Estimation of the Peak Runoff Coefficient on Small Catchments in Slovakia

Runoff is one of the essential indicators for designing measures that retain or safely divert water in the country. However, in the 21st century, rural basins face changes caused by climate or land use. These alters force society to think about the transformation that time brings and to change the view on some parameters of the country's functioning within the outflow processes. The presented contribution deals with runoff changes over the last decades, in which the difference in the values of peak runoff coefficients in small watersheds is analysed using direct and indirect estimation methods. The first part of the study deals with the indirect approximate calculation of the peak runoff coefficient in 128 small catchments up to an area of 150 km2 (approx. 58 mi2 ) located on the territory of the Slovak Republic. Input data consisted of estimated concentration times according to Nash and Kirpich formulae, estimated design values for maximum floods and IDF curves for all available rain gauge stations in each analysed basin. The results were compared to previous studies. In the second part, for two selected basins (Parn Horn Oreany and Bel Liptovsk Hrdok), we estimated the peak runoff coefficient using the direct method. The chosen watersheds represent different landscape structures, where the first is a lowland type, and the second a high-mountain type of landscape. The analysis was based on direct measurements of hourly peak flows from 1989 to 2021. The flood wave parameters for calculating the peak runoff coefficients were obtained by substituting the causative precipitation. These data were subsequently statistically analysed using the Johnson probability distribution. The peak runoff coefficient for a 100-year return period was separately modelled for the summer and winter periods in both basins and compared to the indirect estimation results. The study's results are intended to highlight the differences in the methods used to estimate peak runoff coefficients in small watersheds. Acknowledgements This study was supported by the Slovak Research and Development Agency under Contract VEGA Grant Agency No 1/0782/21. The authors thank the agencies for their research support.