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Hydrological conseąuences of human action..

carrying relatively morę water than before. It cannot be determined from the available data whether this results from the supply from the upper Wieprz river, from which most waters are coiweyed to the Tysmienica basin, or whether a role is also played by increased runoff from its own basin due to hydrotechnical action.

3

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Fig. 11. Totals of annual mean discharge of the Tysmienica river at Tchórzew and Siemień (1951-1990). In relation to totals of annual mean discharge of the Krzna river at Malowa Góra

Lakę water levels have been observed for only a short time. Piaseczno Lakę has the longest observation series, from September 1971³. The diagram of monthly water levels (Fig. 12) shows that there were two periods of high levels — 1975—1976 and 1981—1982 — over 22 years. In the last decade, a decrease in annual maxima and minima has been observed. The lowest level recorded in 1992 was over 70 cm lower than the minima recorded in the first observation period. Mean water level lowering was 0.35 cm/month in the whole period, and sińce 1982 it has increased to 1.1 cm/month. The results of both periods are statistically significant.

c) Hydrographical changes can be recorded directly in the field. The first studies in which attention was drawn to changes were carried out by the end of the 40’s and the beginning of the 50’s (Wilgat 1950, 1954). Most

In the years 1971-1982 readings ofwater levels were performed by the state service. In the following years observations were organized by the Department of Hydrography, Maria Curie-Skłodowska University, Lublin.