analysis of seasonal extreme flows using peaks over threshold method

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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
Page 1: J. Hydrol. Hydromech., 55, 2007, 1,
Page 5 and 6: P. Bača, V. Bačová Mitková As n
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analysis of seasonal extreme flows using peaks over threshold method

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