analysis of seasonal extreme flows using peaks over threshold method

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.
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