QUAESTIONES GEOGRAPHICAE 27A/1 2008

TYPOLOGY OF SPATIAL PATTERNS SEASONALITY IN EUROPEAN RIVERS

FLOW REGIME

DARIUSZ WRZESIŃSKI

Adam Mickiewicz University, Institute of Physical Geography and Environmental Planning, Poznań, Poland

Manuscript received May 13, 2008 Revised version August 5, 2008

Wrzesiński D., 2008. Typology of spatial patterns seasonality in European rivers flow regime. Quaestiones Geographicae 27A/1, Adam Mickiewicz University Press, Poznań, 2008, pp. 87-98, Figs 10. ISBN 978-83-232-1967-5. ISSN 0137-477X. ABSTRACT. The article presents the way in which the North Atlantic Oscillation modifies the features of the hydrological regimes of European rivers. A river regime is defined here as the type and temporal structure of streamflow in an average hydrological cycle. The elements of the structure include so-called hydrological seasons, which were the instrument for the study of the regime as well as the basis of its characterisation and an assessment of its variability. Hydrological seasons were determined via the grouping of elementary time units of the year (pentads) on the basis of similarity of one of their features - the frequency distribution of the river discharge. The determination of the hydrological seasons was carried out on the basis of sets of 20 years with extremely high and extremely low values of Hurrell's NAODJFM index. The analysis embraced 150 rivers of Europe flowing in a variety of environmental conditions. Use was made of daily discharge figures from at least 40-year observation periods spanning the years 1901-2000. The changing number, temporal structure and parameters of the hydrological seasons support the conclusion that each NAO stage produces a different impact on the hydrological regimes of European rivers. The article closes with a classification of the rivers under study in terms of differences in the properties of extreme hydrological seasons in the different NAO stages, followed by a spatial analysis of the results obtained. Key Words: Hydrological regime; typology of river regime; North Atlantic Oscillation impact Dariusz Wrzesiński, Institute of Physical Geography and Environmental Planning, Adam Mickiewicz University, Dzięgielowa 27, 61-680 Poznań, Poland, darwrze@amu.edu.pl

Introduction

Concepts and theories currently developed to account for present-day climate

change commonly emphasise its hydrological consequences. The river discharge,

treated as an element leaving the catchment's water cycle, is determined by climatic

conditions and physico-geographical features of the catchment as well as the human

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impact. Seasonal changes in the discharge are a natural effect of the prevailing

climatic conditions, but multi-year variations may suggest global environmental

changes. Hence, the river discharge can be their simple indicator. The issues often

discussed in works on hydrology are long-term variability, trends and cyclicity (Peel

et al. 2001, Pekárová et al. 2006), as well as stability of the hydrological regime

(Krasovskaia 1995, 1996, Krasovskaia and Sælthun 1997, Arnel 1999, Krasovskaia

and Gottschalk 2002, Wrzesiński 2004, 2005, 2007). Their results are commonly

employed in forecasting models defining the properties of the hydrological regime

under various scenarios of global climate change.

The hydrological regime defines the state and responses of a river system in

relation to the catchment's climatic system and physical-geographic characteristics.

Deviations of climatic elements from average levels caused by, e.g., changes in the

atmospheric circulation modify the streamflow formation conditions, and hence also

the characteristics of the river regime. It is controlled by both precipitation and air

temperature, whose magnitudes show a significant dependence on the intensity of

zonal circulation. A simple indicator characterising the atmospheric circulation over

the north Atlantic is the North Atlantic Oscillation Index (henceforth, NAO), Carleton

1988, Hurrell 1995. Its significance for the formation of the streamflow has been

emphasised by, e.g., Arnell (1997), Shorthouse and Arnell (1997), Kaczmarek (2002,

2003), Pociask-Karteczka et al. (2002-2003), Popova and Shmakin (2003), Pociask-

Karteczka (2006) and Rödel (2006). In winter, the discharges of north European

rivers are positively correlated with the winter NAO index, and those of Southern

Europe are correlated negatively. While in Northern Europe this relation can be

accounted for by an increase in precipitation in a positive stage of the NAO and in

Southern Europe by its decrease then, in Central and Western Europe the

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dependence of winter precipitation on the NAO is rather weak. A strong relation does

exist, however, between the NAO indices and air temperatures. Air temperature

controls the magnitude of water loss through evaporation in summer and the

development and disappearance of snow cover in winter. In Central Europe during a

warm winter (NAO+) snow cover dwindles, hence meltwater floods are rare and carry

small discharge volumes. In a negative NAO stage, in turn, winters are severe with a

thick snow cover, thus contributing to high and large-volume flood waves

(Styszyńska and Tamulewicz 2004, Wrzesiński 2005, Danilovich et al., 2007,).

The observed effect of the atmospheric circulation on climatic conditions,

mostly precipitation and air temperature, makes also the discharges of European

rivers correlated, to a greater or lesser degree, with the winter NAO index. It follows

from an analysis of the temporal structure of hydrological phenomena and their

variations in a yearly cycle that the varying intensity of zonal circulation can

destabilise the hydrological regime as well. The aim of the paper is to present spatial

differences in hydrological regime variations of rivers in Europe, mainly extreme

phenomena: high- and low-water seasons, brought about by changes in the

atmospheric circulation in the North Atlantic sector.

Study area and source data

The analysis embraced 150 rivers flowing in a variety of climatic and physico-

geographical conditions (Fig. 1). This area is highly interesting due to its diversified

climatic conditions and a variety of conditions of river flow, which has produced five

types of hydrological regime. The changes in the conditions of atmospheric

circulation observed here affect the elements of climate, the consequence of which

are changes in the conditions of river runoff and destabilization of river regime. In the

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study use was made of the daily discharge data of at least 40-year observation series

in 1901-2000. They come from the following sources: the Global Runoff Data Centre

in Koblenz (Germany), HYDRO banque nationale de données pour l'hydrométrie et

l'hydrologie (France), Hydrometeorological Center of Belarus, Institute of

Meteorology and Water Management (Poland), and Sistema Nacional de Informaçăo

de Recursos Hidricos (Portugal).

The rivers selected for the research have diversified catchment areas. Those

under 5,000 km2 constitute 68%, and those under 1,000 km2 - 27% of their total

number. The remaining 32% are rivers with catchments exceeding 5,000 km2. The

largest, in excess of 100,000 km2, account for a mere 4% of the rivers. They all have

daily discharge observation series of at least 40 years. Rivers whose observation

series cover from 60 to 70 years constitute the largest proportion (22%). The shortest

series (from 40 to 50 years) and the longest ones (from 90 to 100 years) contribute

17% each. In the study use was made of NAO values calculated according to J.

Hurrell's (1995) method and obtained by electronic means from

http://www.cru.uea.ac.uk/cru/data/nao.htm.

-20 -10 0 10 20 30 40 50 60

40

50

60

70

Fig. 1. Location of profiles on the rivers under study

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Methods

The method employed in the article to determine a river regime was one

involving the identification of the temporal structure of hydrological phenomena and

their variations in a yearly cycle. Its theoretical foundations can be found in Rotnicka

(1988) and its applications in, e.g., Rotnicka (1988), Gutry-Korycka and Rotnicka

(1998), and Wrzesiński (2004, 2005, 2007). An important point in the research

procedure is distinguishing characteristic stages of the cycle termed hydrological

periods or seasons. It consists in the grouping of elementary time units of the

hydrological year on the basis of the similarity of one of their features. The

elementary time unit adopted is a five-day period of time, or a pentad, and the

grouping characteristic is variable x, which is presented in the form of a discharge

frequency distribution.

The result of the testing procedure of the similarity of water discharge

distributions is a quadratic similarity matrix of the yearly set of pentads in which the

rows and columns are designated by the numbers of the pentads in chronological

order. The matrix is pictured as a diagram presenting relations (links) holding among

the pentads in terms of the similarity of their feature. A river regime is defined here as

the type and temporal structure of river discharge in a normal hydrological cycle. The

elements of the structure are so-called hydrological periods, which provide a tool for

the study of a regime and a basis for its classification. An illustrative matrix diagram

depicting the structure of the elementary time units of the year and the method of

distinguishing hydrological periods is shown in Figure 2. Along its diagonal, groups of

pentads of similar discharge frequency distribution can be noted; it is those groups

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that form the so-called hydrological periods. Thus, a hydrological period is a time

segment displaying a uniform structure of links among pentads in terms of the

conformity of discharge distributions.

0

200

400

600

800Q [m3s -1]

SWQ

SSQ

GGSQ

DGSQSNQ

1%

5%

10%

25%

50%75%90%99%

A

B

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 731 1 4 7 7 8 # 7 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 8 # 7 6 8 72 4 1 7 5 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 8 6 6 7 # 5 6 # 93 7 7 1 9 # # # # # # # # # # # # # # # # # # # # # # # # # # 5 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 7 7 # # # 6 # 94 7 5 9 1 7 # 8 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 9 6 6 8 9 8 5 9 85 8 # # 7 1 9 8 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 9 7 8 # # # # # #6 # # # # 9 1 # # # # # # # # # # # # # # # # # # # # # # # # # # # 8 # # # # # # # # # 7 # # # # # # # # # # # # # # # # # # # # # 9 9 # # # 9 # #7 7 # # 8 8 # 1 9 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 9 # # 6 6 78 # # # # # # 9 1 9 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #9 # # # # # # # 9 1 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #

10 # # # # # # # # # 1 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #11 # # # # # # # # # # 1 6 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #12 # # # # # # # # # # 6 1 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #13 # # # # # # # # # # # # 1 9 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #14 # # # # # # # # # # # # 9 1 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #15 # # # # # # # # # # # # # # 1 8 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #16 # # # # # # # # # # # # # # 8 1 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 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# # 8 7 5 6 1 8 # 971 6 6 6 5 # 9 6 # # # # # # # # # # # # # # # # # # # # # # # 7 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 8 5 # # 8 1 5 472 8 # # 9 # # 6 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 8 8 # # 5 1 473 7 9 9 8 # # 7 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 8 7 # 9 4 4 1

1

2

I II III V IIV

Fig. 2. Hydrological periods of the Warta river at Poznań A - Hydrograph of water discharges of specified probability of occurrance, with characteristic

discharge and boundaries of the middle discharge zone: SWQ - average high water, SSQ - average middle water, SNQ - average low water, GGSQ - upper boundary of the middle discharge zone, DGSQ - lower boundary of the middle discharge zone

B - Diagram of a similarity matrix: 1 - similar distributions, 2 - probably similar distributions, I, II ... V - successive hydrological periods.

Hydrological periods were determined twice: once on the basis of a 20-year

set of discharge from years with extremely low values of Hurrell's NAODJFM index

(NAO-), and then on the basis of a 20-year set from years with extremely high values

(NAO+). Each hydrological period was described by means of six variables: the start

and end of the period, its duration (T), the coefficient of variation (Cv), the coefficient

of skewness (S), and the coefficient of discharge (W) defining abundance of the

period. The types of hydrological period were established on the basis of the

7

coefficient of discharge (W) according to the following criteria: W < 0.5 - a deep low;

0.5<W<0.75 - an average low; 0.75<W<0.9 - a shallow low; 0.9<W<1.1 - a normal

period; 1.1<W<1.25 - a low flood; 1.25<W<1.50 - an average flood; 1.50<W<1.75 - a

high flood; and W<1.75 - a very high flood.

Next, average characteristics of high- and low-flow seasons in a positive and a

negative NAO stage were determined, compared, and the differences calculated.

The research closed with a classification of the rivers in question carried out in terms

of differences in the properties of the high- and low-flow seasons in the two NAO

stages. In the classification procedure, use was made of Ward's method of

hierarchical grouping. The variables describing hydrological periods were differences

in the flow magnitude, duration, and starting date of a given period. Their changes

made it possible to determine the areas with the most variable and most stable

parameters of the hydrological periods of European rivers in the different NAO

stages.

In the graphic presentation use was made of the Surfer 7 programme and the kriging

technique was applied to draw up the range and run of the isopleths showing

differences in the values of the analysed variables.

Features of hydrological seasons in the different NAO stages Features of low-water seasons

On almost half of the rivers under study, no significant differences are observed

in the starting dates of low-water seasons between a negative and a positive NAO

stage. The most stable starting dates, with differences ranging up to 15 days, are

noted on most of the rivers of Eastern Europe as far as the Urals, Finland, Ireland

and Scotland, western Germany, and southern France - mostly the western slope of

the Massif Central, the eastern Pyrenees, and Corsica, as well as the Balkans. In a

positive NAO stage, later starting dates of low-water seasons (up to a month) can be

8

observed on the rivers of southern and northern Sweden, the Scandinavian

Mountains, the Alps, and Iceland. In turn, in this NAO stage low-water seasons begin

up to 60 days earlier in Central Europe (eastern Germany, south-western Poland, the

Czech Republic, and Slovakia) and even up to 90 days earlier on the Pyrenean

Peninsula (Fig. 3).

-20 -10 0 10 20 30 40 50 60

40

50

60

70

-90

-75

-60

-45

-30

-15

0

15

30

45

60

75

90

Difference[days]

Fig. 3. Differences in the starting dates of the low-water season between a negative and a positive

NAO stage in days

Two areas can be distinguished in Europe in terms of differences in the duration

of low-water seasons on the rivers in the different NAO stages. There is a compact

area in the northern part of the continent embracing the eastern part of the drainage

basin of the Baltic Sea where rivers are characterised by markedly longer low-water

seasons in a negative NAO stage, and another covering western and southern

Europe as well as the eastern parts of the East European Plain as far as the Urals,

where low-water seasons on the rivers last longer in a positive NAO stage (Fig. 4). In

a negative NAO stage, distinctly longer low-water seasons, even by more than 60

days, are recorded on the rivers from north-eastern Poland through the Baltic states

as far as southern Finland, as well as on the coast of Norway. The differences in the

duration of low-water seasons are only a bit smaller, up to 60 days, on the rivers of

9

western Russia and in Belarus. In a positive NAO stage, low-water seasons are

markedly longer, more than 60 days, on the rivers of the Iberian Peninsula and in the

belt extending from eastern Germany through the Czech Republic and eastern

Austria to Hungary and Croatia.

-20 -10 0 10 20 30 40 50 60

40

50

60

70

-150

-120

-90

-60

-30

0

30

60

90

120

150

180

Difference[days]

Fig. 4. Differences in the duration of low-water seasons between a negative and a positive NAO stage

To determine low-flow runoff in the different NAO stages, use was made of the

average values of discharge coefficients. The values of the discharge coefficient in a

NAO- stage were taken as 100%. Figures over 100% meant that the runoff was

higher in a positive stage, and those under 100%, in a negative stage. About a half of

the rivers under study do not display major variations in low-flow runoff in the years

with extreme NAO values; the differences are smaller than 10%. In a positive NAO

stage, higher runoff figures in low-water seasons can be observed on the

Scandinavian rivers of central Sweden and western Finland emptying into the North

and Norwegian Seas. Low-flow runoff of those rivers amounts to 125%, and

sometimes even more than 150%, of the figures recorded in a negative NAO stage

(Fig. 5). The runoff is also up to 25% higher in a positive NAO stage on the rivers of

10

Eastern Europe, Finland, Iceland and the Jutland Peninsula, as well as on some of

the Alpine rivers and those on the western coasts of the Iberian Peninsula and

France. In a negative NAO stage, low-flow runoff is higher (10% to 20%) on the rivers

of Scotland, northern Ireland and England, and those in the Carpathians, the

Balkans, and the Apennine Peninsula.

-20 -10 0 10 20 30 40 50 60

40

50

60

70

60

80

100

120

140

160

180

H HNAO+ NAO-/ [%]

Fig. 5. Comparison of the runoff of low-water seasons in the different NAO stages (runoff in NAO- = 100%) Features of high-water seasons

The most stable starting dates of high-water seasons occur on one-third of the

rivers under study. The difference between those dates in a negative and a positive

NAO stage is smaller than 10 days. In a negative NAO stage, high flows start

distinctly later, more than a month, on the rivers of Central Europe (Poland, the

Czech Republic), the Baltic countries (Lithuania, Latvia, Estonia), southern Finland,

central France, Norway, and Scotland (Fig. 6). In turn, they start 10 to 50 days earlier

in this NAO stage mainly on the rivers of southern and eastern Europe and those in

the basins of the Rhine and middle Danube, as well as in the northern part of the

Jutland Peninsula.

11

-20 -10 0 10 20 30 40 50 60

40

50

60

70

-90

-75

-60

-45

-30

-15

0

15

30

45

60

75

90

105

120

Difference[days]

Fig. 6. Differences in the starting dates of the high-water season between a negative and a positive NAO stage in days

The spatial distribution of areas with similar differences in the duration of high-

water seasons shows, as in the case of low flows, that there are two regions again

(Fig. 7). The first embraces rivers of Central Europe (Poland and the Czech

Republic), the Baltic states, Finland, Belarus, and western Russia, where high-water

seasons are longer in a positive NAO stage. The greatest differences, in excess of

90 days, are recorded on the rivers from north-eastern Poland to southern Finland.

The other region includes southern Europe, where longer high-water seasons are

characteristic of a negative NAO stage. On the rivers of the Iberian Peninsula,

southern France and the central part of the Danube basin, high-water seasons in a

negative NAO stage can even last more than 90 days longer than in a positive stage.

12

-20 -10 0 10 20 30 40 50 60

40

50

60

70

-150

-120

-90

-60

-30

0

30

60

90

120

Difference[days]

Fig. 7. Differences in the duration of high-water seasons between a negative and a positive NAO stage

As in the case of low-water seasons, the magnitude of high-flow runoff was

assessed by using average values of discharge coefficients. Here the spatial picture

of differences is clearer. In a positive NAO stage, high-flow runoff can be more than

20%, and locally even 40%, higher, mostly on the rivers of Northern Europe from

Norway to northern Russia, and those of Germany, Iceland, Ireland, Scotland and

Wales (Fig. 8). In a negative NAO stage, high-flow runoff is usually up to 20% higher

on the rivers of Southern and Central Europe, the East European Plain, most of the

French, Alpine and Carpathians rivers, and those in eastern England.

-20 -10 0 10 20 30 40 50 60

40

50

60

70

40

60

80

100

120

H HNAO+ NAO-/ [%]

Fig. 8. Comparison of the runoff of high-water seasons in the different NAO stages (runoff in NAO- = 100%)

13

Considering variations in the values of parameters of low- and high-water

seasons in the different NAO stages, the rivers that must be regarded as the most

sensitive to changes in the conditions of atmospheric circulation are those of

Southern, Northern and Central Europe. On the rivers in the south of Europe, mainly

the Iberian Peninsula, in a positive NAO stage low-water seasons start markedly

earlier, last longer, and their runoff values are higher than in a negative NAO stage.

As to high-water seasons on those rivers, their starting dates do not vary much in the

both stages, but they are distinctly shorter and their runoff values lower in a positive

NAO stage. The situation is reversed on Scandinavian rivers. In a positive NAO

stage their low-water seasons start later, last shorter, and have higher runoff values

than in a NAO- stage. In turn, the starting dates and duration of high-water seasons

do not change, but their runoff figures are markedly higher. On the rivers of Central

Europe, the Baltic states and southern Finland, the differences in the starting dates of

low-water seasons are not wide, but in a NAO+ stage they are distinctly shorter and

the runoff usually higher than in a NAO- stage. In turn, high-water seasons on those

rivers in a positive NAO stage begin much earlier and last decidedly longer, but the

runoff is clearly lower than in a negative NAO stage.

Typology of rivers in terms of differences in low-flow periods

On the basis of the grouping of rivers by change in the parameters of their low-

flow periods, also four groups were distinguished, from A to D, with 9 subgroups.

However, since the rivers assigned to the various groups are often scattered

throughout the continent, it was pointless to present the distribution of the typological

classes graphically.

Group A embraces rivers whose low-water flows are decidedly higher in a positive

NAO stage. The lows then usually start later (subgroups A2 and A3), and their

14

duration does not change (subgroup A1) or is markedly shorter (subgroups A2 and

A3). This type is represented by some Portuguese rivers, those of western Norway,

northern France, as well as Finland, Sweden, Lithuania and Estonia.

In group B a distinction was made between subgroups B1 and B2. Subgroup B1

contains rivers with a slight, ca. 10%, domination of low-water flow and an earlier

low-flow starting date in a positive NAO stage. In both NAO stages the duration of

low-flow seasons is basically the same. The rivers of this group are found in Central

Europe, from the Warta to the Dnieper, some are in the Alps and Pyrenees, and on

the Jutland Peninsula. Flows in low-water seasons in subgroup B2 are distinctly

higher, more than 20%, in a positive NAO stage, and the seasons are 1 month longer

then. Most rivers showing this type of change are situated in central and southern

Norway.

Group C comprises rivers with higher flows in low-water seasons in a negative NAO

stage, in subgroup C1 even over 20%. No major differences in the starting dates and

duration of lows can be observed between the two NAO stages. It is only rivers of

subgroup C1 which are characterised by longer, up to 2 months, lows in a negative

NAO stage. They are situated in the British Isles, Germany, the Alps, and central

Norway. There is a distinct clustering of subgroup C2 rivers in northern Norway,

Russia and Iceland.

Rivers of group D (subgroups D1 and D2) display low-water flows statistically 10%

higher in a negative NAO stage, while the starting dates of lows are then decidedly

delayed, by more than 1 month. The duration of those periods is markedly shorter, by

more than 2 months in subgroup D1 and more than 1 month in subgroup D2. Rivers

of this group are largely located in Central Europe (Germany, Czech Republic,

Hungary), and also southern England, the Eastern Alps, and the south of Europe.

15

Typology of rivers in terms of differences in high-flow periods

On the basis of the grouping of rivers by change in the parameters of their high-flow

periods, four basic groups can be distinguished, from A to D, with a total of 10

subgroups. Their spatial location is presented in Fig. 10. Owing to the small number

of rivers analysed in the eastern part of the East European Plain, the presented

division into groups in this part of the continent is only approximate. Differences in

the parameters of the analysed features in the selected groups is presented in Fig. 9.

DIFFERENCES IN STARTING DATES OF FLOOD SEASONS

Me 25%-75% Min. - Max.A1 A2 A3 B1 B2 C1 C2 D1 D2 D3

GROUPS

-30

-20

-10

0

10

20

30

% O

F A

YE

AR

DIFFERENCES IN DURATION OF FLOOD SEASONS

Me 25%-75% Min. - Max. A1 A2 A3 B1 B2 C1 C2 D1 D2 D3

GROUPS

-40

-30

-20

-10

0

10

20

30

40

50

60%

OF

A Y

EAR

DIFFERENCES IN FLOWS IN FLOOD SEASONS (FLOW IN NAO- = 100%)

Me 25%-75% Min. - Max.A1 A2 A3 B1 B2 C1 C2 D1 D2 D3

GROUPS

20

40

60

80

100

120

140

160

%

Fig. 9. Differences in the parameters of flood seasons in the river groups

Group A embraces rivers situated in the north of the continent - those of Norway,

Sweden, central and northern Finland, northern Russia, Iceland, Ireland, western

England and Scotland, and also central Germany and Denmark. They are

characterised by a flow of greater volume, even up to 40%, during high-water

seasons in a positive NAO stage. Subgroup A1 includes rivers with floods in a

positive NAO stage higher by an average of 10% to a maximum of 30%. On average,

16

high-flow periods start then almost 3 weeks earlier and last markedly longer,

statistically a month. This group is represented by rivers on the coast of Norway, in

central Finland and in Scotland. Subgroup A2 contains rivers with the biggest

difference in the high-water flow between the two NAO stages. In a positive stage it is

statistically 22% higher, but in some cases it can be even 40% higher. There are no

marked differences in the starting dates and duration of the flood period. Rivers of

this subgroup are primarily those of central Norway, northern Russia and Iceland.

Subgroup A3 comprises rivers with about 10% higher floods in a positive NAO stage.

In both stages the staring dates of high-flow seasons are similar, but they last

distinctly shorter, from an average of 2-3 weeks to a maximum of more than two

months, in a positive NAO stage. Rivers of this group can be found primarily in

western England, Ireland, and from the Alpine Foreland through central Germany to

Denmark.

-20 0 20 40 60

40

50

60

70

GROUPSABCD

Fig. 10. Typology of flow river regimes by change in the parameters of flood periods in the different NAO stages

17

Placed in group B are rivers of Central and Eastern Europe up to the Oka, as well as

those of Corsica and the Apennine Peninsula. Rivers of subgroup B1 do not occupy

a compact area; they are dispersed throughout southern Sweden and central

Norway. In a negative NAO stage their flood seasons display higher flows (about

10%), start statistically 1 month later and last shorter, sometimes even by 2-3

months. Subgroup B2 is represented by rivers of Central Europe, western and

southern Finland, Eastern Europe up to the Oka, Corsica, and the Apennine

Peninsula. They differ from rivers of subgroup B1 in having a distinctly higher flow in

a negative NAO stage, 30% on average, while high-flow seasons start then more

than 3 weeks later and are about 1 month shorter.

Group C comprises rivers displaying the smallest differences in the analysed

parameters of high-flow seasons between a negative and a positive NAO stage.

They can be found in eastern England as far as Scotland, western France, the

Pyrenees, the Alps, central Germany and the East European Plain, from the White

Sea coast to the Urals. High-flow periods of subgroup C1 rivers are characterised by

about 10% higher floods in a negative NAO stage. There are no major differences

between the two stages in the staring dates and duration of those periods. In

subgroup C2 flood flows and their staring dates are similar in the two NAO stages,

but their duration is markedly longer, up to an average of 1 month, in a negative NAO

stage. The group is represented mainly by Alpine and Pyrenean rivers, and those of

southern England and central Germany.

Group D includes rivers of the Iberian Peninsula, southern France and the Massif

Central, as well as the area extending from the Eastern Alps to the Balkans.

Subgroup D1 comprises rivers of central and southern Portugal. They stand out for

their much higher (40 to 60%) floods in a negative NAO stage, and while the starting

18

dates of high-flow seasons stay the same, their duration in that stage is markedly

extended, by 45 days on average. Rivers of subgroup D2 are characterised by about

15% higher flood flows in a negative NAO stage. The average starting date of high-

flow seasons does not change much, but they are statistically more than 3 months

longer than in a positive stage. This group embraces rivers of northern Portugal,

most of those in the Massif Central, rivers of the Eastern Alps, the Balkans, and

some of the central Danube basin. Rivers of subgroup D3 are characterised by the

same flow in both NAO stages and a decidedly earlier start, by more than a month, of

the high-flow season in a negative stage and its much longer duration then, by more

than 2 months. The rivers can be found in the south of France, the west Danube

basin, and the Caucasus.

Conclusions

In terms of changes in the magnitude of flow, both in the low- and high-water periods,

the rivers most susceptible to variations in the atmospheric circulation conditions

include:

- those in Northern Europe (the Scandinavian Peninsula) which, in a positive

NAO stage, show: an increase in the flow, both at low and high water; usually a lack

of change in the starting dates and duration of high-water seasons; and a usually

later start and shorter duration of lows;

- those in Central and Eastern Europe, characterised by a decided increase in

the flood flow and a slight decrease in the low-water flow in a negative NAO stage.

The starting dates of flood periods are then markedly delayed and their duration

shortened. These regularities tend to diminish eastwards, however, and the observed

differences in the parameters of high-water seasons in the two NAO stages become

19

smaller;

- those in Southern Europe whose flow in flood seasons is decidedly higher and

their duration markedly longer in a negative NAO stage.

The remaining area can be considered stable in terms of changes in high- and low-

water flows in the two NAO stages. Its eastern boundary runs roughly longitudinally

across eastern Germany south to northern Italy embracing the Alpine region, then

farther west to the Mediterranean coast. It bypasses southern France and the Massif

Central. The western boundary of this region is the Pyrenees.

The presented method of analysis of the hydrological regime is also possible to

employ in a research on the stability of regime parameters in the changing climatic

conditions brought about by both, natural factors and human activity.

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