Extreme groundwater levels in Central Poland in 1951-2000

W artykule omówiono skład fIzykochemiczny opadów atmosferycznych, wód gruntowych i roztworów porowych. Wody (roztwory) porowe uzyskano za pomocą metody ciśnieniowej z prób gruntów gliniasto-piaszczystych i piaszczystych, pobranych w regionie łódzkim i bełchatowskim. Ustalenie składu jonowego wód pozwoliło na określenie ich typów hydrochemicznych oraz wzajemnych związków między poszczególnymi rodzajami wód. Wskazano również na czynniki antropogeniczne wpływające na zmiany składu wód opadowych, porowych i gruntowychThe aim of this study is to present an analysis of sequence of extreme annual groundwater levels in central part of Poland. Data were obtained from 55 groundwater levels (Fig. I),which were monitored in Institute of Meteorology and Water Management. Every sequence has its own theoretically fitted distribution, counted autocorrelation and the number of statistically significant, subsequent coefficients (with shifts from I to 5).Annual minima are best approximated (Fig. 3) by Fisher's-Tippet's distribution (E3). Furthermore, in some cases log-normal, two- and three-parametric distributions (LN2 and LN3) were fitted. It is clearly seen in spatial distribution (Fig. 4) in Pilica river basin that the participation of Fisher's-Tippet's distribution is relatively smaller in this region than in the others. Mean autocorrelation of annual minima (Fig. 5) is equal to 0.59, which means that the levels have big inertia. Annual maxima are best approximated by (Fig. 6) log-normal, twoparametric distribution (LN2). Except this one, in some cases: Gumbell (El), three-parametric log-normal (LN3) and Pearson type III distribution (P3) are fitted. On the north of the investigated area in the spatial arrangement the concentration of LN3 distribution can be clearly seen. The sequences of maximum groundwater levels are characterized by much smaller inertia than sequences of minimum values. Looking at their spatial arrangements (Fig. 8) it cannot be omitted that high RaI values (> 0.8) concern the same region, but not the same wells as in case of minima. Low RaI values « 0.2) are rather evenly distributed. Basing on the approximated distributions, water levels of given probability (of 50% and 10%) of exceeding (for maximum values) and unreachables (for minimum values) were calculated. It allowed to create diagrams (Fig. 9) of the frequency of week-long floods and low flows during the long lasting period. After slightly dry 50s the shortages were supplemented and wet years began (late 60s to the beginning of 80s). Following years belong to a dry period with its apogee in 1990-1993. Afterwards we observe some years of supplementation of aquifers and the beginning of floody period just before the beginning of a new millennium. As in case of other hydrometeorological characteristics we can observe its cyclic nature