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Przepływy ekstremalne wybranych rzek środkowej Polski w latach 1951-2000

Title variants
Extreme discharges of selected rivers in Central Poland in 1951-2000
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Artykuł powstał w wyniku prac badawczych prowadzonych w ramach Projektu Zamawianego MNiI (K091/P04/2004/11).
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).
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