A New Empirical Approach to Calculating Flood Frequency in Ungauged Catchments: A Case Study of the Upper Vistula Basin, Poland

The aim of the work was to develop a new empirical model for calculating the peak annual flows of a given frequency of occurrence (Q(T)) in the ungauged catchments of the upper Vistula basin in Poland. The approach to the regionalization of the catchment and the selection of the optimal form of the empirical model are indicated as a novelty of the proposed research. The research was carried out on the basis of observation series of peak annual flows (Q(max)) for 41 catchments. The analysis was performed in the following steps: statistical verification of data; estimation of Q(max) flows using kernel density estimation; determination of physiographic and meteorological characteristics affecting the Q(max) flow volume; determination of the value of dimensionless quantiles for Q(T) flow calculation in the upper Vistula basin; verification of the determined correlation for the calculation of Q(T) flows in the upper Vistula basin. Based on the research we conducted, we found that the following factors have the greatest impact on the formation of flood flows in the upper Vistula basin: the size of catchment area; the height difference in the catchment area; the density of the river network; the soil imperviousness index; and the volume of normal annual precipitation. The verification procedure that we performed made it possible to conclude that the developed empirical model functions correctly.