CHANGES OF THE HYDROLOGICAL REGIME OF RIVERS OF NORTHERN AND CENTRAL EUROPE IN VARIOUS PERIODS OF THE NORTH ATLANTIC OSCILLATION

ABSTRACT. The article presents the impact of the north Atlantic atmospheric circulation on the hydrological regimes of rivers of Northern and Central Europe. A river regime is defined as the type and temporal structure of streamflow in an average hydrological cycle. The elements of the structure include so-called hydrological periods, 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 periods were determined via the grouping of elementary time units of the year (pentads) on the basis of the similarity of one of their features (the frequency distribution of the river discharge). The determination of hydrological periods was carried out on the basis of sets of 20 years with an extremely high and an extremely low NAO index, as well as 20-year periods from the 1901-2000 interval. The varying number and temporal structure of the hydrological periods thus distinguished allowed conclusions about differences in the variability of hydrological regimes of Scandinavian and Central European rivers in the various NAO circulation periods. Dariusz Wrzesiński, Institute of Physical Geography and Environmental Planning, Adam Mickiewicz University, Dzięgielowa 27, 61-680 Poznań, Poland, darwrze@amu.edu.pl

CHANGES OF THE HYDROLOGICAL REGIME OF RIVERS OF NORTHERN AND CENTRAL EUROPE IN VARIOUS PERIODS OF THE NORTH ATLANTIC

OSCILLATION

DARIUSZ WRZESI Ń SKI

Adam Mickiewicz University, Institute of Physical Geography and Environmental Planning, Pozna ń ,

Poland

Manuscript received Revised version

Wrzesi ń ski D., 2005 Changes of the hydrological regime of rivers of Northern and Central Europe in

various periods of the North Atlantic Oscillation Quaestiones Geographicae 24, Adam Mickiewicz

University Press, Pozna ń , 2005, pp , Figs 7, Table 2 ISBN 83-232-0454-3 ISSN 0137-477X.

ABSTRACT The article presents the impact of the north Atlantic atmospheric circulation on the hydrological regimes of rivers of Northern and Central Europe A river regime is defined as the type and temporal structure of streamflow in an average hydrological cycle The elements of the structure include so-called hydrological periods, 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 periods were determined via the grouping of elementary time units of the year (pentads) on the basis of the similarity of one of their features (the frequency distribution of the river discharge) The determination

of hydrological periods was carried out on the basis of sets of 20 years with an extremely high and an extremely low NAO index, as well as 20-year periods from the 1901-2000 interval The varying number and temporal structure of the hydrological periods thus distinguished allowed conclusions about differences in the variability of hydrological regimes of Scandinavian and Central European rivers in the various NAO circulation periods

Introduction

The hydrological regime defines the state and responses of a river system with relation to the climatic system and the catchment's physical-geographic characteristics Variations in the streamflow over the year strongly depend on climatic conditions Deviations of climatic elements from average levels caused by, e.g., changes in the atmospheric circulation modify the conditions in which the streamflow forms, 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) It describes the pressure gradient between the Icelandic Low and the Azores High Its value, especially in winter, determines the intensity of zonal air circulation over the north Atlantic and the European continent In a positive stage of the NAO (NAO+), the pressure gradient is greater and zonal circulation intensifies, which means a greater inflow of moist maritime polar air over the European continent (Hurrell 1995, Hurrell, van Loon 1997, Rogers 1997, Serrese et al 1997, Trigo et al 2002) Western and Central Europe experience then mild winters with frequent thaws and impermanent snow cover The summer is then moderately warm, cloudy, with low-intensity but frequent rains In the negative stage (NAO-), the westerly flow is restricted or stopped and there appear meridional circulation flows The areas over which air masses flow from the north experience cooling, while those to which air masses come from the south enjoy marked amelioration

The occurrence of warm and wet winters as well as frosty and snowy ones in Northern and Central Europe crucially affects the discharge of rivers in those regions

In winter, the discharges of north European rivers are positively correlated with the winter NAO index, and those of Southern Europe are correlated negatively (Shorthouse, Arnell 1997) 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, in Central and Western Europe the dependence of winter precipitation on the NAO is rather weak (Kaczmarek 2002) A strong relation does exist, however, between the NAO indices and air temperatures 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 the negative NAO stage, in turn, winters are severe with

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

In the article a study is made of the effect of the NAO on the temporal structure of hydrological phenomena and their variations in a yearly cycle and on the stability of the characteristics of hydrological regimes The determination of the number and temporal structure of the hydrological periods distinguished in long time series (of about 100 years) is intended to help evaluate changes in the hydrological regimes of the rivers in various circulation periods

Study area and source materials

The effect of the intensity of zonal circulation on river discharges is especially pronounced in Northern and Central Europe Statistically significant differences in runoffs in the various NAO stages can be observed from the winter months till the summer In a NAO+ stage the runoffs are higher in Northern Europe, e.g the Vosso (Fig 2) and in the NAO- stage, in spring in Central Europe, e.g the Warta (Fig 3) The strength of the discovered links is similar, but their nature is different For this reason these regions seem to be particularly interesting for a study of the effect of the intensity of zonal circulation on the characteristics of a hydrological regime

The analysis embraced 17 rivers flowing in a variety of environmental conditions obtaining in those regions (Fig 1) Use was made of the daily discharge figures for the period from 1901 to 2000 They come mostly from the resources of the Global Runoff Data Center The NAO values employed, calculated according to J Hurrell's (1995) method, were obtained from the NOAA through the Internet

Figure 1 Study area

1 – the Tana at Polmak, 2 – the Lakselv at Skoganvarre, 3 – the Kalix at Råktfors, 4 – the Fusta at Fustvatn, 5 – the Oulu at Lake Lenfad outlet, 6 – the Öre at Torrböle, 7 – the Kyrön near the mouth, 8 – the Nilakka near Vesanto, 9 – the Gaula at Haga Bru, 10 – the Lagen at Losna, 11 – the Etna at Etna, 12 – the Vosso at Bulken, 13 – the Helge Å at Torsebro, 14 – the Warta at Pozna ń , 15 – the Oder at Eisenhüttenstadt, 16 – the Labe at Dresden, 17 – the Jizera at Turice

Figure 2 Mean runoff of the Vosso river in 20 years with the lowest (NAO-) and 20 years with the highest (NAO+) NAO values

Figure 3 Mean runoff of the Warta river in 20 years with the lowest (NAO-) and 20 years with the highest (NAO+) NAO values

Methods of study

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, 1991) An important point in the research procedure is distinguishing characteristic stages of the cycle that were termed hydrological periods It consists in the grouping of elementary time units of the hydrological year on the basis of the

VOSSO - BULKEN

0

10

20

30

40

50

60

70

80

90

100

pentads

NAO- NAO+

WARTA - POZNA Ń

0

1

2

3

4

5

6

7

pentads

NAO- NAO+

similarity of one of their features The elementary time unit adopted is a five-day period of time, or a pentad, and the grouping criterion, the river discharge Thus, the calendar year consists of 73 pentads, each represented by five discharge figures The nearly secular discharge measurement series under study were divided into 20-year intervals In such a 20-20-year observation period, a 100-element set of discharge values is obtained for each of the pentads The grouping characteristic is described

by variable x, which is presented in the form of a discharge frequency distribution

The similarity of water discharge distributions was established using the non-parametric Kolmogorov-Smirnov test The result of the testing procedure 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 term

of the similarity of their feature 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 4 Along its diagonal, groups of similar pentads can be noted; it is those groups 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 A river regime is defined here as a kind and temporal structure of river discharges in a normal hydrological cycle The elements of the structure are hydrological periods, which provide a tool for the study of a regime and a basis for its classification

In the initial stage of the research, a comparison was made of the characteristics

of river regimes in periods with low and high winter values of the NAO Of the 100-year study period, two sets of 20 100-years were selected, one with the lowest and the other with the highest NAO values In the next stage, the 100-year measurement

0 200 400 600

800

SWQ

GGSQ

DGSQ SSQ

1% 5% 10%

25% 50% 90%

SNQ

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 73

1 1 4 7 7 8 11 7 10 15 27 28 30 36 43 30 38 40 53 50 49 48 43 38 28 28 24 19 10 25 19 18 32 32 32 36 32 25 15 26 26 24 27 27 29 32 32 37 34 32 32 32 37 30 26 24 13 10 10 10 8 11 7 6 8 7

2 4 1 7 5 11 11 10 13 18 30 32 33 39 46 32 38 43 56 53 52 48 43 38 28 25 16 10 25 25 25 32 32 32 36 32 25 15 24 29 29 24 27 27 29 32 32 37 34 32 32 32 37 30 26 24 13 8 6 6 7 11 5 6 11 9

3 7 7 1 9 13 10 10 16 30 31 33 39 46 37 45 47 56 55 53 55 50 45 35 32 18 18 18 14 5 14 16 27 27 29 30 32 29 25 18 15 26 31 31 25 25 25 25 30 27 25 25 25 30 19 17 15 10 10 7 7 14 18 12 6 11 9

4 7 5 9 1 7 13 8 15 17 28 32 33 37 44 30 36 41 54 51 50 46 41 36 29 29 25 24 24 24 12 27 19 34 34 34 38 34 27 17 25 25 26 29 29 31 34 34 39 36 34 34 34 39 32 28 26 24 15 9 6 6 8 9 8 5 9 8

5 8 11 13 7 1 9 8 12 18 28 33 34 37 44 32 40 42 54 51 50 50 45 40 36 36 32 31 31 31 27 13 19 17 15 14 18 30 30 30 34 30 13 15 18 19 25 25 27 30 30 35 32 30 30 30 35 28 24 12 9 7 8 15 16 15 12 11 15

6 11 11 10 13 9 1 12 24 25 33 34 39 43 41 49 51 56 59 57 59 54 49 39 36 29 28 28 28 24 10 10 14 8 13 17 17 25 14 7 15 16 16 18 26 26 19 15 15 15 15 12 9 9 18 16 9 10 12

7 7 10 10 8 8 12 1 9 12 27 28 29 36 29 37 39 46 47 45 47 42 37 28 28 24 19 11 26 18 33 33 33 37 33 26 16 26 26 25 28 28 30 33 33 38 35 33 33 33 38 31 27 25 17 12 11 9 10 10 6 6 7

8 10 13 16 15 12 9 1 9 24 29 36 29 33 46 43 42 39 34 29 27 27 18 19 30 34 28 26 25 29 41 41 41 45 41 34 24 26 27 30 33 36 36 38 41 41 46 43 41 41 41 46 39 35 33 31 17 15 15 15 15 15 14 15 13

9 15 18 17 18 24 12 9 1 12 19 28 25 27 38 35 34 35 30 25 18 18 17 19 19 18 34 38 32 30 32 33 45 45 45 49 45 38 28 31 36 36 37 40 40 42 45 45 50 47 45 45 45 50 43 39 37 35 28 25 19 17 16 18 16 16 14

1027 30 30 28 28 25 12 1 15 16 16 18 26 28 33 36 34 36 31 26 27 24 30 30 30 33 37 31 31 39 40 44 44 44 48 44 37 32 29 34 34 36 39 39 41 44 44 49 46 44 44 44 49 42 40 40 40 40 37 32 27 25 27 30 24 19

1128 32 31 32 33 33 27 19 15 1 6 19 29 31 36 39 37 39 34 29 28 28 36 38 38 33 33 33 30 34 38 38 39 40 41 41 41 45 41 38 34 36 35 35 35 37 39 40 41 41 46 43 41 41 41 46 40 40 40 40 40 38 38 38 36 33 33 33 33 30

1230 33 33 33 34 34 28 24 16 6 1 13 18 15 25 30 33 31 33 28 26 29 30 37 39 39 34 34 34 36 40 39 39 42 43 47 47 47 51 47 40 35 37 39 39 39 42 42 44 47 47 52 49 47 47 47 52 45 43 43 43 43 40 39 39 37 34 34 34 34 31

1336 39 39 37 37 39 29 29 16 19 13 1 9 10 13 15 17 18 15 17 25 36 30 31 33 39 39 39 49 53 47 45 48 49 60 60 60 64 60 53 43 45 45 49 52 55 55 57 60 60 65 62 60 60 60 65 58 54 52 50 49 46 41 36 34 36 39 33 29 30

1443 46 46 44 44 43 36 36 28 18 9 1 14 15 15 15 16 15 16 16 18 32 43 37 38 40 46 46 46 53 57 51 49 55 56 64 64 64 68 64 57 48 49 49 53 56 59 59 61 64 64 69 66 64 64 64 69 62 58 56 56 56 53 48 43 41 43 46 40 35 37

1530 32 37 30 32 41 29 17 18 15 10 14 1 8 11 24 18 13 16 19 29 27 29 29 33 38 40 51 55 49 47 46 50 62 62 62 66 62 55 45 47 49 51 54 57 57 59 62 62 67 64 62 62 62 67 60 56 54 52 43 39 34 32 27 29 32 33 31 30

1638 38 45 36 40 49 37 29 25 26 29 13 15 8 1 10 19 13 16 26 30 34 36 36 41 46 48 59 63 57 55 54 58 70 70 70 74 70 63 53 55 55 59 62 65 65 67 70 70 75 72 70 70 70 75 68 64 62 60 51 40 40 40 34 31 33 41 39 38

1740 43 47 41 42 51 39 33 27 28 31 25 15 15 11 10 1 13 10 9 11 14 29 40 34 35 38 43 48 50 61 65 59 57 56 60 72 72 72 76 72 65 55 57 57 61 64 67 67 69 72 72 77 74 72 72 72 77 70 66 64 62 53 50 45 42 38 40 43 43 41 40

1853 56 56 54 54 56 46 46 38 33 36 30 17 15 24 13 1 7 4 16 26 28 42 53 47 48 50 56 56 56 66 70 64 62 65 66 77 77 77 81 77 70 60 62 62 66 69 72 72 74 77 77 82 79 77 77 77 82 75 71 69 67 66 63 58 53 51 53 56 50 46 47

1950 53 55 51 51 59 47 43 35 36 39 33 16 10 7 1 4 13 25 39 50 44 45 47 53 56 58 69 73 67 65 64 68 80 80 80 84 80 73 63 65 65 69 72 75 75 77 80 80 85 82 80 80 80 85 78 74 72 70 63 60 55 50 48 50 53 51 49 48

2049 52 53 50 50 57 45 42 34 34 37 31 18 15 19 9 4 4 1 12 24 38 49 43 44 46 52 54 56 67 71 65 63 62 66 78 78 78 82 78 71 61 63 63 67 70 73 73 75 78 78 83 80 78 78 78 83 76 72 70 68 62 59 54 49 47 49 52 49 47 46

2148 48 55 46 50 59 47 39 35 36 39 33 16 18 13 11 16 13 12 1 10 12 26 37 32 38 46 51 56 58 69 73 67 65 64 68 80 80 80 84 80 73 63 65 65 69 72 75 75 77 80 80 85 82 80 80 80 85 78 74 72 70 61 50 50 50 41 37 43 51 49 48

2243 43 50 41 45 54 42 34 30 31 34 28 15 16 13 16 14 26 10 1 8 16 27 27 33 41 46 51 53 64 68 62 60 59 63 75 75 75 79 75 68 58 60 60 64 67 70 70 72 75 75 80 77 75 75 75 80 73 69 67 65 56 45 45 45 36 32 38 46 44 43

2338 38 45 36 40 49 37 29 25 26 29 26 17 18 16 28 25 24 12 8 1 14 25 28 36 41 46 48 59 63 57 55 54 58 70 70 70 74 70 63 53 55 55 59 62 65 65 67 70 70 75 72 70 70 70 75 68 64 62 60 51 40 40 40 31 27 33 41 39 38

2428 28 35 29 36 39 28 27 18 28 29 25 32 19 26 29 42 39 38 26 16 14 1 11 12 18 26 31 36 38 49 53 47 46 50 50 60 60 60 64 60 53 43 49 54 54 52 55 55 57 60 60 65 62 60 60 60 65 58 54 52 50 41 33 30 30 31 29 28

2528 25 32 29 36 36 28 27 18 27 28 30 36 43 29 30 40 53 50 49 37 27 25 11 1 9 15 28 33 35 46 50 44 46 50 50 57 57 57 61 57 50 40 49 54 54 49 52 52 54 57 57 62 59 57 57 57 62 55 51 49 47 38 33 29 28 28 26 25

26 25 32 29 24 17 36 37 30 37 27 34 34 47 44 43 32 27 12 9 1 6 14 19 24 26 37 41 35 42 46 46 48 49 51 52 48 41 34 45 50 50 44 43 43 45 48 48 53 50 48 48 48 53 46 42 40 38 29 29 25 24 17 16 17 17

27 18 24 31 28 19 38 39 31 38 29 36 35 48 45 44 38 33 28 18 15 6 1 8 13 18 31 40 34 41 45 45 47 48 50 46 42 35 33 44 49 49 43 39 40 43 43 42 47 44 42 42 42 47 40 36 34 32 26 28 24 16 15 16 19 16

28 18 24 31 28 19 24 38 39 33 40 29 36 38 50 47 46 46 41 36 26 14 8 1 6 10 18 25 40 34 41 45 45 47 48 50 44 34 27 33 44 49 49 43 39 40 43 43 41 39 36 35 34 34 39 32 29 26 27 26 28 24 16 15 16 19 16

29 18 24 31 28 18 30 33 34 39 46 33 41 43 56 53 52 51 46 41 31 28 19 13 6 1 5 18 25 40 34 41 45 45 47 48 50 44 30 27 33 44 49 49 43 39 40 43 43 41 34 35 35 31 30 34 30 29 25 27 26 28 24 16 15 16 19 16

30 19 16 14 27 24 19 18 30 33 34 39 46 38 46 48 56 56 54 56 51 46 36 33 24 18 10 5 1 14 36 30 37 41 41 43 44 46 40 26 29 40 45 45 39 35 36 39 39 37 29 31 31 27 26 29 26 25 24 19 15 19 13 15 16 12

31 10 10 5 12 13 10 11 19 30 33 34 39 46 40 48 50 56 58 56 58 53 48 38 35 26 18 18 14 1 11 16 27 27 29 30 32 26 15 15 26 31 31 25 25 25 27 24 27 16 14 12 10 10 8 8 17 15 7 11 10

32 14 19 10 30 34 33 30 36 49 53 51 59 61 66 69 67 69 64 59 49 46 37 31 25 25 11 1 15 10 16 25 19 11 10 8 19 24 24 18 14 15 18 18 16 16 13 11 11 11 16 10 10 10 10 10 19 19 19 28 32 26 18

3325 25 27 14 26 34 38 37 34 40 53 57 55 63 65 70 73 71 73 68 63 53 50 41 40 40 40 36 15 1 8 8 9 10 10 10 10 11 10 13 10 8 9 9 5 7 9 10 10 10 12 10 10 10 10 12 10 11 15 13 14 32 36 30 24 25

34 19 19 16 17 8 28 32 31 38 39 47 51 49 57 59 64 67 65 67 62 57 47 44 35 34 34 34 30 16 10 8 1 7 11 11 13 14 16 17 13 7 4 10 15 15 9 8 9 10 13 13 18 15 13 13 13 18 11 10 10 10 10 17 17 17 26 30 24 16 18 19

35 18 19 15 13 18 26 30 31 38 39 45 49 47 55 57 62 65 63 65 60 55 46 46 42 41 41 41 37 16 8 7 1 8 9 15 15 15 19 15 14 8 5 8 8 7 10 10 12 15 15 17 15 15 15 13 12 16 14 15 15 17 18 25 28 25 25

36 25 27 14 17 25 32 39 39 42 48 55 46 54 56 65 64 62 64 59 54 50 50 46 45 45 45 41 27 9 11 8 1 4 16 16 16 16 18 12 13 7 7 8 11 11 13 16 16 18 16 16 16 15 16 18 19 17 29 30 29 26 25 29

37 25 27 18 17 29 33 40 40 43 49 56 50 58 60 66 68 66 68 63 58 50 50 46 45 45 45 41 27 10 11 9 4 1 12 12 12 16 15 18 12 14 8 8 5 7 7 9 12 12 17 14 12 14 15 17 15 16 18 19 18 29 31 29 26 25 29

3832 32 29 34 30 33 41 45 44 41 47 60 64 62 70 72 77 80 78 80 75 70 60 57 48 47 47 47 43 29 10 13 15 16 12 1 1 3 4 17 17 15 15 11 8 8 7 4 4 6 15 12 12 16 17 17 17 18 30 30 30 39 43 37 29 31 32

3932 32 30 34 30 33 41 45 44 41 47 60 64 62 70 72 77 80 78 80 75 70 60 57 49 48 48 48 44 30 10 14 15 16 12 1 1 2 4 18 17 15 15 11 8 9 8 5 5 7 16 13 13 17 18 18 18 19 30 30 30 39 43 37 29 31 32

4032 32 32 34 30 33 41 45 44 41 47 60 64 62 70 72 77 80 78 80 75 70 60 57 51 50 50 50 46 32 25 10 16 15 16 12 3 2 1 6 17 15 15 11 8 11 10 7 7 9 18 15 15 19 25 24 30 30 30 39 43 37 31 31 34

4136 36 29 38 34 25 37 45 49 48 45 51 64 68 66 74 76 81 84 82 84 79 74 64 61 52 46 44 44 40 26 19 11 17 19 16 4 4 6 1 14 17 19 19 15 12 9 9 7 4 4 12 9 9 13 14 14 14 15 19 17 30 34 34 43 47 41 33 35 36

4232 32 25 34 30 33 41 45 44 41 47 60 64 62 70 72 77 80 78 80 75 70 60 57 48 42 34 30 26 11 10 13 15 16 15 17 18 14 1 7 17 15 19 19 13 9 10 13 13 11 5 5 5 1 1 5 2 6 8 10 19 30 30 30 39 43 37 29 31 32

4325 25 18 14 26 34 38 37 38 40 53 57 55 63 65 70 73 71 73 68 63 53 50 41 35 27 27 15 10 13 7 14 18 18 17 7 1 10 17 16 12 13 16 16 14 12 9 8 7 7 12 5 3 3 3 12 32 36 30 24 25

44 15 15 15 17 13 7 16 24 28 32 34 35 43 48 45 53 55 60 63 61 63 58 53 43 40 34 33 33 33 29 15 8 10 4 8 12 12 17 17 17 17 10 1 11 16 16 10 12 12 14 17 17 19 17 17 17 15 11 9 8 8 13 13 13 26 14 17

45 24 26 15 16 26 31 29 36 37 45 49 47 55 57 62 65 63 65 60 55 49 49 45 44 44 44 40 26 19 8 10 5 13 14 15 15 15 19 15 17 11 1 6 6 9 10 13 14 15 15 17 15 15 15 14 15 19 17 18 16 28 29 28 25 24 28

4626 29 31 25 18 26 27 36 34 35 39 45 49 49 55 57 62 65 63 65 60 55 54 54 50 49 49 49 45 31 24 9 15 8 7 8 15 15 15 19 19 16 6 1 4 7 10 10 12 15 15 17 15 18 19 19 24 25 26 33 34 33 30 29 33

4726 29 31 25 19 26 30 36 34 35 39 49 53 51 59 61 66 69 67 69 64 59 54 54 50 49 49 49 45 31 24 9 15 8 7 8 11 11 11 15 19 16 6 4 1 6 10 9 8 11 11 17 14 14 18 19 19 19 24 25 26 33 34 33 30 29 33

4824 24 25 26 15 25 33 37 36 35 39 52 56 54 62 64 69 72 70 72 67 62 52 49 44 43 43 43 39 25 18 5 9 7 8 5 8 8 8 12 13 16 10 9 7 6 1 4 5 5 8 8 13 10 8 12 13 13 13 14 18 16 17 31 35 29 24 24 27

4927 27 29 25 16 28 36 40 39 37 42 55 59 57 65 67 72 75 73 75 70 65 55 52 43 39 39 39 35 14 7 8 10 11 7 8 9 11 9 9 12 12 10 10 10 4 1 4 5 5 5 10 7 7 8 9 10 11 12 14 14 15 25 25 25 34 38 32 24 26 27

5027 27 29 25 16 28 36 40 39 39 42 55 59 57 65 67 72 75 73 75 70 65 55 52 43 40 40 40 36 15 9 9 10 11 7 7 8 10 9 10 13 12 13 10 9 5 4 1 3 5 5 10 7 6 9 10 10 10 11 15 13 14 25 25 25 34 38 32 24 26 27

5129 29 25 31 27 18 30 38 42 41 40 44 57 61 59 67 69 74 77 75 77 72 67 57 54 45 43 43 43 39 25 18 10 10 12 13 9 4 5 7 7 13 16 14 14 12 8 5 5 3 1 3 3 11 8 8 12 13 13 13 14 18 16 17 27 27 27 36 40 34 26 28 29

5232 32 25 34 30 33 41 45 44 41 47 60 64 62 70 72 77 80 78 80 75 70 60 57 48 43 43 43 39 25 18 10 13 15 16 12 4 5 7 4 13 16 17 15 15 11 8 5 5 3 1 2 11 8 8 12 13 13 13 14 18 16 19 30 30 30 39 43 37 29 31 32

5332 32 25 34 30 33 41 45 44 41 47 60 64 62 70 72 77 80 78 80 75 70 60 57 48 42 41 41 37 16 10 13 15 16 12 6 7 9 4 11 14 17 15 15 11 8 5 5 3 2 1 9 6 6 10 11 11 11 12 16 14 19 30 30 30 39 43 37 29 31 32

5437 37 30 39 35 26 38 46 50 49 46 52 65 69 67 75 77 82 85 83 85 80 75 65 62 53 47 39 34 29 27 16 12 18 17 15 16 18 12 5 12 17 13 10 10 11 11 9 1 3 5 5 5 5 7 11 13 15 24 35 35 35 44 48 42 34 36 37

5534 34 27 36 32 35 43 47 46 43 49 62 66 64 72 74 79 82 80 82 77 72 62 59 50 44 36 35 31 24 13 10 15 17 18 14 12 13 15 9 5 9 19 17 17 14 10 7 7 8 8 6 3 1 2 4 5 5 5 8 10 12 32 32 32 41 45 39 31 33 34

5632 32 25 34 30 33 41 45 44 41 47 60 64 62 70 72 77 80 78 80 75 70 60 57 48 42 35 35 31 11 10 13 15 16 12 12 13 15 9 5 8 17 15 15 14 8 7 6 8 8 6 5 2 1 4 5 5 5 6 10 10 19 30 30 30 39 43 37 29 31 32

5732 32 25 34 30 33 41 45 44 41 47 60 64 62 70 72 77 80 78 80 75 70 60 57 48 42 34 31 27 11 10 13 15 16 14 16 17 19 13 1 7 17 15 18 18 12 8 9 12 12 10 5 4 4 1 1 5 3 6 8 10 19 30 30 30 39 43 37 29 31 32

5832 32 25 34 30 33 41 45 44 41 47 60 64 62 70 72 77 80 78 80 75 70 60 57 48 42 34 30 26 11 10 13 15 16 15 17 18 14 1 7 17 15 19 19 13 9 10 13 13 11 5 5 5 1 1 5 2 6 8 10 19 30 30 30 39 43 37 29 31 32

5937 37 30 39 35 26 38 46 50 49 46 52 65 69 67 75 77 82 85 83 85 80 75 65 62 53 47 39 34 29 27 16 12 18 17 18 14 5 12 19 13 10 10 13 13 11 5 5 5 5 5 1 7 11 13 15 24 35 35 35 44 48 42 34 36 37

6030 30 32 28 19 31 39 43 42 40 45 58 62 60 68 70 75 78 76 78 73 68 58 55 46 40 32 30 26 10 10 11 13 15 15 17 18 14 2 5 15 14 19 19 13 11 10 13 13 11 7 5 5 3 2 7 1 4 6 8 17 28 28 28 37 41 35 27 29 30

6126 26 19 28 24 15 27 35 39 40 40 43 54 58 56 64 66 71 74 72 74 69 64 54 51 42 36 29 29 25 16 10 11 10 12 16 16 18 19 15 6 3 11 15 14 12 11 14 14 12 11 8 6 6 6 11 4 1 4 4 13 24 24 24 33 37 31 25 26

6224 24 17 15 25 33 37 40 40 43 52 56 54 62 64 69 72 70 72 67 62 52 49 40 34 26 25 14 10 15 10 16 25 19 8 3 9 19 24 24 18 14 15 18 18 16 13 10 10 8 8 13 6 4 1 2 11 31 35 29 24

63 15 15 31 35 40 40 43 50 56 52 60 62 67 70 68 70 65 60 50 47 38 32 27 27 12 10 13 10 14 18 18 17 10 3 8 17 16 14 13 16 16 14 15 12 10 10 10 15 8 4 2 1 9 29 33 27 19

64 13 13 10 15 12 15 28 40 40 43 49 56 43 51 53 66 63 62 61 56 51 41 38 29 26 26 26 10 10 14 10 15 19 19 24 19 12 8 18 17 15 14 17 19 19 24 19 19 19 24 17 13 11 9 1 11 11 13 24 18 16 19

65 10 8 10 9 9 12 17 17 25 37 38 40 46 53 39 40 50 63 60 59 50 45 40 33 33 29 28 28 28 24 10 19 17 15 17 18 30 30 30 34 30 13 16 25 27 30 30 35 32 30 30 30 35 28 24 11 1 5 10 12 13 12 13 18 16

66 10 6 7 6 7 9 12 15 32 38 39 41 48 34 40 45 58 55 54 50 45 40 30 29 25 24 24 24 8 19 17 17 30 30 30 34 30 13 25 25 25 27 30 30 35 32 30 30 30 35 28 24 11 5 1 5 9 13 8 8 13 11

67 10 6 7 6 8 9 11 15 19 27 38 39 36 43 32 40 42 53 50 49 50 45 40 30 28 24 19 8 19 17 18 30 30 30 34 30 13 26 26 25 27 30 30 35 32 30 30 30 35 28 24 13 10 5 1 9 13 7 5 8 8

68 8 7 14 8 15 18 9 15 17 25 36 37 34 41 27 34 38 51 48 47 41 36 31 17 16 16 16 15 17 28 32 26 25 29 29 39 39 39 43 39 32 28 33 33 31 34 34 36 39 39 44 41 39 39 39 44 37 33 31 29 12 9 9 1 4 5 10 8 7

69 11 11 18 9 16 10 15 16 27 33 34 36 43 29 31 40 53 50 49 37 32 27 16 15 15 15 19 32 36 30 28 30 31 43 43 43 47 43 36 26 29 34 34 35 38 38 40 43 43 48 45 43 43 43 48 41 37 35 33 24 13 13 13 4 1 6 14 12 11

70 7 5 12 8 15 16 10 15 18 30 33 34 39 46 32 33 43 56 53 52 43 38 33 17 16 16 16 13 15 26 30 24 25 29 29 37 37 37 41 37 30 28 33 33 29 32 32 34 37 37 42 39 37 37 37 42 35 31 29 27 18 12 8 7 5 6 1 8 11 9

71 6 6 6 5 12 9 6 14 16 24 33 34 33 40 33 41 43 50 51 49 51 46 41 31 28 19 19 19 15 7 18 16 26 26 29 29 31 33 29 14 25 30 30 24 24 24 26 29 29 34 31 29 29 29 34 27 19 16 13 8 5 10 14 8 1 5 4

72 8 11 11 9 11 10 6 15 16 19 33 34 29 35 31 39 41 46 49 47 49 44 39 29 26 16 11 24 25 25 31 31 31 35 31 24 14 24 29 29 24 26 26 28 31 31 36 33 31 31 31 36 29 25 18 13 8 8 12 11 5 1 4

73 7 9 9 8 15 12 7 13 14 30 31 30 37 30 38 40 47 48 46 48 43 38 28 2517 16 16 16 12 10 25 25 29 29 32 32 34 36 32 251728 33 33 27 27 27 29 32 32 37 34 32 32 32 37 30 26 24 19 16 11 8 7 11 9 4 4 1

1 2

Figure 4 Hydrological periods on the Warta in the Pozna ń gauging profile

A - Hydrogram of water discharges of specified probability of occurrence, with characteristic discharge and boundaries of medium discharge zone: SWQ - average peak discharge, SSQ- average medium discharge, SNQ - average low discharge, GGSQ - upper boundary of medium discharge zone, DGSQ - lower boundary of medium discharge zone

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

series were divided into 20-year intervals (1901-1920, 1911-1930, 1921-1940, , 1981-2000) for which hydrological periods were distinguished It allowed an evaluation of long-term changes in the hydrological regimes of selected rivers seen against changes in the NAO index over that time interval

The next step was the application of classification methods to reveal the structure

of similarity among the hydrological period distinguished, which made it possible to identify types of hydrological periods To this end, use was made of Ward's method

of hierarchical grouping Each hydrological period was described by means of four variables characterising the frequency distribution of water discharge: medium discharge (d5), the coefficient of variation (Cv), the coefficient of skewness (s), and the coefficient of discharge (w) defining water abundance of a hydrological period

Typology of hydrological periods

The grouping procedure allowed 12 types of hydrological periods to be distinguished; their characteristics are presented in Table 1

Table 1 Characteristics of the types of hydrological periods

Characteristics of explanatory variables Type of period

(N – number)

Name of explanatory variable

Mean Standard

deviation (s)

Variation coefficient (C v )

I - Deep stable low

water

(N- 20)

d 5

C v

s

w duration (days)

0.61 0.49 2.14 0.34 118.80

0.21 0.17 1.72 0.14 53.32

0.35 0.36 0.80 0.41 0.45

0.31 0.24 0.43 0.09 30.0

1.10 0.82 7.54 0.57 200.0

II - Deep unstable

low water

(N – 7)

d 5

C v

s

w duration (days)

0.44 1.14 4.68 0.30 99.71

0.19 0.15 1.25 0.11 51.24

0.43 0.14 0.27 0.35 0.51

0.16 0.99 3.25 0.15 30.0

0.72 1.40 6.63 0.46 177.0 III – Average low

water

(N – 16)

d 5

C v

s

w duration (days)

0.63 1.08 2.84 0.58 70.81

0.17 0.57 1.29 0.08 31.89

0.27 0.53 0.45 0.13 0.45

0.41 0.25 0.75 0.41 27.0

0.97 2.18 4.94 0.72 145.0

IV - Shallow low

d 5

C v

0.95 0.59

0.23 0.18

0.24 0.30

0.70 0.35

1.52 0.95

water

(N – 17)

s

w duration (days)

2.55 0.80 118.76

2.11 0.08 58.17

0.83 0.11 0.49

0.50 0.70 33.00

7.42 1.00 210.0

V - Normal stable

period

(N – 26)

d 5

C v

s

w duration (days)

1.31 0.66 2.23 1.05 62.19

0.24 0.12 1.08 0.11 33.97

0.18 0.19 0.48 0.10 0.55

0.94 0.46 0.76 0.90 15.00

1.76 0.86 5.58 1.28 142.0

VI – Normal

unstable period

(N – 18)

d 5

C v

s

w duration (days)

1.48 1.05 2.80 1.04 83.44

0.50 0.17 1.72 0.15 50.70

0.34 0.16 0.61 0.14 0.61

0.68 0.88 1.17 0.85 15.00

3.04 1.52 8.58 1.30 150.0 VII – Low stable

rise

(N – 29)

d 5

C v

s

w duration (days)

1.84 0.53 1.26 1.45 47.10

0.40 0.12 0.46 0.17 23.22

0.22 0.23 0.37 0.12 0.49

1.24 0.38 0.62 1.20 18.00

2.81 0.88 2.18 1.78 102.0 VIII – Low unstable

rise

(N – 7)

d 5

C v

s

w duration (days)

1.67 1.21 2.10 1.49 31.29

0.54 0.15 0.40 0.09 12.98

0.32 0.13 0.19 0.06 0.41

0.74 1.03 1.52 1.34 20.00

2.38 1.43 2.55 1.63 52.00

IX – Average

stable rise

(N – 18)

d 5

C v

s

w duration (days)

2.96 0.63 1.75 2.00 39.72

0.86 0.27 0.85 0.14 20.59

0.29 0.43 0.49 0.07 0.52

1.16 0.22 -0.01 1.77 20.00

4.62 1.46 3.24 2.28 95.00 X- Average

unstable rise

(N – 5)

d 5

C v

s

w duration (days)

2.61 1.05 1.74 1.92 19.60

0.89 0.15 0.32 0.11 7.60

0.34 0.15 0.18 0.06 0.39

1.77 0.86 1.36 1.78 15.00

3.83 1.29 2.19 2.03 33.00

XI – High stable

rise

(N – 11)

d 5

C v

s

w duration (days)

4.72 0.51 0.92 2.73 32.55

1.46 0.18 0.69 0.24 16.49

0.31 0.36 0.75 0.09 0.51

2.96 0.27 -0.32 2.40 10.00

7.35 0.88 2.22 3.04 60.00 XII – Disastrous

rise

(N – 12)

d 5

C v

s

w duration (days)

9.25 0.62 1.05 4.19 25.75

2.39 0.09 0.50 0.55 9.20

0.26 0.14 0.47 0.13 0.36

6.20 0.49 0.27 3.25 15.00

13.90 0.77 1.86 5.02 40.00

The sequence of the types of hydrological periods in the yearly cycle is presented in Table 2 and Fig 6 Three groups of rivers can be distinguished in them which differ in the sequence and temporal structure of the hydrological periods and their types: the rivers of Norway and Sweden, those of Finland, and those of Central Europe This regional division is clearly the result of the geographical location and

specific environmental conditions, mainly climatic, obtaining in the catchments of the rivers in question

Scandinavian rivers differ from Central European ones in that they usually have regimes with a greater number of hydrological periods (6 or even 7) and markedly different dates of their occurrence Owing to a higher seasonal variability of their discharges, the types of hydrological periods distinguished are extreme in terms of their water abundance Hence their regimes feature sharp contrasts, with a deep winter low-water stage and a very high late-spring rise In regime terms, they do not form a uniform group, since the flow of each river is controlled by its own individual natural conditions, as has been mentioned earlier The Helge Å in southern Sweden and Finnish rivers (mostly the Oulu and Nilakka) stand out among them for the clearly distinct temporal structure of their hydrological periods In comparison with the Scandinavian streams, those in Central Europe have much less contrasting regimes, with a high spring rise and a shallow summer-autumn low-water stage

Table 2 Characteristics of the types of hydrological periods at extreme values of the NAO Index

River

Q m

3 s

-1 ]

Q m

3 s

-1 ]

I 359 98 105 0.483 0.474 0.86 0.090 I 351 95 110 0.658 3.715 1.18 0.123 III 99 130 32 1.742 2.833 8.95 0.935 VI 96 125 30 1.515 2.890 9.66 1.009 XII 131 150 20 0.621 0.694 50.28 5.253 XII 126 155 30 0.740 1.314 42.71 4.463

XI 151 165 15 0.671 1.189 23.96 2.503 XI 156 165 10 0.883 1.836 26.48 2.767

VI 166 298 133 1.032 2.608 11.42 1.193 VI 166 295 130 1.233 3.655 9.69 1.013

II 299 358 60 0.983 2.965 2.73 0.286 III 296 350 55 1.501 4.941 4.71 0.492

II 359 110 117 1.395 5.577 10.57 0.327 III 331 110 145 1.317 3.157 18.61 0.575

V 111 145 35 0.739 0.986 39.14 1.210 V 111 135 25 0.746 0.761 41.50 1.283

IX 146 180 35 0.492 0.784 67.11 2.074 XI 136 168 33 0.502 0.638 77.74 2.403 VII 181 203 23 0.531 0.966 43.35 1.340 VII 169 223 55 0.455 0.953 52.30 1.617

V 204 250 47 0.668 1.495 29.10 0.899 V 224 305 82 0.823 1.854 37.02 1.144 VII 251 305 55 0.726 1.391 45.28 1.400 VI 306 330 25 1.205 2.470 28.01 0.866

III 306 358 53 1.194 3.479 18.61 0.575

I 361 100 105 0.820 3.471 10.92 0.139 II 306 103 163 1.322 6.155 20.55 0.264

G u VI 101 130 30 1.161 1.889 82.42 1.049 VIII 104 125 22 1.219 2.548 119.03 1.515