analysis of seasonal extreme flows using peaks over threshold method

J. Hydrol. Hydromech., 55, 2007, 1, 16–22
ANALYSIS OF SEASONAL EXTREME FLOWS USING
PEAKS OVER THRESHOLD METHOD
PETER BAČA, VERONIKA BAČOVÁ MITKOVÁ
Ústav hydrológie SAV, Račianska 75, 831 02 Bratislava 3, Slovenská republika; mailto: baca@uh.savba.sk
The paper presents review of occurrence frequency of extreme hydrological events in a small agricultural
basin in the flysh region over the period of 40 years (1964/65 – 2003/04). No human alterations were carried
out in the basin over the period studied and hence influence of climatic changes on intensification of
water cycle could be evaluated. Peaks over threshold (POT) method was applied separately to winter and
summer hydrological events. In comparison with annual maximum series, all mean daily discharge values
over a certain thresholds were taken into account for POT frequency analysis. The frequency of POT events
has decreased in the past two decades (1984/85 – 1993/94 and 1994/95 – 2003/04). The results reveal that
the most extreme values occurred in the 1964/65 – 1973/74 decade, mostly caused by summer rainfall. High
event frequency also emerges in the 1974/75 – 1983/84 decade, especially in winter seasons as the consequence
of snow melting. It is most likely that influence of climatic changes has not resulted yet in increase
of occurrence frequency of POT events in the basin studied over the 40 years period.
KEY WORDS: Flood Frequency Analysis, Peaks Over Threshold (POT) Method, Runoff Changes, Seasonal
Extreme Flows.
Peter Bača, Veronika Bačová Mitková: VYUŽITIE METÓDY (POT) PRIETOKOV NAD ZVOLENOU
PRAHOVOU HODNOTOU PRI ANALÝZE EXTRÉMNYCH SEZÓNNYCH PRIETOKOV. Vodohosp.
Čas., 55, 2007, 1; 27 lit., 4 obr.
Príspevok sa zaoberá frekvenciou výskytu extrémnych hydrologických udalostí na malom poľnohospodárskom
povodí vo flyšovom pásme počas 40-ročnej periódy (1964/65–2003/04). Počas tohto obdobia
nebolo povodie ovplyvnené významnými zmenami ľudskej činnosti a preto mohol byť zhodnotený
vplyv klimatických zmien na intenzifikáciu hydrologického cyklu. POT metóda bola aplikovaná zvlášť pre
zimné a letné hydrologické udalosti. V porovnaní so sériami ročných maxím, všetky priemerné denné prietoky
nad určitou prahovou hodnotou boli zahrnuté do POT frekvenčných analýz. Frekvencia POT udalostí
(nad zvolenou prahovou hodnotou) klesla v posledných dvoch dekádach (1984/85–1993/94 a 1994/95–
2003/04). Výsledky ukazujú, že najextrémnejšie hodnoty priemerných denných prietokov, spôsobené prevažne
letnými prívalovými zrážkami sa objavili v dekáde 1964/65–1973/74. Vysoké hodnoty prietokov,
spôsobené prevažne topením snehovej pokrývky sa vyskytli tiež v dekáde 1974/75–1983/84. Je nanajvýš
pravdepodobné, že v študovanom povodí za obdobie 40-tich rokov klimatické zmeny nemali zatiaľ vplyv
na zvyšovanie frekvencie extrémnych udalostí.
KĽÚČOVÉ SLOVÁ: analýza frekvencie povodní, POT metóda, zmeny v odtoku, sezónne extrémne prietoky.
Introduction
In connection with climatic changes, greater attention
is given to occurrence frequency of extreme
hydrological events. Labat et al. (2004) conclude
that an increase in world continental runoff from
major rivers for the period 1920 – 1995 provides
the first experimental data-based evidence demonstrating
the link between the global warming and
the intensification of the global hydrological cycle.
Higher frequency of extreme flow occurrence and
increase in runoff in different regions in the latter
half of the 20th century have been published by
many authors (Foster et al., 1997; Milly et al.,
2002; Peterson et al., 2002; Werritty, 2002; Walter
et al., 2004; Micevski et al., 2006).
Zhang et al. (2006) show that higher occurrence
frequency of annual maximum water level over
time is not influenced by the single factor like climatic
change, but by multiple factors like human
16

Analysis of seasonal extreme flows using Peaks over threshold method
activities. Vörösmarty and Sahagian (2000) noted
that increases in runoff may be attributable to human
alterations such as conversion from forest to
agricultural land uses. In contrast, reforestation
could result in decreases in runoff. However, in
spite of abandonment of cultivated farmlands and
conversion to forest or pasture, increases in runoff
due to intensification of the water cycle has been
documented by Milly and Dunne (2001).
In other cases, measured data indicates decrease
or no changes in magnitude and flood frequency
(Aizen et al., 1997; Zhang et al., 2001; Holko and
Kostka, 2005). Sharma and Shakya (2006) concluded
that the magnitude of flood is decreasing but
its frequency and duration are increasing.
Although climate change is a global phenomenon
the trends and impact may be different at a local
scale. Analysis should be done at the local level
rather than at the large scale (Sharma and Shakya,
2006). Flash floods caused by intensive rainfall
events occurred in the last years in small basins in
Slovakia (Grešková, 2005). The results presented
by Svoboda and Pekárová (1998) show that in the
Carpathian flysh region an intensive rainfall over
a small catchment sufficiently wetted by the antecedent
even moderate precipitation can cause 500-
year return period flood which can not be effectively
reduced neither by retention capacity of the
shallow soil nor by interception of a dense and
healthy forest.
The annual maximum series approach is the most
frequently used method for probabilistic assessment
of design flows. The annual maximum series considers
only one value per year, i. e. annual maximum
(AM). However, the use of an AM series may
involve some loss of information. For example, the
second or third peak within a year may be grater
than maximum flow in other years and yet they are
ignored (Kite, 1977; Chow et al., 1988). This situation
is avoided in the peaks over threshold (POT)
method where all peaks above a certain base value
are considered (Bayliss, 1999; Rao and Hamed,
2000; Black and Burns, 2002; Ouarda et al., 2006).
Moreover, POT method allows detection of
changes in the frequency of occurrence of extreme
floods or hydrological events in different seasons.
The purpose of the paper is to analyse changes in
occurrence frequency of extreme hydrological
events in a small basin in the flysh region over the
period of 40 years. POT method was applied separately
to winter and summer hydrological halfyears.
Study area
The study focuses on the agricultural micro-basin
Rybárik, near Považská Bystrica (Western Slovakia,
Central Europe) (Fig. 1). The Rybárik basin is
a part of the experimental Mošteník brook basin
and is bounded by latitude 49°06′N and 49°07′N;
RYBÁRIK
Jel ové
Legend
Gauging Station
Forest
Watershed Boundary
0 100 200 300 400 500 m
Fig. 1. The experimental Rybárik basin.
Obr. 1. Experimentálne povodie Rybárik.
17

P. Bača, V. Bačová Mitková
and longitude 18°24′E and 18°25′E. The Mošteník
basin is a part of the Váh River catchment, which is
the main tributary of the Danube River from the
territory of Slovakia. The area of the Rybárik basin
is 0.12 km 2 . The mean elevation of the basin is
about 400 m a. s. l. (min 375 m a. s. l., max 434 m
a. s. l.) with slope angles of 8 – 25%. Length of the
Jelšové stream from the spring, which is found in
the Rybárik basin, to the outlet section is 255 m and
average gradient is 9.1%. The geological conditions
in this micro-basin are characterized by flysh substrates
(alternating layers of clay and sandstones).
Soils are clay loams and are classified as Cambisols.
The Rybárik basin is mainly arable, only
10.4% of the area is covered by forest. Mean annual
temperature is 8.09 °C, mean annual precipitation is
738 mm and the runoff averages 231 mm year -1 .
Water discharge data and methods
Discharge is measured continuously by weir fitted
with recording gauge. Water-level stage of the
Jelšové stream is under steady state conditions during
major period of the year. Discharge values are
low and range between 0.04 – 0.5 l s -1 . However,
water level increasing occurred during hydrological
events resulted from snow melting (in winter and
spring months) and from rainfall (mainly in summer
months). Annual peak discharges range between
10.9–364.2 l s -1 (over the period of 40 hydrological
years 1964/65 – 2003/04) (Pekárová et al.,
2005).
Although discharge is measured continuously,
due to lacking archival data (instantaneous peak
discharges) only values of daily mean discharge
were analysed. Two different peaks over threshold
(POT) records were created, one for winter halfyears,
in which increases in flows are mostly
caused by snow melting or combination of snow
melting with rainfall, and one for summer halfyears,
in which increases in flows results from intensive
rainfall. This allowed detection of changes
of occurrence frequency of POT data (mean daily
discharge values) in different seasons and provided
higher homogeneity of single POT records.
For each record, POT data were extracted using
thresholds selected to give, on average, 1.0 and 0.2
exceedances per season, respectively, over the 40-
year period (1964/65 – 2003/04). Generally, flood
frequency analysis considers only one value per
year or season. However, application of such approach
may mask years or decades when the most
extreme values occurred. The value of upper
threshold (0.2 exceedance per year) was selected to
show in which decades the highest mean daily discharge
values occurred. As shown in Figs 2 and 3
selected value of the upper threshold emerges as
satisfactory for this purpose. In order to provide
independence of POT data the following criterions
were used:
[%]
80
70
60
50
40
30
20
10
0
POT 1
POT 0,2
1965-1974 1975-1984 1985-1994 1995-2004
Fig. 2. Portion (in percentages) of mean daily discharge
exceeding thresholds yielding, on average, 1.0 and 0.2 events
per winter hydrological year in single decades over
the 40-years period in the Rybárik basin.
Obr. 2. Percentuálny podiel priemerných denných prietokov
nad zvolené prahové hodnoty, 1,0- resp. 0,2-krát v priemere za
zimný hydrologický polrok v jednotlivých dekádach
za 40-ročné obdobie v povodí Rybárik.
[%]
80
70
60
50
40
30
20
10
0
POT 1
POT 0,2
1965-1974 1975-1984 1985-1994 1995-2004
Fig. 3. Portion (in percentages) of mean daily discharge
exceeding thresholds yielding, on average, 1.0 and 0.2 events
per summer hydrological year in single decades over
the 40-years period in the Rybárik basin.
Obr. 3. Percentuálny podiel priemerných denných prietokov
nad zvolené prahové hodnoty, 1,0- resp. 0,2-krát v priemere za
letný hydrologický polrok v jednotlivých dekádach za 40-ročné
obdobie v povodí Rybárik.
18

Analysis of seasonal extreme flows using Peaks over threshold method
− the value of mean daily discharge extracted is
higher than the values on preceding day and next
day;
− for winter POT data it is considered that the values
of mean daily discharge between two days on
which POT data were recorded do not change
significantly over time and remain low during
a few days;
− for summer POT data, when basin response to
rainfall is fast, time interval between days on
which POT data were recorded is three days at
least and the values of mean daily discharge between
two days on which POT data were recorded
do not change significantly over time and
remain low during a few days; minimum value of
mean daily discharge in this time interval is low.
Results and discussion
Time series plots have been used to reveal the
temporal distribution of the values of mean daily
discharge above the thresholds selected in two records.
In winter seasons at the lower threshold
(68 l s -1 km -2 ), the frequency of occurrence of hydrological
events caused by snow melting does not
change remarkably in single decades (Fig. 2). The
highest event frequency emerges in the 1974/75 –
1983/84 decade. However, at the upper threshold
(156 l s -1 km -2 ) a contrast between the 1964/65–
1983/84 and 1984/85 – 2003/04 periods is evident
with the highest event frequency in the 1974/75 –
1983/84 decade. Fig. 2. reveals that the most extreme
values of mean daily discharge occurred in
the 1964/65 – 1983/84 period.
In summer half years, event frequency varies
markedly over decades at both thresholds (the
lower one is 52 l s -1 km -2 and the upper one is 130
l s -1 km -2 ) with more decreasing trends at the upper
threshold (Fig. 3). The most extreme values of
mean daily discharge especially occurred in the
1964/65 – 1973/74 decade, while in the 1974/75 –
2003/04 period, the event frequency was very low.
When both seasons are considered, on average,
the 1964/65 – 1973/74 decade emerges as the decade
with the highest event frequency (mostly in
summer and less in winter) using data extracted
above the upper threshold. High event frequency at
the upper threshold also emerges in the 1974/75 –
1983/84 decade (especially in winter). Comparison
of the values of mean daily discharge shows that
higher values occurred during snow melting (Fig.
4). The four highest values, however, were recorded
during summer extreme hydrometeorological
events. It is obvious that these values occurred
over the 1964/65 – 1973/74 decade. It is confirmed
that the most extreme discharge values were recorded
over this decade.
350
300
winter
summer
250
Q [l.s -1 .km 2 ]
200
150
100
50
0
0 5 10 15 20 25 30 35 40
Fig. 4. The ascending line of 40 the highest values of mean daily discharge for summer and winter hydrological years over
the 40-years period in the Rybárik basin.
Obr. 4. Vzostupný rad 40-tich najvyšších hodnôt priemerných denných prietokov pre zimné a letné hydrologické polroky za 40-ročné
obdobie v povodí Rybárik.
19

P. Bača, V. Bačová Mitková
As noted above, only the values of mean daily
discharge were used for analysis. Linsley et al.
(1949) reveal certain practical deficiencies of mean
flows. The maximum mean daily flows never
equals the peak flow, and only occasionally will the
maximum 24-hr period coincide with a calendar
day. The maximum daily flow rarely, therefore,
represents the maximum 24-hr flow. Random timing
of a hydrograph with respect with calendar days
frequently results in a maximum mean daily flow
on a day other than on which the actual instantaneous
peak flow occurred. Secondary flood peaks
may be masked by the averaging of flow to compute
mean daily values.
In order to compute missing instantaneous peak
values, relationship for predicting the missing data
from the recorded mean daily flow for the day on
which the instantaneous value occurred can be used
(McCuen and Beighley, 2003). However, satisfying
records of instantaneous peak values were not
available.
In spite of the uncertainties regarding to mean
daily data, it may be concluded that occurrence
frequency of extreme events decreased over the 40
years period in the study Rybárik basin. Human
alterations such as conversion of forest to agricultural
land uses or others, which can be related to
changes in event frequency (Huntington, 2006),
have not been carried out over the 40 years period.
Thus, it is most likely that influence of climatic
changes on increase in runoff, which are suggested
to take place in northern hemisphere, based on the
results of modelling studies (Manabe et al., 2004),
has not been recorded yet in the Rybárik basin. The
results of this study are consistent with the conclusions
of the analysis in mountains catchments in
Central Slovakia over the 1962 – 2001 period published
by Holko and Kostka (2005). Similarly, in
Canada, increasing temperature combined with
almost no change in precipitation, resulted in no
change in annual streamflow from 1947 to 1996 for
most regions (Zhang et al., 2001). The basins studied
in North America were selected because of
minimal human perturbations to the water cycle.
Conclusion
This review reports changes in occurrence frequency
of extreme mean daily discharge values
over the 1964/65 – 2003/04 period in the small
agricultural Rybárik basin placed in the flysh region.
Peaks over threshold method was used for
detection of changes in discharge values over
thresholds selected separately for hydrological
events caused by snow melting in winter seasons
and rainfall in summer seasons in single decades.
The frequency of POT events has decreased in
the past two decades (1984/85 – 1993/94 and
1994/95 – 2003/04). The results reveal that the
most extreme values of mean daily discharge occurred
in the 1964/65 – 1973/74 (mostly in summer)
and 1974/75 – 1983/84 (especially in winter)
periods. The former decade emerges as the decade
with the most extreme mean daily discharge values
recorded over the period studied. However, higher
values generally occurred during snow melting.
Human alterations, which can result in increase
or decrease in flood frequency, have not been carried
out over the 40 years period. The results presented
imply that influence of climatic changes on
intensification of hydrological cycle has not been
recorded yet in the Rybárik basin. The role of climatic
changes and its influence on flood frequency
still remains questionable. The next research should
also include catchments where flash floods caused
by intensive rainfall events occurred in the last
years. As suggested, not only climatic changes but
also human alterations such as conversion of forest
to agricultural land uses should be taken into account
in flood frequency analysis.
Acknowledgements. This research was supported by
the Science and Technology Assistance Agency
(Slovakia) under contract no. APVT-7804 and by
the Science Granting Agency (Slovakia) under contract
no. VEGA-5055.
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Received 11. August 2006
Scientific paper accepted 7. December 2006
VYUŽITIE METÓDY (POT) PRIETOKOV
NAD ZVOLENOU PRAHOVOU HODNOTOU
PRI ANALÝZE EXTRÉMNYCH
SEZÓNNYCH PRIETOKOV
Peter Bača, Veronika Bačová Mitková
V súvislosti s klimatickými zmenami je väčšia pozornosť
venovaná frekvencii výskytu extrémnych hydrologických
situácií. Labat a kol. (2004) sa domnieva,
že zvyšovanie odtoku z veľkých kontinentálnych riek
počas rokov 1920–1995 poskytuje prvý dôkaz, ktorý
demonštruje súvislosť medzi globálnym otepľovaním
a intenzifikáciou globálneho hydrologického cyklu. Na
zvýšenú frekvenciu výskytu extrémnych prietokov
a zvyšovanie odtoku v rozdielnych regiónoch poukazujú
viacerí autori (Foster a kol., 1997; Milly a kol., 2002;
Peterson a kol., 2002; Werritty, 2002; Walter a kol.,
2004; Micevski a kol., 2006).
Podľa Zhanga a kol. (2006) nie je zvyšovanie frekvencie
výskytu ročných maximálnych vodných stavov
spôsobený jediným faktorom akým je klimatická zmena,
ale komplexom faktorov, akými sú zmeny ľudských
aktivít. Vörösmarty a Sahagian (2000) uvádzajú, že
zvyšovanie odtoku môže byť pripísané napríklad
odlesňovaniu. Naopak, znovu zalesňovanie poľnohospodárskych
území môže znížiť hodnoty odtoku. Na
druhej strane merané údaje nepoukazujú na zmeny alebo
zvyšovanie veľkosti a frekvencie povodní (Aizen a kol.,
1997; Zhang a kol., 2001; Holko a Kostka, 2005).
Hoci klimatické zmeny sú globálnym problémom, ich
vplyv a trendy sú rozdielne v závislosti od veľkosti
územia. Analýzy by mali byť uskutočnené radšej
v menšej ako vo veľkej mierke (Sharma a Shakya,
2006). Náhle povodne (Flash floods) spôsobené
prívalovými zrážkami sa vyskytli na Slovensku
v posledných rokoch na malých povodiach (Grešková,
2005). Výsledky, ktoré prezentujú Svoboda a Pekárová
(1998) ukazujú, že, intenzívne zrážky v regióne Karpatského
flyša, ktoré spadnú na malé povodie nasýtené
predchádzajúcimi výdatnými zrážkami, môžu vyvolať
povodeň s dobou opakovania 500 rokov.
Najčastejšie sa pri pravdepodobnostných frekvenčných
analýzach vychádza z maximálnych ročných
prietokov. Uvažuje sa teda len s jednou hodnotou za rok.
Takýto prístup nezahŕňa do analýz prietoky, svojou
veľkosťou v roku druhé alebo tretie v poradí, ktoré môžu
byť však väčšie ako maximálne prietoky v iných rokoch
(Kite, 1977; Chow a kol., 1988).
Preto je výhodné použiť metódu POT, pri ktorej sú do
analýz zahrnuté všetky hodnoty prietokov nad zvolenú
prahovú hodnotu (Bayliss, 1999; Rao a Hamed, 2000;
21

P. Bača, V. Bačová Mitková
Black a Burns, 2002; Ouarda a kol., 2006). Okrem toho
umožňuje táto metóda identifikovať zmeny vo frekvencii
výskytu extrémnych prietokov v rozdielnych sezónach
roka.
Cieľom príspevku bolo analyzovať zmeny vo frekvencii
výskytu extrémnych hydrologických situácií na
poľnohospodársky využívanom malom povodí Rybárik
vo flyšovom pásme pri Považskej Bystrici v období 40-
tich rokov 1964/65–2003/04. POT metóda bola aplikovaná
zvlášť pre zimné a letné hydrologické polroky.
Povodie Rybárik je súčasťou experimentálneho povodia
potoka Mošteník, ktoré bolo založené ako Prírodné
hydrologické laboratórium v roku 1958 a je časťou povodia
rieky Váh, hlavného prítoku rieky Dunaj na území
Slovenska. Územie potoka Mošteník patrí do
Strážovskej vrchoviny, ktorú možno zaradiť do
vrchovinnej a čiastočne stredohorskej oblasti Slovenska.
Plocha povodia Rybárik je 0,12 km 2 s priemernou nadmorskou
výškou 400 m n. m. (min 375 m n. m., max 434
m n. m.). V povodí sa nachádza prameň potoka Jelšové,
ktorého dĺžka po záverečný profil povodia je 255 m a
priemerný sklon 9,1 %. Tvar povodia je výrazne vejárovitý,
priemerný sklon povodia je 14,9 %. Z geologického
pohľadu je povodie súčasťou flyšového
pásma so striedajúcimi sa ílovými a pieskovcovými
vrstvami. Pôdny druh je zastúpený ílovito-hlinitými
pôdami a pôdny typ hnedozemou. Spôsob využívania
krajiny je prevažne poľnohospodársky, len 10,4 %
z celkovej rozlohy povodia je pokryté lesom. Priemerná
ročná teplota je 8,09 °C, priemerný ročný úhrn zrážok je
738 mm, priemerná ročná odtoková výška je 231
mm/rok, bilančný výpar 507 mm a koeficient odtoku
0,31 (za 40-ročné obdobie rokov 1965–2004).
V záujmovom povodí Rybárik existuje v záverečnom
profile vybudovaný merný priepad s limnigrafom,
určený na kontinuálny záznam vodného stavu. Pomocou
merných kriviek prietokov získaných tarovaním bol
vyhotovený kontinuálny záznam prietokov. Údaje
získané za 40-ročné obdobie rokov 1965–2004 ukazujú,
že vodný stav potoka Jelšové je väčšiu časť roka
ustálený. Hodnoty prietokov vody sú nízke a pohybujú
sa od 0,04 l s -1 do približne 0,5 l s -1 . K výraznému
zvyšovaniu vodného stavu dochádza počas hydrologických
situácií vyvolaných topením snehovej
pokrývky (v zimných a jarných mesiacoch) a intenzívnymi
zrážkami (vo vegetačnom období). Maximálne
ročné kulminačné prietoky sa pohybujú od 10,9 l s -1 do
364,2 l s -1 . Podrobnejšie informácie o režime odtoku
podáva Pekárová a kol. (2005).
Hoci boli vodné stavy merané kontinuálne, analyzované
boli len priemerné denné hodnoty vzhľadom na
chýbajúce archívne údaje o okamžitých vrcholových
prietokoch. Kvôli lepšej homogenite údajov bola POT
metóda aplikovaná zvlášť pre zimné a letné hydrologické
polroky. Boli zvolené prahové hodnoty prietokov tak,
aby sa prietoky nad prahovými hodnotami vyskytli
v priemere 1.0-, resp. 0.2-krát za sezónu. Nezávislosť
vybraných hodnôt priemerných denných prietokov bola
zaistená nasledujúcimi kritériami:
− hodnota vybraného priemerného denného prietoku je
vyššia ako hodnoty v predchádzajúcom a nasledujúcom
dni;
− pre zimné polroky sa uvažovalo, že hodnoty priemerných
denných prietokov sa v čase medzi dvoma
prietokmi vybranými pre POT analýzy významne
nemenili a boli nízke;
− pre letné polroky, kedy je reakcia povodia na
prívalové zrážky rýchla, bol zvolený časový interval
medzi dvoma prietokmi vybranými pre POT analýzy
prinajmenšom tri dni a minimálna hodnota priemerného
denného prietoku v tomto čase bola nízka.
Použitie priemerných denných hodnôt prietokov
prináša so sebou určité neistoty. Kulminačné prietoky sa
totiž nemusia vyskytovať v dňoch s najväčšími priemerným
prietokomi (Linsley a kol., 1949).
Výsledky na obr. 2 a 3 ukazujú, že frekvencia výskytu
priemerných denných prietokov nad zvolenou vyššou
prahovou hodnotou (0,2-krát za sezónu) klesla
v posledných dvoch dekádach (1984/85–1993/94 a
1994/95–2003/04). Najextrémnejšie hodnoty sa vyskytli
v rokoch 1964/65–1973/74 (predovšetkým v letných
polrokoch) a 1974/75–1983/84 (najmä počas topenia
snehovej pokrývky). V prvom desaťročí sa počas letných
prívalových vĺn, spôsobených intenzívnymi zrážkami
vyskytli 4 najvyššie hodnoty denných prietokov za celé
skúmané obdobie (obr. 4). Všeobecne sa však vyššie
hodnoty priemerných denných prietokov objavujú
v zimných polrokoch.
Počas skúmaného obdobia 40-tich rokov nebolo povodie
ovplyvnené významnými zmenami ľudskej činnosti,
ktoré by mohli vyvolať zmenu vo frekvencii
výskytu extrémnych hydrologických udalostí. Je preto
nanajvýš pravdepodobné, že vplyv klimatických zmien
na intenzifikáciu hydrologického cyklu, predpokladaný
na severnej pologuli (Manabe a kol., 2004), nebol zatiaľ
v skúmanom povodí Rybárik zaznamenaný. Podobné
výsledky z horských povodí stredného Slovenska
prinášajú aj Holko a Kostka (2005) a z vybraných povodí
v Kanade Zhang a kol. ( 2001).
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