Nevenka O`ani}
Josip Rubini}
ANALYSIS OF DISCHARGE FLUCTUATION TRENDS IN THE MOUNTAIN PART
OF THE KUPA RIVER CATCHMENT AREA
SUMMARY
The upper part of the Kupa river catchment area is situated in the mountain massif of Gorski
Kotar, with the highest peaks reaching about 1500 m. It is in this mountain area that the Kupa
catchment area, and hence the catchment area of the Black Sea, is only some 15 km away from
the Adriatic Sea, and therefore directly exposed to its climatic influence. Therefore, this area is
the most abundant in rainfall in Croatia, with rainfall reaching even 4000 mm per annum. With
that respect, water resources of the upper part of the Kupa catchment area, in addition to high
enviromental value, have an extremely important water supply and hydropower potential which
is at present utilized only to a minor extent. However, in accordance with the growing water and
power demand, larger use of water resources from this part of the catchment area is planned.
Such plans are being intensified by the fact that, opposite to the global growing trend of water
demand, the contrary trends are also present - the falling trends of mean annual discharges in
most watercourses throughout the wider region. The situation is similar in the upper part of the
Kupa catchmant area, where the falling trend of mean annual rainfall has been particularly
noticed in the past ten-year period. Therefore, the approach to planning of the area development
and finding of optimum ways to meet the expected water demands should take into account also
the falling trend of discharges in watercourses and sources in the area.
Key words: the Kupa river catchment area, discharge fluctuation trends, changes of water
regime
1.
INTRODUCTION
The analyses of annual discharge fluctuations belong to the basic hydrological data in assessing
the capacity of water resources in a given area. The analyses carried out in the upper part of
Kupa river catchment area have shown a considerable falling trend of mean annual discharges
during the past three decades. The presence of this trend has been somewhat neglected by the
public due to the fact that during the last ten years, when the largest decrease of annual
discharges was noticed, there were at the same time some very high, even catastrophic floods in
some areas. The obvious examples are the flood of the ^abranka river on September 13-14,
1988, and of the Kupica on September 13-14, 1993. However, such short extreme flood events
cannot have essential influence on the general trend of discharge fluctuations in the analysed
area.
The paper shows the results of analyses carried out at several typical rain and hydrological
gauging stations on the Kupa river and its tributaries in the mountain part of the catchment area.
The analysis covers the joint 30-year period of stations operation, i.e. 1965-1994. Fig. 1 shows
the position of the rain gauges and the hydrological gauging stations.
2.
COURSE OF ANNUAL RAINFALL AND MEAN ANNUAL DISCHARGES
The upper part of the Kupa catchment area, due to its geographic position and orographic
properties, is comparatively heterogeneous regarding the annual rainfall which ranges from
1500 to 4000 mm. To illustrate the oscillations of annual rainfall in this area, the analysis
included two stations - station Mrzla Vodica (751 ma.s.l.) situated on the periphery of the
catchment area, with rainfall properties typical for the most upstream part of the Kupa basin,
and the station Skrad (675 m a.s.l.) representing the more downstream part of the analysed area.
Table 1 shows their typical values, while the course of modular values of annual rainfall and
their trends are shown in Fig. 2.
modular values of annual rainfall
1.6
TREND-MRZLE VODICE
y = -0.004x + 1.3091
1.4
1.2
1.0
Mrzla Vodica
0.8
TREND-SKRAD
y = -0.0057x + 1.4545
Skrad
0.6
65 67 69 71 73 75 77 79 81 83 85 87 89 91 93
years
Fig. 2
Course of modular values of annual rainfall (1965-1994)
The observed trends of annual rainfalls also influenced the regime of discharges in the area. The
hydrological stations selected for the analysis were: Hrvatsko on the Kupa river (370 sq.km. of
catchment area), Brod na Kupi on the tributary Kupica (291 sq.km.), and Luke on the
downstream tributary Gornja Dobra (175 sq.km.). Their typical hydrological parameters are also
shown in Table 1. Fig. 3 shows the course of modular values of the mean annual discharges.
Table 1
Basic data of the analysed gauging stations (1965-1994)
station
RAIN GAUGES
Mrzla Vodica
Skrad
(mm)
(mm)
HYDROLOGICAL GAUGING STATIONS
HrvatskoBrod na Kupi- Hrvatsko - Kupa
Kupa
Kupica
(cu.m./sec)
(cu.m./sec)
(cu.m./sec)
20,00
13,467
7,010
2688,4
1690,3

378,8
249,5
3,95
2,564
1,47
Cv
Annual max.
Annual min.
0,142
3589,9
2054,9
0,148
2110,0
1169,0
0,197
419,0
2,11
0,160
18,50
8,40
0,210
9,70
4,60
modular values of mean annual discharges
Mean annual
values
1.5
1.4
1.3
1.2
1.1
1.0
L uke-D obra
0.9
0.8
H rvatsko-Kupa
0.7
Brod na KupiKupica
0.6
0.5
65 67 69 71 73 75 77 79 81 83 85 87 89 91 93
years
Fig. 3
Modular values of mean annual discharges in typical hydrological
gauging stations in the upper part of the Kupa catchment area
(1965-1994)
Presentation of typical modular annual values of rainfalls and discharges show a falling trend of
annual values in all stations. In rain gauge station Mrzle Vodice this trend is 0.40 percent per
annum, and in Skrad 0.57 percent. It may be noticed that in the same period of analysis the
calculated falling trends of mean annual discharges are even more evident. In the station
Hrvatsko-Kupa the falling trend of mean annual discharges is 1.39 percent per annum, while in
the stations Brod na Kupi-Kupica and Luke-Dobra the trends are the same, i.e. 1.34 percent per
annum.
The analysis of correlation between modular values of mean annual discharges from the
considered stations was also carried out, resulting in a very high degree of coincidence. The
coefficient of linear correlation of interrelations of modular values of mean annual discharges in
the stations Hrvatsko and Brod na Kupi is k=0.93, for stations Hrvatsko and Luke it is k=0.77,
and Brod na Kupi and Luke k=0.71. A comparatively high degree of correlation has also been
observed in the analysis of modular values of annual rainfalls and mean annual discharges - for
the relation between the Kupa discharge at Hrvatsko and rainfall in Mrzle Vodice k=0.77, for
the relation between the Kupica discharge and rainfall in Skrad k=0.81, and for the relation
between the Gornja Dobra discharge in Luke and rainfall in Skrad k=0.62.
It may be seen that dry periods occur in series of several years in succession. In order to
determine whether the hydrological system of analysed watercourses “remembers” wet or dry
intervals in time periods longer than one season, i.e. over one year, autocorrelation analysis of
the time series has also been caried out. The analysis was carried out in steps up to four years,
and its results are shown in Table 2.
Table 2
Results of autocorrelations analysis of annual rainfalls and mean annual
discharges in the upper part of the Kupa catchment area (1965-1994)
station
parameter
Mrzla Vodica (rainfall)
Skrad
(rainfall)
Hrvatsko - Kupa
(discharges)
Brod na Kupi - Kupica
(discharges)
Luke - Dobra
(discharges)
1 year
AUTOCORRELATION STEP
2 year
3 year
4 year
-0.08
0.36
0.14
-0.017
-0.11
0.27
0.18
0.27
0.17
0.41
0.28
0.23
0.17
0.45
0.26
0.25
0.14
0.39
0.12
0.07
It may be seen from Table 2 that no significant autocorrelation has been determined for any of
the stations subject to the analysis. Still, although statisticaly important autocorrelations of time
series have not been proved, it has been determined that within the analysed time step up to 4
years the calculated autocorrelation coefficients retain positive values. This, in part shows that
their time series have a tendency of slow changes of values, or a certain inertia in changes.
3.
ANALYSIS OF CHANGES IN THE HYDROLOGICAL REGIME
In order to obtain a more detailed picture of the occurrence of falling trends of mean annual
discharges, the analysis of time series by RAPS method (Rescaled Adjusted Partial Sums) has
been carried out on the example of hydrological stations Hrvatsko - Kupa, Brod na Kupi -
Kupica, and Luke - Dobra. The results of this analysis are shown in Fig. 4 where prevailingly
wet periods “W”, prevailingly dry “D”, and neutral “N” periods are indicated. Noticeably high
coincidence is evident, indicating their mutual interconnections.
Fig. 4
Transformed series of mean annual discharges in typical
hydrological stations (1965-1994)
It may be sean in Fig. 4 that the last decade was characterized by a long lasting dry period. In
addition to reduced annual rainfall and thus also reduced discharges, the reasons of this
phenomenon should also be sought in the changes of the intra-annual water regime. Therefore, a
comparison has been made between mean monthly rainfalls and discharges for the entire period
of analysis (1965-1994) and for the last decade (1985-1994), which is shown in Fig. 5.
400
70
350
60
300
50
250
40
200
30
150
20
100
10
50
0
Mrzle Vodice
(1965-1994)
rainfalls (mm)
discharges (m 3/s)
80
Mrzle Vodice
(1985-1994)
Hrvatsko-Kupa
(1965-1994)
Hrvatsko-Kupa
(1985-1994)
0
1
2
3
4
5
6
7
8
9
10
11
12
months
Fig. 5
Mean monthly rainfalls in the rain gauging station Mrzle Vodice and
an monthly discharges in the hydrological station Hrvatsko-Kupa
It may be seen that during the last decade the changes occured resulting, in almost all months
except October and November, in rainfalls and discharges lower than the average. Regarding
the absolute discharge balance, the runoff deficit is particulary marked during the winter months
(December-March). However, from the standpoint of the use of water resources, even more
concern is raised by the deficit occuring during the dry summer period (July-September).
4.
CONCLUSIONS
The results of the hydrological analyses, although modest with regard to the number of stations
involved, have indicated the presence of clear falling trends of both the annual rainfalls and,
even more, the mean annual discharges in watercourses from the mountain part of the Kupa
catchment area. Also, it has been noticed that the last decade was characterized by an
expressedly dry period.
Such trends and phenomena make it necessary for future water mnagement development plans
to be based, instead of the usual individual hydrological parameters, on more complex
hydrological data which include also the assumptions of possible further occurrence of such
unfavourable hydrological conditions. With respect to the exceptionally abundant water
resources of the upper part of Kupa catchment area, as well as to the high water quality in areas
not exposed to human influence, the Kupa water resources will have, in future, to meet water
requirements of the wider region. Therefore, it becomes imperative to preserve and to increase
the possibility of regulation of the water regime in the upper part of the catchment area, through
construction of storage reservoirs, as well as by taking care not to disturb the biological balance
with the existing vegetation contributing to alleviation of the surface runoff regime.
5.
LITERATURE
1. Bonacci,O., Bonacci,T.: Drought Periods Identification at the Osjek Climatologic Station
(Croatia), XVIII. Conference of the Danube Countries, Graz, 1996.
2. O`ani},N., Rubini},J.:
Problemi pra}enja stanja i gospodarenja vodnim resursima na
primjeru sliva Rje~ine (Problems of Situation Monitoring and Water Resources Management on
the Example of the Rje~ina Catchment Area), Hrvatska vodoprivreda 50, Zagreb, 1996.
3. Rubini},J., O`ani},N.:
Prirodne hidrolo{ke zna~ajke povr{inskih vodnih pojava (Natural
Hydrological Properties of Surface Water Phenomena), basic data for County physical plan
(unpublished), Rijeka, 1997.
kontakt adrese autora:
dr.sc. Nevenka O`ani}, dipl.in`.gra|.
HRVATSKE VODE, VGO Rijeka, Ciottina 17b, 51000 RIJEKA
Josip Rubini}, dipl.in`.gra|.
HRVATSKE VODE, VGI Labin, Zelenice 18, 52220 LABIN