Maksymalny czas trwania głębokich niżówek letnich w środkowej Polsce i jego uwarunkowania

Abstract

Maximum duration of extreme summer flows in central Poland was estimated on the base of hydrometric data including twenty four hour flow series from the 1966-1983 period, which were published by IMGW. There were 29 water gauges located in Warta, Pilica and Bzura drainage basins (Fig. l) shortlisted for the analysis. Low flow periods were sectioned basing on a border flow SNQ - a principle was that the period with flows below the borderline had lasted at least 5 days, while low flows followed in succession had to be divided by periods of flows higher than SNQ being minimum 3 days long. Maximum summer flows were at that time very differentiated and their length fluctuated from 10 to 282 days (Fig. 2). According to that some kind of spatial order can be outlined here, the longest low flow periods appeared in Great Poland and Warsaw - Berlin Glacial Valley (Fig. 3). Maximum length of low flow was significantly correlated with the parameters, which illustrated flow recession time, mean time of low flow or extreme time constitution in total low flow and extreme low flow time (Fig. 4, 8). The parameters of maximum length probability distribution were estimated as well. On the base of those calculations was estimated the probability of not achieving the maximum, which was empirically confirmed. Values fluctuated from 90-99.2% usually being about 96% (Fig. 6). The greatest probability of lengthening of maximum low flow periods had Great Poland Rivers (Mogilnica, Wrześnica, Lutynia), while the smallest probability was characteristic for rivers, which origins are in uplands (Warta - Poraj, Pilica - Przedbórz).