analysis of seasonal extreme flows using peaks over threshold method

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.
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