2006/1 PAGES 10 – 18 RECEIVED 18. 2. 2006 ACCEPTED 15. 4. 2006 S. KOHNOVÁ, L. GAÁL, J. SZOLGAY, Associate Prof. Silvia Kohnová, PhD. Prof. Ján Szolgay, PhD. Associate Prof. Kamila Hlavčová, PhD. K. HLAVČOVÁ Department of Land and Water Resources Management Faculty of Civil Engineering, SUT Bratislava Radlinského 11, 813 68 Bratislava REGIONAL ESTIMATION Tel: 02/59274623,02/5974499 E-mail:[email protected] [email protected] OF DESIGN 10-DAY [email protected] Mgr. Ladislav Gaál PRECIPITATION TOTALS IN Czek Hydrometeorological Institute Libuš Na Šabatce 17 THE UPPER HRON REGION 143 06 Praha 4 – Komořany, ČR [email protected] ABSTRACT KEY WORDS In the paper 10-day maximum precipitation totals from 23 rain gauges from the period • extreme precipitation, 1961 to 2000 in the upper Hron River basin in Slovakia were analysed. A combined • L-moments, method, based on statistical criteria and on the evaluation of evaporation and runoff • regional frequency analysis, conditions during long precipitation events, has been used for the selection of the 10- • 10-day maximum precipitation totals. day precipitation totals. N-year values of the 10-day annual maximum precipitation totals were estimated separately in the summer and winter seasons using a methodology developed by Hosking and Wallis (1997). The regional distribution function was selected using a L-moments diagram. Finally, a comparison of the resulting N-year maximum precipitation totals estimated from these regional distributions with the at–site analyses was performed. INTRODUCTION or heavy rainstorms. In Great Britain, the Flood Studies Report (NERC, 1975) and subsequently the Flood Estimation Handbook An essential part of flood risk assessment in engineering hydrology (FEH, 1999) were broad studies lasting for decades on the methods is the issue of intensive and long-lasting precipitation events. Such of estimating extraordinary flood events, which also included several-day precipitation events can, under special synoptical methods for estimating design precipitation for durations of hour circumstances, bring extraordinary precipitation amounts to to 8 days at any location in Great Britain. The German KOSTRA watersheds, which can result in severe floods and cause damage project (Malitz, 1999; Bartels, et al. 1997), the Italian VAPI project to human property and human lives as well. Examples of such (Ferrari, 1994), the HIRDS system of New Zealand (Thompson, natural disasters have also been observed in the very recent past 2002) and the Australian guide to rainfall and runoff (Pilgrim, 1987) in Central Europe, including flood events in Slovakia, the Czech are other examples of such complex national studies on flood or Republic, Austria and Germany (July 1997; August, 2002). The precipitation risk assessment. reliable estimation of such intensive, several-day precipitation totals A new perspective on design rainfall estimating has recently been therefore plays a key role in flood risk assessment, and particular developed by the National Weather Service (NWS) at National attention has been paid to this issue for decades worldwide. Oceanic and Atmospheric Administration (NOAA) in the United In most of the national meteorological offices of the world, there States: the NOAA Atlas Vols. 1 and 2 (Bonnin, et al.; 2004a, 2004b) has been an intensive effort to develop complex statistical methods are exclusively available via the Precipitation Frequency Data in order to cope better with natural hazards such as flash floods Server through the internet (http://dipper.nws.noaa.gov/hdsc/pfds) 10 20056 SLOVAK UNIVERSITY OF TECHNOLOGY kohnova.indd 10 23. 10. 2006 15:09:39 2006/1 PAGES 10 — 18 - they no longer exist in the form of a classic, hard-copy handbook. precipitation analyses have been carried out, for example, in the NOAA Atlas 14, Vol. 1 (Bonnin, et al.; 2004a) contains precipitation United States (Schaeffer, 1990; Guttman, 1993; Sveinsson, et al., frequency estimates for the semi-arid areas in the south-west of 2002), Canada (Pilon, et al., 1991; Adamowski, et al., 1996) and the U.S., while NOAA Atlas 14, Vol. 2 (Bonnin, et al.; 2004b) South Africa (Smithers and Schulze, 2001). Fowler and Kilsby focuses on the Ohio River basin and the 14 surrounding states in (2003) concentrated on regional analysis of extreme precipitation the northeastern parts of the U.S. Thanks to the convenience of the for a duration of 1 to 10 days in the United Kingdom: the authors Internet, precipitation frequency estimates for 5-minute through examined changes in the shape of regional growth curves in the 60-day durations at average recurrence intervals of 2 years through last decades of the 20th century. Gellens (2002) investigated k-day 1000 years can be easily achieved for any location of interest among precipitation totals (k = 1… up to 30) for Belgium, while the whole the states featured in the analysis. Similar work has been undertaken territory of the country was treated as a single region for all the for the state of Alabama (not included in the aforementioned combinations of the 11 durations and 3 seasons (calendar year, warm studies) by Durrans and Kirby (2004). The Alabama Rainfall and cold hydrological half-year) analyzed. In the Czech Republic, Atlas is an internet-based server for providing design information the regional frequency analysis of several-day precipitation totals on the characteristics of extreme precipitation, which include has been processed recently: Kyselý, et al. (2004) presented methods intensity-duration-frequency (IDF) curves and 24-hr design storm and principles for the delineation of homogeneous groups for annual hyetographs. maximum precipitation totals of 1, 3, 5 and 7 day durations. The most promising methodology for estimating design rainfall In Slovakia, 2-day and 5-day precipitation totals were analysed in recent decades has turned out to be the so-called index flood in the upper Hron Region in Stehlová, et al. (2001) and Jurčová, methodology in combination with L-moments. An index value et al. (2002). Gaál and Lapin (2002) analyzed k-day precipitation approach assumes that the region is homogeneous, i.e., the frequency totals in a 100-year series from Hurbanovo. In Lapin, et al. (2003), distributions of the observations from all the sites in the region are an at-site analysis of 1-, 2-, and 5-day precipitation totals and identical, apart from a site-specific scaling factor (i.e., Hosking and a subsequent geostatistical regionalization was undertaken for 557 Wallis, 1997). In other words, an estimate of an amount of rainfall sites in Slovakia. with a given return period for a selected site is obtained by the In the past, only daily maximum precipitation totals were usually product of the index value from this site (usually, the average value evaluated in Slovakia (Šamaj et al., 1985). As the previous examples of the sample) and the desired quantile of the regional growth curve illustrate, it is necessary to deal with rainfalls of various durations. that is common for all of the sites in the region. Issues involving testing new methodologies used for rainfall L-moments represent a unique tool within the field of design value frequency analysis and acquiring experience concerning their estimation. L-moments are statistical characteristics analogous to applicability under the various physical/geographic conditions of conventional (product) moments – they can describe any statistical Slovakia are also of interest. The number of new types of acceptable properties of the probability distributions (such as the location, probability distribution functions and parameter estimation methods scale, skewness, kurtosis or others). However, they are computed implies that the use of a single distribution function (for instance, from a linear combination of the ascending ordered data sample. the almost exclusively used Gumbel or the 3-parameter Pearson The superiority of L-moments over conventional moments has distributions – Šamaj, et al., 1985) may not be appropriate in the been proved in a number of studies (Sankarasubramanian and future. Srinivasan, 1999; Peel, et al., 2001). For example, L-moments do In this paper, problems of selecting 10-day precipitation totals not have sample size related boundaries and are less sensitive to are discussed and a case study on fitting the appropriate regional the presence of extraordinary values (’’outliers’’) in the data sample distribution functions to 10-day maximum precipitation values in as conventional moments (Hosking, 1990). Vogel and Fenessey the upper Hron region is presented. (1993) proved the advantage of using L-moments ratio diagrams in the identification of a parent statistical distribution from which the data sample might have originated. A detailed overview of the METHODOLOGY FOR THE SELECTION OF regional frequency analysis based on L-moments can be found in 10-DAY MAXIMUM PRECIPITATION TOTALS the handbook of Hosking and Wallis (1997). Several examples of the successful adoption of the aforementioned In previous studies (e.g., Stehlová, et al., 2001; Jurčová, et al., methods can be found in the various regions of the world; however, 2002), the selection of k-day precipitation totals (k = 2, and 5 days) most of the studies are devoted to an analysis either of short was based on the requirement of recording daily precipitation duration rainstorms or to 1-day precipitation totals. Such regional totals higher than 0.0 mm for each day of the selected k-day REGIONAL ESTIMATION OF DESIGN 10-DAY PRECIPITATION TOTALS IN THE ... 11 kohnova.indd 11 23. 10. 2006 15:09:40 2006/1 PAGES 10 — 18 period. However, when calculating k-day precipitation totals for sample L-Cv ratios. The properties of the simulated homogeneous k higher than 6 days from several locations in Slovakia we came pooling group are compared to the sample L-Cv ratios as to the conclusion, that, especially in the lowlands, the number of such continuous events dramatically decreases with increasing k (Kohnová, et al., 2005).