Spatial Variability of Average Annual and Monthly Minimum River Flow in Poland

Abstract

The aim of this article is to analyse the spatial variability of SNQ, the average annual minimum river flow, as well as SNQm (m = 1, 2, …12), the average monthly minimum river flow in Poland. The data were obtained from the Institute of Meteorology and Water Management – National Research Institute (IMWM-NRI) in the form of the daily flow series from the period between 01 Nov 1990 and 31 Oct 2020 from 433 gauging cross-sections located within the territory of Poland. The results of the analyses are presented on maps of the physiographic regions of Poland (the Coastlands, the Lakelands, the Lowlands, the Highlands, the Carpathians and the Sudety Mountains). In order to compare SNqm – the unit average minimum monthly flow between the physiographic regions, the Kruskal-Wallis test with the Dunn (Bonferroni) adjustment was performed. In order to evaluate the spatial variability of the SNqm, the hypothesis was verified for each gauging station that the Spearman correlation coefficient between the SNqm and the zero point of the gauge was different from zero. The SNqm flow changed over the year. As expected, the highest values were observed in March and April, and the lowest in July and August. Regardless of the month, the rivers in the central part of Poland (the Lowlands) were less water abundant than those in other regions of the country while the greatest flows were observed in the mountain rivers. Statistically, no difference was observed between the SNqm in the Coastlands, the Carpathians and the Sudety Mts., and in nearly all of the months between the SNqm in the Lakelands and the Lowlands. In the whole territory of Poland, the river flow was dependent on the altitude of the catchment, while the strongest correlation was observed in the mountain regions.