Acta geographica Slovenica, 58-2, 2018, 7–18
CHANGES IN DISCHARGE REGIMES
OF RIVERS IN CROATIA
Ivan Čanjevac, Danijel Orešić
The Krka River upstream from Skradin Falls, Croatia.
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DOI: https://doi.org/10.3986/AGS.2004
UDC: 911.2:556.535(497.5)
COBISS: 1.01
Changes in discharge regimes of rivers in Croatia
ABSTRACT: This paper presents the results of the first comprehensive national analysis of changes in dis-
charge regimes of rivers in Croatia. Seven types of discharge regimes have been defined for rivers in Croatia.
We analyzed the changes in discharge regimes of all types, comparing the standard period from 1961 to 1990
with the most recent period from 1990 to 2009. We found evidence of a redistribution of discharge through-
out the year, an increase in autumn and winter discharges (especially for rivers dominantly fed by snowmelt),
and a decrease in summer discharge values. Furthermore, we detected a change in the month of the appear-
ance of mean discharge maxima and minima. In most cases, the changes can be explained by changes in
the regime of climate elements (temperature, precipitation, and evapotranspiration). The results are con-
sistent with those from upstream countries; that is, Slovenia, Austria, and Bosnia and Herzegovina.
KEYWORDS: geography, hydrology, discharge, river regime, module coefficients, Croatia
Spremembe v pretočnem režimu hrvaških rek
POVZETEK: V članku so predstavljeni izsledki prve pregledne nacionalne analize sprememb v pretočnem
režimu rek na Hrvaškem. V analizi je bilo ugotovljenih sedem različnih vrst pretočnih režimov. Avtorja
sta pri vsaki preučevala spremembe, pri čemer sta primerjala obdobji med letoma 1961 in 1990 ter 1990
in 2009. Ugotovitve so pokazale spremembe v porazdelitvi pretoka čez leto, povečanje jesenskega in zimskega
pretoka (zlasti pri rekah, na pretok katerih večinoma vpliva taljenje snega) ter zmanjšanje pretoka v poletnih
mesecih. Poleg tega se je spremenil tudi mesec najnižjega in najvišjega povprečnega rečnega pretoka. V večini
primerov lahko spremembe pojasnimo s spremembami podnebnih dejavnikov (temperature, padavin in
evapotranspiracije). Izsledki se ujemajo z rezultati analiz, opravljenih v državah gorvodno (tj. Sloveniji,
Avstriji ter Bosni in Hercegovini).
KLJUČNE BESEDE: geografija, hidrologija, pretok, rečni režim, koeficienti modulov, Hrvaška
Ivan Čanjevac, Danijel Orešić
University of Zagreb, Faculty of Science, Department of Geography
canjevac@geog.pmf.hr, doresic@geog.pmf.hr
This paper was submitted for publication on June 2nd, 2015.
Uredništvo je prejelo prispevek 2. junija 2015.
Ivan Čanjevac, Danijel Orešić, Changes in discharge regimes of rivers in Croatia
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0 50 100 km25
Alpine nival – pluvial regime
Dinaric pluvial – nival regime
Peripannonian pluvial – nival regime
Pannonian pluvial – nival regime
Pannonian pluvial regime
Mediterranean pluvial – nival regime
Mediterranean pluvial regime
Other regimes
Legend
Content by: Ivan Čanjevac
Map by: Ivan Čanjevac
Figure 1: Types of discharge regimes in Croatia (Čanjevac 2013).
9
Acta geographica Slovenica, 58-2, 2018
1 Introduction
The aim of this study was to assess recent changes in discharge regimes in Croatia. What is the effect of
(evident) climate change or oscillation on water resources in Croatia? Are there any significant changes in
the discharge regimes of rivers in Croatia? Is it possible to draw any general conclusions regarding the change
in discharge regimes that could confirm climate change observations?
Discharge regimes have traditionally been a major hydrogeographical research topic (Pardé 1933; Ilešič 1947;
Grimm 1968; Riđanović 1993; Hrvatin 1998; Frantar 2003; Frantar and Hrvatin 2005). This is probably because
they are the result of complex physical geographical characteristics, as well as the socio-geographical devel-
opment of the upstream area. Although station-specific discharge regimes are mainly the result of the climate
characteristics of the basin area, climate change or oscillations can and usually are reflected in discharge regimes.
This study is the first comprehensive nationwide assessment of changes in discharge regimes.
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The study covers rivers and stations in Croatia with available data. Croatia has heterogeneous physi-
cal-geographical characteristics. It lies at the intersection of the Pannonian Plain, the Dinaric Alps, and
the Adriatic Sea, covering an area of 56,594 km2 with around 4.3 million inhabitants (Croatian … 2014).
Its diverse climate, geological, and geomorphological conditions result in a variety of discharge regimes
of rivers (Čanjevac 2013). For the purpose of this study, we compare the standard period (1961–1990) with
the most recent twenty-year period (1990–2009). We are aware of possible different conclusions due to
different lengths of time series, but we consider a twenty-year period long enough to avoid interannual
oscillations (Dukić 1984; Shaw et al. 2010).
The discharge regimes of rivers in Croatia were studied by Ilešič (1947) and Riđanović (1993), who
covered only large rivers. Regional analysis of discharge and river regimes was carried out for rivers in the
Lika region (Pejnović 1991), the catchment of the Krapina River (Orešić 1995), and the catchments of the
Rječina and Mirna Rivers (Knežević 2001, 2004). In addition, the discharge regimes of some rivers in Croatia
were analyzed within larger-scale studies (Belz et al. 2004; Kovacs 2010). All of these studies cover parts
of Croatia and did not seek to obtain the wider picture necessary to understand processes and changes
for the entire Croatian area and for the correlation with neighboring and upstream countries. Therefore,
for methodological reasons and spatial correlation, we also analyze the research findings from nearby coun-
tries; namely, Slovenia (Hrvatin 1998; Ulaga 2002; Frantar 2003, 2005, 2007; Frantar and Hrvatin 2005;
Brilly et al. 2007; Ulaga et al. 2008), Austria (Fürst et al. 2008, 2010), Germany (Bormann 2010), Hungary
(Belz et al. 2004), and Bosnia and Herzegovina (Hidrološka studija površinskih voda BiH 2012). Those stud-
ies analyze recent changes in river discharge and discharge regimes in their respective countries and for
rivers in the wider area of the Danube Basin. Due to changes observed for rivers linked to changes in snow-
fall and snow cover duration, Swiss research is also analyzed (Birsan et al. 2005).
2 Methods
This study uses the mean monthly discharge data from 116 hydrological stations in Croatia (Meteoro -
logical…2014). The main selection criterion was the length of time series without gaps. We chose seventy-six
stations on fifty-one rivers for our analysis of the discharge regime change. All of the data were obtained
from the Meteorological and Hydrological Service of Croatia.
According to the recent typology of discharge regimes (for the period 1990–2009), there are seven types
of discharge regimes in Croatia (Čanjevac 2013).
The changes in each of the types were analyzed. A comparison and analysis of discharge regimes for
two periods (1961–1990 and 1990–2009) were made comparing the values of module coefficients. This
paper presents changes of discharge regimes in the characteristic (typical) stations for each type.
3 Results
3.1 Alpine nival-pluvial regime
Rivers with an Alpine nival-pluvial regime in Croatia are the Drava, the Mura, and the Danube. The regimes
of these rivers and the uniformity of discharge in particular are affected by the construction of dams for
large hydropower plants. The mechanisms of water input and change in characteristics of this regime type are
shown based on the example of the Terezino Polje station on the Drava River. The regimes of the 1961–1990
standard period were compared to the period (1990-2000) for which the recent typology (Čanjevac 2013)
was made (Figure 2).
There is evidence of changes and a transition from a nival (nival-glacial; Riđanović 1993) regime in
the 1961–1990 period towards a nival-pluvial regime in the last period. This can be concluded from the
decreased values of module coefficients in the high-water period, when the main input comes from snow
and glacier melt, and increased coefficient values in the low-water period from September to January. Between
the two periods studied, the mean monthly discharge value at the annual level decreased by 10%, from
533.37 to 480.02 m3/s. Indirectly, we conclude that feeding by snowmelt is lessening (there is a decrease
in the retardation effect) and feeding by rainfall is increasing, which corresponds to processes of climate
element change in the basins analyzed (Gajić-Čapka and Cesarec 2010).
10
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Figure 2: Module coefficient values of the Drava at the Terezino Polje station; an Alpine nival-pluvial regime.
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Figure 3: Module coefficient values of the Kupa River at the Hrvatsko station; a Dinaric pluvial-nival regime.
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Acta geographica Slovenica, 58-2, 2018
3.2 Dinaric pluvial-nival regime
The Kupa, Čabranka, Kupica, Gornja Dobra, and Vitunjčica rivers are characterized by a Dinaric pluvial-nival
regime. In order to illustrate the changes in this regime type, we chose the Kupa River and the Hrvatsko
station (Figure 3). There is a notable decrease in mean monthly discharge values for the Kupa at that sta-
tion by about 8% from 20.65 m3/s to 18.94 m3/s.
Through a parallel analysis of the regimes in the two periods, an increase in module coefficients was
recorded from September to February in the second period, with the most marked increase in October.
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Figure 4: Module coefficient values of the Bednja River at the Tuhovec station; a Peripannonian pluvial-nival regime.
12
On the other hand, there was a fall in the values of module coefficients from March to August, with the
most marked one occurring in May. It is precisely this pronounced decrease in discharge in May that indi-
cates a decrease in snowmelt proportion in the total input of the river. This is the cause of increased discharges
in the winter months, whereas those of the spring months are decreasing due to a smaller amount of snow,
which also melts earlier. Another change noticed is that the autumn maximum is more marked than the
spring one, whereas they were equal in the standard period. Generally, the autumn months have become
wetter and the spring months have become drier. Furthermore, the decrease in discharge in August, which
is around one-third of the average annual discharge, is a bit alarming. Due to such changes, in the twen-
ty-year period observed the difference between the months with the highest and lowest discharge has
increased significantly.
3.3 Peripannonian pluvial-nival regime
This type of regime includes over 30% of the stations analyzed. Rivers with a Peripannonian pluvial-nival
regime are the Bednja, Krapina, Krapinčica, Horvatska, Sutla, Lonja, Bjelovatska, Gliboki, Koprivnica, Sava
(stations: Podsused Žičara, Zagreb, Jasenovac), Bregana, Gradna, Trnava, Kupčina, Mrežnica, and Kupa
(Jamnička Kiselica station). The three stations included on the Sava show characteristics of its upper and
middle course in Croatia. This regime type is the most heterogeneous one and shows the diversity of cli-
mate conditions of runoff at both the meso-regional and micro-regional levels.
The Bednja River at the Tuhovec station was chosen to assess the change in this type of regime (Figure 4).
The mean monthly discharge of the Bednja at the Tuhovec station between the two periods fell from 6.62
to 4.84 m3/s (i.e., 27%).
Through a comparative analysis of the regimes, an increase in coefficient values from September to
January was observed, along with a decrease from February to August. December showed the most marked
increase in module coefficients, whereas the decrease, on the other hand, was most pronounced in March.
Such changes indicate dryness in the spring and summer months, probably caused by reduced precipi-
tation, but most of all by the increase in mean monthly temperatures and evapotranspiration values
(Pandžić et al. 2009). The results are indicative, but in order to confirm these causal connections any future
research will require an analysis of available climate elements' data series.
The Sava regime in Zagreb is somewhat different from other streams and requires a more detailed analy-
sis (Figure 5).
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Figure 5: Module coefficient values of the Sava River at the Zagreb station; a Peripannonian pluvial-nival regime.
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Acta geographica Slovenica, 58-2, 2018
Comparing the Sava regime in the two periods, one can observe changes as a consequence of climate
element changes, primarily in Slovenia (Brilly et al. 2007; Ulaga et al. 2008). The decreased discharge in late
spring (May) and early summer (June) is evidently a consequence of a reduced amount of snow in the total
amount of precipitation in the drainage area. The decreased discharge values in the summer months are
a result of atmospheric pressure, rising evapotranspiration rates, and a decreasing amount of precipita-
tion (Šegota and Filipčić 2007). Higher autumn discharges are a consequence of the prevailing rain input
in comparison to snow. The same trend was detected for the Sava River in Slovenia (Frantar 2003). On
the whole, the mean discharge value has dropped by about 11%, from 310.12 m3/s to 274.25 m3/s. This
drop in discharge is marked in all months except October and December. This trend in falling mean annual
discharge values has been detected since the mid-1920s (Šegota and Filipčić 2007; Bonacci and Oskoruš 2011).
However, this trend (of falling mean discharges) has accelerated in the last twenty-year period, and it needs
to be taken into consideration in further water management and planning as well as the construction of
five new hydropower plants on the lower course of the Sava River in Slovenia.
3.4 Pannonian pluvial-nival regime
Rivers with a Panonnian pluvial-nival regime are the Bijela, Orljava, Toplica (Daruvarska), Glina, Sava (Župan-
ja Stepenica station), Voćinka, Vučica, Biđ, Lonđa, and Sunja. These are mainly smaller streams in the
Panonnian part of Croatia. This group includes the downstream regime of the Sava at the Županja sta-
tion, which is somewhat different from the regime of the Sava in the upper and middle part of its course
in Croatia. Unfortunately, we do not have data series long enough for a sufficient number of stations with
this regime, and so it is impossible to draw general conclusions about the changes specific to this type of
regime. Nevertheless, it is possible to perform an analysis of the Sava regime change at the Županja Stepenica
station, for which there is a data series long enough (Figure 6).
Mean monthly discharge for the  1961–1990 period was 1,134.16  m3/s, whereas the discharge for
the 1990–2009 period was about 10% lower, at 1,026.28 m3/s. Such a relatively large fall is the result of an
absolute fall from January to September and a relative fall during the months before and after the spring
maximum, which is in April. Except for similar changes in the upper stream part of the basin in Croatia
and Slovenia, this is a consequence of the reduced amount of snowmelt in the input of main right-side
tributaries (draining the Bosnian mountains), which affects the Sava regime downstream of Jasenovac
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Figure 6: Module coefficient values of the Sava River at the Županja stepenica station; a Pannonian pluvial-nival regime.
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Figure 7: Module coefficient values of the Česma River at the Narta station; a Pannonian pluvial regime
14
(Hidrološka … 2012). Due to this station's large drainage area, which encompasses different climate areas
and runoff conditions, the relationships in this case are among the most complex in Croatia.
3.5 Pannonian pluvial regime
The Česma, Ilova, and Kutina rivers have a Pannonian pluvial regime. The basic changes in the case of
this regime type are shown in the example of the Česma River at the Narta station (Figure 7). The mean
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Figure 8: Module coefficient values of the Krka River at the Skradinski Buk Gornji station; a Mediterranean pluvial-nival regime.
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Acta geographica Slovenica, 58-2, 2018
monthly discharge between the two periods observed decreased by about 7%, from 5.2 m3/s to 4.84 m3/s.
The highest increase in module coefficients is typical for autumn and partly for the winter months; for
example, in December the increase moved from 1.44 to 1.95. On the other hand, the biggest decrease is
significant for February, March, and the summer months.
The relative decrease in July and August is particularly significant. Due to such redistribution of dis-
charge in the course of the year, the primary maximum shifted from February to December, and the minimum
moved from September to August or even July. The changes in the colder part of the year are probably
a consequence of the increase in the amount of autumn precipitation, and the decreases in July and August
could also be explained by continuation of the already determined increase in temperature and rise in evap-
otranspiration during the warmest part of the year (Zaninović and Gajić-Čapka 1999). These changes, together
with the decrease in the highest module coefficient values and an even greater relative fall in the lowest
values, suggest that in the following period in this area water shortages and drought in the summer months
can be expected more frequently.
3.6 Mediterranean pluvial-nival regime
This regime type is characteristic of 22% of the stations analyzed, which are mainly located on karst streams
belonging to the Adriatic Basin. These are the Cetina, Vrljika, Jadro, Ombla, Krka (the Skradinski Buk Gornji
station), Zrmanja, Lika, Novčica, Istrian Mirna, Pazinčica, and Rječina Rivers. This type includes two rivers
of the Black Sea Basin: the (Donja) Dobra and Korana rivers. The characteristics of this regime and its
changes in relation to the standard period are analyzed based on the example of the Krka River at the
Skradinski Buk Gornji station (Figure 8). First, it is necessary to point out that in the case of the Krka River
the mean monthly discharge values also decreased, by about 12%, from 54.16 m3/s to 47.4 m3/s.
In addition, one can observe a transition from a simple regime with one maximum and one minimum
towards a regime with a marked second maximum in April and minimum in February and March. The
increase in module coefficients has been recorded from November to January, but was especially marked
in December and April. On the other hand, a significant decrease is characteristic for February and March.
Apart from that, there are smaller decreases in the summer months, which can be alarming considering
that a major part of the rivers with this regime have a crucial role in water supply.
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Ivan Čanjevac, Danijel Orešić, Changes in discharge regimes of rivers in Croatia
3.7 Mediterranean pluvial regime
According to the typology made by Čanjevac (2013), only three stations on two streams belong to this regime:
the Čikola River (stations: Ružić 1 and Ključice) and the Jadova River in Lika. These streams are threat-
ened by a significant decrease in mean monthly values of discharge over the last twenty years. For the Čikola
River (the Ružić 1 station) it amounts to nearly 25%, from 5.04 to 3.85 m3/s. These data are alarming because
the Čikola is an important stream in the water supply system of the town of Drniš and the surrounding area.
Such a high decrease in discharge corresponds to a general trend in the Mediterranean (Ludwig et al. 2009).
The changes in regime refer to a decrease in the coefficients in the autumn months, shorter and more intense
duration of the winter maximum, and a more variable spring discharge (Figure 9).
4 Discussion
The analysis of changes in discharge regimes in the last twenty-year period reflects a complex picture, and
in most cases this could be related to changes in climate elements. Over the last century, lowland (i.e., cen-
tral and eastern) Croatia has generally shown a mild increase in mean annual air temperature, a decrease
in the precipitation quantity and soil humidity, and an increase in potential evapotranspiration with a sta-
tistically significant trend since 1987 (Zaninović and Gajić-Čapka 1999). Furthermore, the precipitation
variability in continental Croatia increases towards the east (Maradin 2011; Maradin and Filipčić 2012).
Larger rivers in the continental part (the Sava, Drava, and Mura rivers) exhibited a change in discharge
regime primarily as a consequence of changing conditions in the upstream countries; that is, Slovenia
(Frantar 2003, 2005, 2007) and Austria (Fürst et al. 2008, 2010). In these cases, the basin size and strong
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 sta-
bilized the already steady discharge regime, the consequences of climatic changes are impossible to detect.
There is evidence of increased discharge; namely, values of module coefficients in the spring, especially
for rivers fed by snowmelt. Together with the evidence of an increase in mean annual temperature, this
suggests that due to warmer winters with less snow (for the change in snow cover, see Gajić-Čapka 2011)
16
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Figure 9: Module coefficient values of the Čikola River at the Ružić 1 station; a Mediterranean pluvial regime.
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there is a change in the ratio of solid to liquid precipitation in favor of rainfall. This reduces the retarda-
tion effect of the colder months on water resources, the amount of runoff is distributed, and the spring
maxima shift toward the winter months. Such trends and processes are consistent with those in the Alps
(Frantar 2005; Birsan et al. 2005; Fürst et al. 2010). A decrease in the values of module coefficients in the
summer months is noticed at all stations analyzed. The most pronounced decrease is for rivers with sim-
ple Pannonian and Mediterranean pluvial regimes. This is probably the result of their higher sensitivity
to precipitation variability and climate oscillations in general. In addition, for the entire Mediterranean
there is a significant trend of a decrease in mean discharges over the last fifty years (Ludwig et al. 2009).
The nearly uniform trend of the module coefficient increase in the autumn months is a consequence of
the aforementioned trend of a change in the ratio of solid to liquid precipitation in favor of rainfall. Winter
months showed an increase in module coefficients in most cases, but also the occurrence of a secondary
minimum, usually in February.
5 Conclusion
Our comprehensive study of changes in discharge regimes of rivers in Croatia comparing the 1961–1990
and 1990–2009 periods yields the following general conclusions presented at the seasonal level:
• A decrease in module coefficients in the spring months, especially for rivers fed by snowmelt.
• A decrease in module coefficients in the summer months at all stations analyzed. The most pronounced
decrease is for rivers with a simple Pannonian pluvial regime and especially a Mediterranean pluvial regime.
• A nearly uniform increase in module coefficients in the autumn months.
• An increase in module coefficients in winter months in most cases, but also the occurrence of a secondary
minimum, usually in February.
The changes are in general the result of changes in climate elements (mainly a temperature rise) between
the two periods. The most pronounced are changes in snow occurrence and accumulation processes. Given
that many rivers in Croatia have a transboundary character, the conclusion above is supported by the results
of studies in upstream countries. Projections of a further decrease in snow cover duration and thickness
in the Alpine area in the twenty-first century have to be taken into consideration for further planning
(Steiger 2010). The intensity of change for relatively lower elevations (from 600 to 1,300 m) is particularly
stressed.
5 References
Belz, J. U., Goda, L., Buzás, Z., Domokos, M., Engel, H., Weber, J. 2004: Flow regime of river Danube and
its catchment – an update of Chapter II of the Danube Monograph. Koblenz.
Birsan, M.-V., Molnar, P., Burlando, P., Pfaundler, M. 2005: Streamflow trends in Switzerland. Journal of
hydrology 314, 1–4. DOI: http://dx.doi.org/10.1016/j.jhydrol.2005.06.008
Bonacci, O., Ljubenkov, I. 2008: Changes in flow conveyance and implication for flood protection, Sava
River, Zagreb. Hydrological processes 22-8. DOI: http://dx.doi.org/10.1002/hyp.6688
Bonacci, O., Oskoruš, D. 2011: Hidrološka analiza sigurnosti Zagreba od poplave vodama rijeke Save u novim
uvjetima. Hrvatske vode 19-75.
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