analysis of seasonal extreme flows using peaks over threshold method

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.
REFERENCES
AIZEN V.B., AIZEN E.M., MELACK J.M., DOZIER J., 1997:
Climatic and hydrologic changes in the Tien Shan, Central
Asia. J. Clim., 10, 1393–1404.
BAYLISS A.C., 1999: Catchment descriptors. Flood estimation
handbook, vol. 5., Institute of Hydrology, Wallingford.
BLACK A.R., BURNS J.C., 2002: Re-assessing the flood risk
in Scotland. The Sci. of the Total Environ., 294, 169–184.
CHOW V.T., MAIDMENT D.R., MAYS L.W., 1988: Applied
hydrology. McGraw-Hill, New York, NY.
FOSTER M.J., WERRITTY A., SMITH K., 1997: The nature,
causes and impacts of recent hydroclimatic variability in
Scotland and Northern Ireland. Proceedings, Sixth National
Hydrology Symposium. Wallingford: British Hydrological
Society.
GREŠKOVÁ A., 2005: Analysis of selected basin parameters
from the point of wiev of flash flood occurrence. Geographical
J., 57, 2, 131–144.
HOLKO L., KOSTKA Z., 2005: Analýza maximálneho denného
prietoku v horských povodiach stredného Slovenska.
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