floods should become more frequent and intense"; observations do not always confirm this expectation The book will be of interest to those interested in floods and flood risk, including research scientists and educators, students, engineers, planners, risk reduction specialists, staff of specialized national and international agencies, and the media.