Contemporary changes of mean annual and seasonal river discharges in Croatia

Abstract

This paper presents the first comprehensive nationwide trend detection of mean discharges in Croatia. Over the last 30 years, global climate change has manifested itself largely in the rise of the average annual temperature, which has resulted in the changes of evapotranspiration and precipitation patterns. The consequences of those changes have been varied spatial impacts on river discharge (flow). We analysed the changes in mean river discharges in Croatia at 53 gauging hydrological stations over the most recent period from 1990 to 2009. To assess the trend, the Kendall-Theil (Sen) non-parametric trend test was carried out for the yearly and seasonal mean discharge values. We found evidence of redistribution of discharge throughout the year, an increase in autumn and winter discharges (especially on the rivers dominantly fed by snowmelt) and a decrease in summer discharge values. Furthermore, we detected a change in the month of the appearance of mean discharge maxima and minima. In most cases the changes can be explained by the changes in the regime of climate elements (temperature, precipitation, evapotranspiration) although that information should be treated with care because of the shortness of the time-series. The results are consistent with the ones from the upstream countries, i.e. Slovenia and Austria.

Full text

7
HRVATSKI GEOGRAFSKI GLASNIK 77/1, 7 − 27(2015.)
UDK 556.535(497.5)
911.2:556](497.5)
Original scientifi c paper
Izvorni znanstveni članak
Contemporary Changes of Mean Annual
and Seasonal River Discharges in Croatia
Ivan Čanjevac, Danijel Orešić
This paper presents the fi rst comprehensive nationwide trend detection of mean dis-
charges in Croatia. Over the last 30 years, global climate change has manifested itself
largely in the rise of the average annual temperature, which has resulted in the changes
of evapotranspiration and precipitation patterns. The consequences of those changes have
been varied spatial impacts on river discharge (fl ow). We analysed the changes in mean
river discharges in Croatia at 53 gauging hydrological stations over the most recent period
from 1990 to 2009. To assess the trend, the Kendall-Theil (Sen) non-parametric trend test
was carried out for the yearly and seasonal mean discharge values. We found evidence
of redistribution of discharge throughout the year, an increase in autumn and winter dis-
charges (especially on the rivers dominantly fed by snowmelt) and a decrease in summer
discharge values. Furthermore, we detected a change in the month of the appearance of
mean discharge maxima and minima. In most cases the changes can be explained by the
changes in the regime of climate elements (temperature, precipitation, evapotranspira-
tion) although that information should be treated with care because of the shortness of
the time-series. The results are consistent with the ones from the upstream countries, i.e.
Slovenia and Austria.
Key words: discharge, trends, Croatia, rivers, hydrology, geography
Novije promjene srednjih godišnjih i sezonskih protoka
rijeka u Hrvatskoj
U radu se iznose rezultati prve sveobuhvatne analize suvremenih promjena srednjih
mjesečnih protoka rijeka u Hrvatskoj. U posljednjih tridesetak godina globalne promjene
klime uglavnom se očituju kroz porast srednje godišnje temperature zraka, što za po-
sljedicu ima promjene u prostornoj i vremenskoj distribuciji padalina i vrijednosti eva-
potranspiracije. Te promjene imaju različite prostorne posljedice na protoke i protočne
(riječne) režime rijeka. U ovom istraživanju analizirane su promjene srednjih protoka
rijeka u Hrvatskoj na 53 hidrološke stanice od 1990. do 2009. Za analizu trenda na godiš-
njoj i sezonskoj razini upotrijebljen je Kendall-Theilov (odnosno Senov) test. Utvrđeno
je postojanje preraspodjele protoka unutar godine, povećanje jesenskih i zimskih srednjih
protoka (osobito na rijekama sa značajnijim udjelom sniježnice) i smanjenje ljetnih pro-
toka. Uočene su i promjene u mjesecu javljanja maksimuma i minimum srednjih protoka,
koje se u većini slučajeva mogu objasniti promjenama u režimu klimatskih elemenata
(temperature, padalina i evapotranspiracije) iako rezultate treba uzimati s dozom opreza
zbog kratkoće analiziranog niza. Rezultati istraživanja uglavnom se poklapaju s rezultati-
ma sličnih istraživanja u nama uzvodnim zemljama (Slovenija i Austrija).
Ključne riječi: protok, trend, Hrvatska, rijeke, hidrologija, geografi ja

8
Hrvatski geografski glasnik 77/1 (2015.)
INTRODUCTION
Over the last 30 years, global climate change has largely manifested itself in the average
annual temperature rise, resulting in evaporation and precipitation pattern change. These changes
have different spatial impacts on river discharge (streamfl ow) regimes. River discharges are the
most reliable component of the hydrological cycle (from the measurement standpoint) and the
analysis of long-term data can give us valuable information about trends and variations of climate
in the study area (Chiew and McMahon, 1996).
According to FAO, Croatia is a country rich in water resources with around 24 000 m3/year/
capita (FAO, 2012). Although Croatia has a high dependency ratio of around 65% as well, water
resources are among the most vital and strategic natural resources of the country. What is the effect
of (evident) climate change or oscillation on water resources in Croatia? Are there any signifi cant
trends in the discharge regimes of rivers in Croatia? These are the main underlying questions and
the motivation for the study. The answers, together with other studies, should help water managers
and decision-makers in Croatia. This study represents the fi rst comprehensive nationwide assess-
ment of the trends of mean river discharges and should be one of the starting points for future
smaller scale research projects.
RESEARCH AREA AND TIME FRAMEWORK
The research area is Croatia, i.e. rivers in Croatia. More than 50% of the country is covered
by karst with specifi c hydrological processes. Mountainous, western and southern Croatia largely
belong to the Dinarides (Dinaric karst) with the surface and subsurface drainage into the Adriatic
Sea. Eastern, northern and central parts of Croatia are largely non-karst areas with a well-developed
surface drainage network belonging to the Danube River Basin. The Danube Basin in Croatia
covers around 2/3 of the country’s area with around 3/4 of the total Croatian population. The
climate is also diverse, with variations of the continental climate in the Danube River Basin and
transition and variations of the Mediterranean climate towards the south (Dalmatia). Precipitation
patterns as the main input in the hydrological cycle are also very diverse. Total annual amounts
of precipitation vary from less than 200 mm/year in the east to more than 3500 mm/year in
the mountainous part of Gorski Kotar (hinterland of the port city of Rijeka). Diverse climate,
geological and geomorphological features result in a variety of discharge regimes of rivers in
Croatia (Čanjevac, 2013).
For the purposes of this research we have decided to use the most recent 20-year period from
1990 to 2009 as the time framework. Although we have also performed some tests for a limited
number of stations for the 50-year period from 1960 to 2009, we have chosen the most recent
20-year period in order to assess contemporary trends and to perform the analysis on as many
stations and rivers as possible. We are aware that taking a 20-year period for the trend analysis
can lead to less reliable conclusions, but the limitations with data availability and gaps on many
gauging stations in Croatia determined our study.
At the beginning of 2012, there were 448 gauging stations under the auspices of the
Meteorological and Hydrological Service of Croatia. This research uses the mean monthly
discharge data from 116 hydrological stations in Croatia. The main selection criterion is the length
of time series without gaps. Through the selection and homogeneity testing we chose 53 stations
on 39 rivers for the performance of statistical tests.

9
Contemporary Changes of Mean Annual and Seasonal River Discharges in Croatia
PREVIOUS RESEARCH
The most signifi cant research activity in the last 15 years refers to different aspects of change
in minimum, average and maximum values of discharge on rivers in Croatia. For instance, research
results have been published for the River Sava by O. Bonnaci and D. Oskoruš (2011), D. Trninić
and T. Bošnjak (2009), K. Pandžić et al. (2009), O. Bonacci and I. Ljubenkov (2008), T. Šegota and
A. Filipčić (2007); for the River Drava by O. Bonacci and D. Oskoruš (2010), and M. Gajić-Čapka
and K. Cesarec (10); for the Rivers Lika and Gacka by O. Bonacci and I. Andrić (2008, 2009); for
the rivers of the Kupa Basin by O. Bonacci and I. Andrić (2010), and K. Žganec (2011); while
the research results of discharge changes for the Dalmatian rivers Ombla and Cetina have been
published by O. Bonacci (1995) and O. Bonacci and T. Roje-Bonacci (2001, 2003). According to
their approach, the aforementioned pieces of research can roughly be grouped in two groups. The
fi rst group focuses primarily on describing the changes in discharge within the analysis of climate
element changes, while the other group (Bonacci and co-authors) largely stresses the signifi cance
of the human impact on the river regime. In his studies, along with other hydrological methods,
O. Bonacci uses the RAPS method (Rescaled Adjusted Partial Sums) for determining changes and
trends in data series, particularly valuable for the visualisation of changes and the segmentation of
long-term data series (Garbrecht and Fernandez, 1994).
For reasons of methodology and spatial correlation (mostly upstream countries), we also
analyse the research results of Slovenian (Ulaga, 2002; Frantar, 2003; Frantar, 2005; Frantar, 2007;
Brilly et al., 2007; Ulaga et al., 2008), Austrian (Fürst et al., 2008; Fürst et al., 2010), German
(Bormann, 2010) and Hungarian (Belz et al., 2004) authors who have studied recent discharge
changes in their respective countries and on rivers in the wider area of the Danube Basin. Due
to the changes observed on rivers, which are linked to the changes in snowfall and snow cover
duration, Swiss authors’ research is also analysed (Birsan et al., 2005).
METHODOLOGY
The sources of uncertainty or inaccuracy in hydrological systems are numerous. The most
important uncertainties are those related to hydrological data, since they are the basis of all re-
search and decision-making. Being aware of those uncertainties is highly important if we want to
interpret the changes in time and space. Data limitations and uncertainties will remain an important
part of hydrology in the foreseeable future (Shaw et al., 2010). Therefore, we chose methods that
have been used on multiple occasions and verifi ed through relevant research worldwide and in
Croatia. The Wilcoxon test was used for the homogeneity analysis while the Mann-Kendall test,
considered appropriate because of its resistance to extreme values that often cause problems in
trend analyses, was used for the signifi cativity analysis of changes in discharge (Helsel and Hirsch,
1992; Kundzewicz, 2004; Kundzewicz and Robson, 2004). The Kendall-Theil (or Sen) method
was used for the estimate of the mean annual change of the mean discharge in the observed period
of time, and also in order to get a better picture considering extreme values (Helsel and Hirsch,
1992; Fürst et al., 2008).
HOMOGENEITY OF DATA
Prior to the statistical trend analysis, it was necessary to determine the homogeneity of data
series. The existence of sudden changes or jumps in data, most frequently caused by the change
in the location of the gauging station, modifi cations of the measurement instruments, building of
dams, or sudden changes in watercourse profi les, was determined by testing the data series for
homogeneity. The testing of homogeneity was done by means of different statistical tests, among

10
Tab. 1 Basic information on the hydrological stations used for the trend analysis
Tab. 1. Osnovne informacije o hidrološkim stanicama upotrijebljenima u analizi trendova
River/stream Station
Elevation of
”0”
(m a. s. l.)
Topographic
catchment area of
the station (km
2
)
Average yearly
discharge in m
3
/s
(1990-2009)
Bednja Tuhovec 162,85 469,54 5,01
Bednja Lepoglava 147,35 546,98 1,35
Biđ Vrpolje 78,56 214 1,16
Bijela Badljevina 137,14 170 1,64
Bregana Bregana Remont 152,33 88,5 1,22
Cetina Han 296,63 836 49,49
Čabranka Zamost II 297,66 103 3,47
Česma Čazma 97,11 2406 14,54
Česma Narta 103,36 880,8 4,84
Donja Dobra Stative Donje 116,47 1008 33,87
Donja Mrežnica Mrzlo Polje 113,97 879 25,49
Drava Botovo 121,55 31.038 456,2
Drava D.Miholjac C.S. 88,57 37.142 496,95
Drava Novo Virje skela 108,87 31.803 469,85
Drava Terezino Polje 100,67 33.916 480,05
Gliboki Mlačine 144,45 84 0,56
Gornja Dobra Luke 353,67 162 6,47
Gornja Dobra Turkovići 323,72 298 10,54
Gradna Samobor 150,53 38,1 0,59
Ilova V.Vukovje 98,65 995 7,3
Jadro Majdan 13,46 - 7,14
Korana Slunj uzv. 212,17 944 8,8
Krapina Kupljenovo 128,88 1150 10,12
Krapinčica Zabok 141,48 192,4 1,83
Krka Skradinski buk g. 45,4 2.103 47,39
Kupa Hrvatsko 285,21 370 18,96
Kupa Kamanje 123,83 2047 65,41
Kupa Kupari 304,43 208 12,95
Kupčina Strmac 155,14 125 1,91
Kupica Brod na Kupi 221,43 251 11,92
Kutina Kutina 96,34 55,3 0,34
Londža Pleternica 114,33 483 1,7
Lonja Lonjica most 103,77 326 1,66
Mirna Buzet 42,07 127 1,92
Mirna Portonski most 1,68 483 5,86
Mura Goričan 138,59 13.148 167,55
Hrvatski geografski glasnik 77/1 (2015.)

11
which the Wilcoxon test is one of the most frequently used (Žugaj, 2000; Prohaska and Ristić,
2002; Biondić, 2005; Reimann et al., 2008; Walford, 2011). The Wilcoxon non-parametric test
is based on comparison and summing up of the ranks of two data series (s1 and s2) and the cal-
culation of the standard aberration of the modifi ed series (s2) (Žugaj, 2000). When applying the
test, the data series that needs to be checked is divided in two parts (s1 and s2) according to the
year when the potential cause of non-homogeneity appeared. If the year when potential changes
appeared is unknown, the time series is usually divided into two equal parts (Prohaska and Ristić,
2002). The series (s1 and s2) have to be subsequent and neither can be shorter than seven years
(Žugaj, 2000). The critical region of the null hypothesis (that there are no signifi cant changes in
data) is covered by the lower and upper levels of confi dence of α = ± 5% (Žugaj, 2000; Biondić,
2005). Analogously, in this case, if the value of the probability coeffi cient p was less than 0,05, the
starting hypothesis that the series was homogenous was rejected with a 95% probability and the
conclusion, according to this statistical test, was that the series was not homogenous.
At the beginning of this research, we included 116 gauging stations. After the analysis and
homogeneity testing of 157 data series (in some cases, after non-homogeneity had been deter-
mined on the entire series, the shorter part of the series was analysed), 79 series were determined
as homogeneous. Finally, 53 out of the 79 series, i.e. stations with homogenous and suffi ciently
long data series, were appropriate for trend analysis (Tab. 1, Fig. 1).
Source: Meteorological and Hydrological Service of the Republic of Croatia
Izvor: Državni hidrometeorološki zavod Republike Hrvatske
Tab. 1 continuation
Tab. 1. u nastavku
River/stream Station
Elevation of
”0”
(m a. s. l.)
Topographic
catchment area of
the station (km
2
)
Average yearly
discharge in m
3
/s
(1990-2009)
Mura M.Središče 156,29 10.891 159,87
Novčica Lički Novi 554,32 - 2,32
Orljava Pleternica most 113,76 745 4,99
Orljava Požega 143,97 437,6 3,29
Pazinčica Dubravica 249,59 59,7 0,72
Rječina Martinovo selo 270,38 19 6,64
Sava Podsused žičara 119,13 12.316 265,65
Sava Rugvica 95,61 12.730 277,47
Sava Zagreb 112,26 12.450 274,23
Sava Županja 76,28 62.891 1.027,1
Sunja Sunja 99,14 225,5 2,73
Sutla Zelenjak 162,46 455 6,01
Toplica Daruvar 151,59 30,6 0,31
Trnava Jendrašiček 150,9 105,13 0,31
Vitunjčica Brestovac 335,82 11 3,21
Vrljika Kamenmost 259,69 - 7,06
Vučica Orahovica 169,95 42,6 0,48
Contemporary Changes of Mean Annual and Seasonal River Discharges in Croatia

12
Fig. 1 Hydrological stations used for the trend analysis
Sl. 1. Hidrološke stanice upotrijebljene u analizi trendova
Hrvatski geografski glasnik 77/1 (2015.)

13
TREND ANALYSIS
The analysis was continued by calculating the linear trend equations in the period from 1990
to 2009 for 53 stations on the total of 39 rivers in Croatia. The analysis covered all the important
Croatian rivers with determined data homogeneity.
A descending trend in the annual discharge was found at 29 stations and it was statistically
signifi cant on six of them. An ascending trend was calculated at 24 stations, out of which two
stations showed a statistically signifi cant trend. The signifi cativity of the aforementioned trends
was analysed by the Seasonal Mann-Kendall test. It is necessary to stress that these are the anal-
ysis results of the mean annual value data series and that a more thorough analysis and a possible
determination of general causes of change required a trimestral, i.e. seasonal analysis, approach.
A more detailed analysis of the values of mean discharge changes on a yearly and season-
al basis in the period analysed last (1990-2009) was done in order to detect spatial differences
and critical periods in a year when the changes in discharge on Croatian rivers are the most pro-
nounced. By doing so, we drew closer to determining the key mechanisms causing the changes.
The Kendall-Theil test proved to be successful when determining the average values of change
within a particular period because it takes the period median into consideration when determining
the values of change, instead of, for instance, the starting point value or some other mean value
(Fürst et al., 2008). This allowed us to obtain more reliable data because extreme values did not
affect the results as much. The statistical signifi cativity was calculated for each change on the
lower and upper level of confi dence of α = ± 5%. The testing was done at the same 53 stations on
39 rivers in Croatia as in the case of linear trends.
RESULTS
Results on an annual basis
On an annual basis, in the period from 1990 to 2009, a decrease in average discharge was
detected at 31 stations, and it was statistically signifi cant at four out of the 31 stations (Fig. 2).
The decrease ranges from the minimal -0.02% (Mura) to over -2.3% (Gliboki) a year. A
pronounced decrease, statistically signifi cant at the same time, was detected on smaller streams
of lowland Croatia (Gliboki, Orljava and Česma). An average increase in discharge on an annual
basis was detected at 22 stations with no results being statistically signifi cant. The largest increase
in all Croatia (1.14% annually) was found on the Gornja Dobra (the Turkovići station).
The next step was the analysis of the results for the basins. The Drava Basin was fully charac-
terised by a decrease in discharge values. The values at the stations on the Drava and Mura actually
indicated stagnation or absence of a marked trend of discharge change in the twenty-year period.
The Bednja, Gliboki and Vučica streams, on the other hand, had a more marked decline in dis-
charge. At the Mlačine gauging station (Gliboki) it was -2.34% annually, which was also the larg-
est decrease of all the analysed stations in Croatia. The data for the Sava River in Croatia showed
a very small increase on an annual basis at the stations around Zagreb in the period from 1990 to
2009, whereas a small decrease was detected in the eastern part of Croatia (the Županja station).
The Sava Basin showed mixed results. The right-hand side of the basin (the Kupa Basin) mostly
showed a more or less marked increase in discharge on a yearly level, while a decrease was usually
detected on the left-hand side of the basin. The River (Gornja) Dobra, at the Turkovići station, had
the most pronounced average increase in discharge (1.14%), whereas the most pronounced (and
statistically signifi cant) decrease was detected on the Rivers Česma (both stations) and Orljava (the
Pleternica Most station). The Istrian Rivers Mirna and Pazinčica had a decrease in discharge, as did
the Novčica River in Lika, while Dalmatian rivers showed no signifi cant changes.
Contemporary Changes of Mean Annual and Seasonal River Discharges in Croatia

14
Hrvatski geografski glasnik 77/1 (2015.)
Fig. 2 Values of mean yearly discharge change (% in comparison with the median) in the 1990-2009 period.
Sl. 2. Vrijednosti srednje godišnje promjene protoka (% u odnosu na medijan razdoblja) od 1990. do 2009.

15
Results on a seasonal basis
A number of studies have shown that trends, i.e. changes in discharge, are more marked and
easier to detect on the level of particular months or seasons (Fürst et al., 2008; Arnell, 1992; Arnell
and Reynard, 1996; Baggaley et al., 2009...). This study has shown the same.
Spring
In the seasonal analysis, spring was defi ned as the March-April-May trimester. The observed
results were more marked than on the annual level. A great majority of stations showed an average
annual increase in discharge (Fig. 3).
Contemporary Changes of Mean Annual and Seasonal River Discharges in Croatia
Fig. 3 Values of mean discharge change in spring (% in comparison with the median) in the 1990-2009 period
Sl. 3. Vrijednosti srednje godišnje promjene protoka u proljeće (% u odnosu na medijan razdoblja) od 1990.
do 2009.

16
The increase was the most pronounced in the Kupa Basin where (Donja) Dobra and (Donja)
Mrežnica exhibited a statistically signifi cant increase in discharge of more than 2% annually on
average within the observed timespan. The Drava Basin showed the least pronounced increase.
Within the observed period, only 10 stations exhibited a decrease in the spring months. These were
once again mainly smaller streams of lowland Croatia and the Rivers Korana and Cetina. When
comparing all the months of the year, March showed the most marked positive average change of
discharge (Fig. 4). A decrease in discharge in March was detected for two rivers only (the Korana
and Cetina), while others largely showed a signifi cant average increase, which was also statistical-
ly signifi cant at 23 stations. The largest increase was detected at the Dubravica on the Pazinčica
River (5, 72%) and Turkovići stations on the Gornja Dobra River (5.6%). In the case of the Rivers
Mura and Drava the increase was the least pronounced.
Fig. 4
Values of mean discharge change in March (% in comparison with the median) in the 1990-2009 period.
Sl. 4.
Vrijednosti srednje godišnje promjene protoka u ožujku (% u odnosu na medijan razdoblja) od 1990. do 2009.
Hrvatski geografski glasnik 77/1 (2015.)

17
Summer
The results for the summer trimester (June, July and August) exhibited more pronounced re-
gional differences (Fig. 5). An increase in discharge was detected on the rivers of north Croatia (the
Drava Basin and parts of the Sava Basin) and south Croatia (the Rivers Krka, Cetina and Vrljika).
However, this increase was statistically signifi cant at only one station (Kupčina-Strmac). There
was a marked decrease on the rivers of east and west Croatia, which was statistically signifi cant
at fi ve stations.
The River Kupčina had the most pronounced discharge increase with the summer average
at over 2%, while the increase was again the least pronounced on the Rivers Drava and Mura. On
the other hand, the decrease was marked on smaller karst rivers (the Kupa and Lika Basin), in
Fig. 5
Values of mean discharge change in summer (% in comparison with the median) in the 1990-2009 period.
Sl. 5.
Vrijednosti srednje godišnje promjene protoka ljeti (% u odnosu na medijan razdoblja) od 1990. do 2009.
Contemporary Changes of Mean Annual and Seasonal River Discharges in Croatia

18
Istria (the Mirna) and in smaller basins of lowland Croatia. The Gliboki and Korana had the most
marked decrease of more than -2.5% annually. There were three more stations with a statistically
signifi cant decrease: Ilova - Veliko Vukovje, Mirna - Portonski most and Novčica - Lički Novi.
Autumn
The values of change in the mean monthly discharge for autumn months (September, October
and November) showed a decrease at nearly all the gauging stations (Fig. 6). These were usually
values smaller than -1% per year.
The decrease was statistically signifi cant at ten stations. The smallest decrease was again
detected on the Rivers Drava and Mura. On the other hand, it was the most pronounced on the
Fig. 6
Values of mean discharge change in autumn (% in comparison with the median) in the 1990-2009 period.
Sl. 6.
Vrijednosti srednje godišnje promjene protoka ujesen (% u odnosu na medijan razdoblja) od 1990. do 2009.
Hrvatski geografski glasnik 77/1 (2015.)

19
Istrian River Mirna, the Rivers Česma and Sutla, the Novčica in Lika, the Biđ in Slavonia and
in the Kupa Basin where the negative change values at some stations were lower than -3.5%
annually. The only rivers showing an increase in discharge in the autumn months in the observed
period were the Sunja (Sunja station) and Korana (Slunj station). November was the month with
the most marked decrease in mean discharge value in the entire particular year in the observed
period (Fig. 7). The decrease was even more marked than the autumn months’ average and it was
statistically signifi cant at half of the stations (23). It was the most marked on smaller streams of
lowland Croatia (Kutina, Česma, Orljava) and on Istrian rivers where it exceeded -3.5%. Once
again, positive values of change were detected only on the Rivers Sunja and Korana.
Contemporary Changes of Mean Annual and Seasonal River Discharges in Croatia
Fig. 7
Values of mean discharge change in November (% in comparison with the median) in the 1990-2009 period.
Sl. 7.
Vrijednosti srednje godišnje promjene protoka u studenome (% u odnosu na medijan razdoblja)
od 1990. do 2009.

20
Winter
As in the case of the summer trimester, the results for the winter trimester (December, Janu-
ary and February) also showed more pronounced regional differences (Fig. 8). The largest increase
was detected on the rivers of west Croatia – the rivers of Istria and the Kupa Basin as well as most
Dalmatian rivers (with the exception of the Cetina River). The increase was statistically signifi cant
only on the Gornja Dobra (the Turkovići station).
On the other hand, a decrease in mean discharge value was detected on most rivers of north
and east Croatia. It was equally marked on larger and smaller rivers but statistically signifi cant only
in the case of two smaller lowland streams (the Gliboki and Trnava). The change in discharge value
in the winter months was normally higher or lower than +/- 1%.
Hrvatski geografski glasnik 77/1 (2015.)
Fig. 8
Values of mean discharge change in winter (% in comparison with the median) in the 1990-2009 period.
Sl. 8.
Vrijednosti srednje godišnje promjene protoka zimi (% u odnosu na medijan razdoblja) od 1990. do 2009.

21
DISCUSSION
The trend analysis for the last twenty-year period refl ects a complex picture and confi rms the
hypothesis about uncertainties in making conclusions and generalising results. Little over a half of
the 53 analysed stations had a negative trend, and on most of them the trend was not statistically
signifi cant. More signifi cant decreases were seen in the case of smaller lowland streams of central
and east Croatia. This is probably related to the sensitivity of smaller basins to climatic change or
oscillation. Over the last century, lowland Croatia has generally shown a mild increase in mean
annual air temperature, a decrease in the precipitation quantity and humidity of soil, and an increase
in potential evapotranspiration, with a statistically signifi cant trend since 1987 (Zaninović and
Gajić-Čapka, 1999). Furthermore, the precipitation variability in continental Croatia increases
towards the East (Maradin, 2011; Maradin, 2012). Larger rivers of the continental part (the Sava,
Drava and Mura) exhibited a small decrease or increase primarily as a consequence of changing
conditions in the upstream countries, i.e. Slovenia (Frantar, 2003; Frantar, 2005; Frantar, 2007)
and Austria (Fürst et al., 2008; Fürst et al., 2010). In these cases, the basin size and intensive
human interventions make the causal relationship of climatic elements change and runoff less
clear, although observed signals are the same. A great number of hydroelectric power plants have
been constructed on all three rivers, especially on the Drava (23) and Mura (26). Given that their
construction has further stabilised the already steady runoff regime, the consequences of climatic
changes are more diffi cult to detect.
A more marked increase in annual discharges was detected only in the Kupa Basin. This is
primarily the result of the trend of climatic elements change, i.e. the average annual temperature
increase, decrease in snow-cover duration and the number of days with solid precipitation in the
area (Gajić-Čapka, 2011). Further seasonal analysis confi rmed such a conclusion. Interestingly,
varied trend combinations (decrease – decrease and decrease – increase) were detected at the
stations which were analysed in both the 50 and the 20-year period. As direct human interventions
in the hydrological cycle in Croatia over the last 20 years have been relatively small, the results
speak in favour of oscillations of climatic elements (precipitation in particular) and their spatial
and temporal characteristics (annual or decadal). A more detailed analysis showed indicative and
clearer change, primarily on the seasonal level in the period from 1990 to 2009.
The seasonal results for the period from 1990 to 2009 support conclusions on the annual
level. The marked trend of increased discharge in the spring, especially on the rivers fed by snow-
melt, as well as the increase in the mean annual temperature, suggest that, due to warmer winters
with less snow, there is a change in the ratio of solid to liquid precipitation in favour of rainfall.
This reduces the retardation effect of the colder months on water resources, the amount of runoff
gets distributed and the spring maxima shift towards the winter months. Such trends and processes
are consistent with those in the Alps (Fürst et al., 2010; Birsan et al., 2005). In addition, the most
marked positive trend of all the months in the year was detected in March. In the case of the Mura
and Drava the positive trend was somewhat less pronounced.
A decrease in discharge in the summer months was the most pronounced in east and west
Croatia and in the mountainous part of Croatia. Several Dalmatian rivers (the Cetina, Krka and
Vrljika) exhibited interesting results with a discharge increase in the summer months, mainly due
to the August increase, although that increase was not statistically signifi cant. At fi rst glance, this
is partly opposite to the results of previous studies, which indicated a temperature increase and a
decrease in the value of surface runoff in the coastal area of Dalmatia, in particular since the be-
ginning of the 1990s (Gajić-Čapka and Zaninović, 2006). However, precipitation trends were not
as pronounced. In addition, the Cetina and Vrljika Rivers, for instance, are fed from the Dinaric
mountains and hinterland. This has to be taken into consideration, since the Hvar meteorological
station, used in the mentioned research, is not representative for those rivers.
Contemporary Changes of Mean Annual and Seasonal River Discharges in Croatia

22
The nearly uniform trend of discharge decrease in the autumn months is the consequence
of the increase in summer temperatures and evapotranspiration values and the reduced amount of
precipitation. Such a decrease is also the direct consequence of the reduced amount of precipitation
in the autumn. This is particularly noticeable in November, the month with the most pronounced
decrease in discharge values at almost all the observed stations. Such November change in dis-
charge is related to the fact that October is the month with a pronounced decrease in the amount
of precipitation and its increased variability (Maradin, 2011). At the same time, autumn is the
season of the highest precipitation variability with values of over 33% in eastern Croatia (Maradin,
2011). The overall trend of drying in summer and autumn months noticed in the eastern part of
Croatia is consistent with the results for eastern (dryer) parts of Austria (Fürst et al., 2008; Fürst
et al., 2010). Winter month trends showed an increase in discharge in the part of Croatia with the
maritime pluviometric regime (Mountainous Croatia, Istria, Dalmatia) on the one hand, and on the
other, mainly a decrease in the part of Croatia with the continental pluviometric regime (with the
exception of the upper course of the Sava River).
The World Meteorological Organization projections state that the decrease in discharge in
the area of south and southeast Europe by the year 2070 will be up to 36% (Trninić and Bošnjak,
2009). Such projections are based on the climate models which take further increase in average
annual temperature on Earth as a starting parameter, due to the increase of greenhouse gases. If the
trends shown in this study continue, we expect that the aforementioned decrease will not equally
affect all the rivers in Croatia. It is more likely that some rivers will show an increase both on a
monthly and annual level. The projections of a further decrease in the snowcover duration and
thickness in the Alpine area in the 21st century have to be taken into consideration as well (Steiger,
2010). The intensity of change for relatively lower altitudes (from 600 to 1300 m) is particularly
stressed. More certain predictions of the overall change in climate elements and their consequenc-
es on the river discharge regimes will require modelling on the basin level. This is made diffi cult
due to the inadequate scale of climate models and the non-existence of several relevant data series
for all the physical geographical features of a basin.
CONCLUSION
Over a half of the 53 analysed stations had a negative trend of annual discharge in the 1990-
2009 period, but it was not statistically signifi cant at most of those stations. More pronounced de-
creases were observed on smaller lowland streams of central and east Croatia. This is related to the
sensitivity of smaller basins to climate change or oscillations. Larger rivers of the continental part
(the Sava, Drava and Mura) exhibited a smaller decrease or increase primarily as a consequence
of modifi ed conditions in the upstream countries. In these cases, the basin size and intensive hu-
man interventions complicate causal relationships of climate elements and runoff. A more marked
increase in annual discharges was observed only in the Kupa Basin. A detailed analysis showed in-
dicative and more uniform signals of change, primarily on the seasonal level in the period between
1990 and 2009. Given the period of analysis and the signifi cance of results, we handle conclusions
with care but they can be used as an indicator of changing conditions on rivers in Croatia. Smaller
scale investigations should follow.
There are several possible consequences for Humankind and the whole ecosystem that the
mentioned changes bring about. Lower summer discharges carry the danger of elevated concentra-
tion of pollutants in watercourses as well as the reduced amount of water available for water supply
and other human activities. This problem is particularly dangerous for the coastal part of Croatia,
where the needs for water are highest in the summer months when the number of users of space and
water resources increases exponentially due to the arrival of tourists. The decreased discharges and
Hrvatski geografski glasnik 77/1 (2015.)

23
increased concentrations of pollutants together with the rise of water temperatures also endanger
and change river ecosystems, affecting plant and animal species. The increased spring discharges,
on the other hand, along with the sudden melting of snow, can increase the risk of fl ooding in the
late winter months and early spring months. These changes in discharge regimes should defi nitely
be taken into account when managing water resources in all sizes of basins.
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SAŽETAK
Novije promjene srednjih godišnjih i sezonskih protoka
rijeka u Hrvatskoj
Ivan Čanjevac, Danijel Orešić
U istraživanju se analiziraju promjene srednjih protoka rijeka u Hrvatskoj na 53 hidrološke
stanice s homogenim nizom podataka od 1990. do 2009. Na većini stanica zabilježen je negativan
trend srednjega godišnjeg protoka, no on je u svega četiri slučaja statistički značajan. Izrazitiji pad
primijećen je kod manjih nizinskih tekućica u unutrašnjosti Hrvatske, posebno u Slavoniji. Vjero-
jatno je da se njihova manja poriječja brže odazivaju na klimatske promjene koje su zabilježene
od kraja 20. stoljeća u nizinskoj Hrvatskoj: blagi porast srednje godišnje temperature, pad količine
padalina i vlage u tlu te porast potencijalne evaporacije (Zaninović i Gajić-Čapka, 1999). Usto
varijabilnost padalina u unutrašnjosti Hrvatske raste od zapada prema istoku, što također utječe na
protoke manjih tekućica.
Glavne rijeke kontinentske, unutrašnje Hrvatske (Sava, Drava i Mura) bilježe male promjene
srednjih godišnjih protoka, što je prije svega posljedica promjena uvjeta otjecanja u uzvodnim
državama Sloveniji (Frantar, 2003, 2005, 2007) i Austriji (Fürst i dr., 2008, 2010). Nešto veća
promjena, odnosno porast srednjih godišnjih protoka, zabilježena je jedino u poriječju Kupe. To
je posljedica porasta srednje godišnje temperature, smanjenja broja sniježnih dana te smanjenja
trajanja sniježnog pokrova u tom području (Gajić-Čapka, 2011).
Analiza sezonskih promjena protoka pokazala je veću povezanost s promjenama klimatskih
elemenata. Postoji jasan uzlazni trend protoka u proljeće, posebno tekućica koje se dijelom hrane
Contemporary Changes of Mean Annual and Seasonal River Discharges in Croatia

26
sniježnicom, što upućuje na utjecaj sve toplijih zima s manje snijega, odnosno sa sve manjim udje-
lom snijega u odnosu na kišu u ukupnim zimskim padalinama. Jasno je da će to dovesti do smanje-
nja efekta retardacije u hladnom dijelu godine, te se protok preraspoređuje, a proljetni maksimum
pomiče prema zimskim mjesecima. Ti trendovi i procesi sukladni su s onima u Alpama (Fürst i
dr., 2010; Birsan i dr., 2005). Od svih mjeseci upravo je u ožujku zabilježen najveći trend porasta
srednjega mjesečnog protoka.
Pad protoka u ljetnim mjesecima najizraženiji je na istoku i zapadu Hrvatske, kao i u pla-
ninskim krajevima. Na nekoliko dalmatinskih tekućica (Cetina, Krka i Vrljika) zabilježen je pak
porast protoka, pretežno zahvaljujući porastu u kolovozu, koji nije statistički značajan. Na prvi
pogled taj je rezultat oprečan rezultatima istraživanjâ koji pokazuju porast srednje godišnje tem-
perature i pad površinskog otjecanja na dalmatinskoj obali od devedesetih godina 20. stoljeća
(Gajić-Čapka i Zaninović, 2006). No u njima je riječ o temperaturnim podacima s obale, odnosno
otoka Hvara, a u ovome istraživanju o tekućicama koje se dijelom hrane iz dinarskoga planinskog
zaleđa, pa i podzemljem.
Gotovo uniformno zabilježen je pad protoka u jesenskim mjesecima, kao posljedica porasta
ljetnih temperatura i evapotranspiracije te smanjenja količine padalina. Smanjenje količine
padalina u jesenskim mjesecima, posebno u listopadu (Maradin, 2011) također izravno utječe
na smanjenje protoka, posebno izraženo u studenome i zabilježeno na gotovo svim razmatranim
stanicama. Ujedno je jesen godišnje doba s najizrazitijom varijabilnošću padalina u Hrvatskoj
(Maradin, 2011). Općeniti trend sušnijih ljetnih i jesenskih mjeseci sukladan je s procesima u
istočnoj Austriji (Fürst i dr., 2008, 2010).
U zimskim mjesecima bilježi se porast protoka na tekućicama u dijelovima Hrvatske s mari-
timnim pluviometrijskim režimom (Gorska Hrvatska, Istra i Dalmacija), a pad protoka (uz iznimku
gornjeg toka Save) na tekućicama u dijelu Hrvatske s kontinentskim pluviometrijskim režimom.
Projekcije Svjetske meteorološke organizacije govore o padu protoka u Južnoj i Jugoistočnoj
Europi od najviše 36% do 2070. godine (Trninić i Bošnjak, 2009). Te se projekcije temelje na kli-
matskim modelima koji predviđaju daljnji porast srednje godišnje temperature zraka na Zemlji kao
posljedicu daljnjeg rasta količine stakleničkih plinova u atmosferi. Nastave li se trendovi izloženi
u razmatranoj studiji, možemo očekivati da to smanjenje protoka neće pogoditi sve tekućice u
Hrvatskoj ravnomjerno. Vjerojatno je da će neke tekuć
ice pokazati povećanje pojedinih mjesečnih
godišnjih protoka i srednjega godišnjeg protoka. Valja uzeti u obzir smanjenje trajanja sniježnog
pokrova i njegove debljine, što se predviđa za područje Alpa u 21. stoljeću, posebno na nižim nad-
morskim visinama od 600 do 1300 m (Steiger, 2010). Preciznija predviđanja promjena klimatskih
elemenata i njihovih posljedica na otjecanje tekućica zahtijevat će modeliranje na razini poriječja,
što je zasad otežano neadekvatnim skalama klimatskih modela i nedostatkom podataka, odnosno
nizova podataka o svim relevantnim prirodnim čimbenicima unutar poriječja.
Što se tiče promjena zabilježenih u ovome radu, nekoliko je mogućih posljedica za društvo
i cijele ekosustave. Niži ljetni protoci nose opasnost povećanja koncentracije raznih polutanata u
našim tekućicama, što uz povećane temperature vode ugrožava riječne ekosustave, te znače i sma-
njenje količine pitke vode raspoložive za vodoopskrbu i druge ljudske djelatnosti. Taj je problem
posebno opasan za primorski dio Hrvatske, gdje su ionako više potrebe za vodom u ljetnim mje-
secima još znatno izraženije zbog turizma. Povećanje protoka u proljeće, naglije topljenje snijega
i veća varijabilnost padalina svakako povećavaju rizik od poplava, posebno od bujičnih brdskih i
gorskih tekućica u kasnu zimu i rano proljeće. Promjene režima otjecanja svakako se moraju uzeti
u obzir pri upravljanju vodnim resursima na svim razinama.
Hrvatski geografski glasnik 77/1 (2015.)

27
Ivan Čanjevac, PhD, Senior Assistant
University of Zagreb, Faculty of Science,
Department of Geography
Marulićev trg 19/II, Zagreb, Croatia
canjevac@geog.pmf.hr
Danijel Orešić, PhD, Associate Professor
University of Zagreb, Faculty of Science,
Department of Geography
Marulićev trg 19/II, Zagreb, Croatia
doresic@geog.pmf.hr
Received (Primljeno): 2015-04-15
Accepted (Prihvaćeno): 2015-06-16
Contemporary Changes of Mean Annual and Seasonal River Discharges in Croatia