Przepływy ekstremalne wybranych rzek środkowej Polski w latach 1951-2000

The following study is an attempt to create a multiaspect statistical analysis of 50-years long sequences of monthly extreme llows of three rivers of central Poland: Warta in Sieradz, Bzura in Sochaczew and Pilica in Przedbórz (1951-2000). Those rivers play leading roles in water management of one of the poorest in surface water regions of country - central Poland. The knowledge of their regime and identification of its changes in time especially due to low and high flows is a basis for future planning and efficient water management in regional scale. Within this work functions of monthly extreme flow distributions were identified and distinct probability quantiles were calculated (fab. 1-2; Attachments I-VI). Looking at flow maxima it was noted that for snow-melt freshets months (lI, Ill, IV) the most characteristic were gamma and Weibull's distributions, while for the months of 'summer freshet (VI, VII, VIII) - gamma and log-gamma functions. In case of monthly minima was slightly different. The greatest number of sequences might have been described by Weibull's distribution, significantly smaller number by log-gamma, last two functions appeared very rarely. The biggest annual specific capacity of probability of exceeding l% characterize Pilica river. It was much bigger than analogous runoffs calculated for Warta and Bzura rivers. For Warta and Bzura annual low llows of probability of exceeding 99% were also 8 times smaller than their ecological discharges. In case of Pilica this difference was about 400% smaller. There were investigated the perennial and seasonal dynamics of extreme llows (attachments VII, IX), changeability of monthly llow amplitudes (Fig. 13, 14) as well the trends and their significance (Tab. 4). Long lasting maximum llow series for months of the same names from cold half-year were investigated in 3 rivers and characterized by negative trends (sometimes statistically significant) - Pilica (XI-IV); Bzura (II, III). The lowest flows period systematically moved from early summer (in 1951-1960) towards late summer or even early autumn (in 1991-2000). This trend was very pronounced and unambiguous. Perennial changeability of warm half-year minimum series was usually smaller than of parallel series in cold half-year. All monthly minima of Warta increased, Pilica's - decreased, while the observed tendencies for Bzura were variegated. Most of the obtained trend lines indicate on tendencies of decreasing in the series of monthly amplitudes, although statistically significant trends are characteristically arranged. Statistically significant trends occur during late winter and early spring (I-II), what is more they concern all studied rivers. The analysis indicate on a small decrease in changeability of flows in investigated rivers, which is characterized by slightly systematic increase of monthly minima and amplitude decrease. However, those phenomena are visible best during cold half-years and concern mainly month of freshets (II, III).