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Z. Michalczyk, M. Łoś

annual flow of this river was much lower than the mean of many years, and in 1991 it decreased to 3.02 m³/s. In the last 15 years a decrease in the amount of the exploited underground water was also recorded, as well as one in the amount of discharged wastewater, which determined the amount of Bystrzyca river flow in its lower course. Most water was exploited in 1981 — 58.3 million m³. In the following years the consumption decreased systematically to a level of 50 million m³ in 1991. Despite decreased underground water consumption, big changes in water circulation were found after 1982, being particularly noticeable in periods of a Iow flow.

Fig. 6. Changes in the years 1977-1991 in annual mean flows of the Bystrzyca river

at Sobianowice (1), in Lublin (2), and water exploitation in Lublin (3)

Big transformations in water circulation are well documented from the Lublin water-gauge records. It closes the Bystrzyca river basin, 748 km² in area, which constitutes 59.1% of the whole collecting area. The mean flows in the Lublin hydrometrie section were in the rangę 54.4—64.9% of the flows at Sobianowice in the years 1977-1981; thus, they were proportional to the catchment basin area. In the following years a considerable decrease in the outflow from the upper Bystrzyca river was noticed. In 1990 the amount of conveyed water in the Lublin profile was only 28.6% of the flow at Sobianowice. In the period 1990—1991, at the Lublin water-gauge profile the mean annual flow was 0.92 and 0.96 m³/s. These are much smaller values than the unimpaired (biological) flow estimated at 1.17 m³/s according to the hydro-biological criteria (Kostrzewa 1977).