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Z. Michalczyk, M. Łoś
The depth of underground waters is very differentiated and is generally related to relief In natural conditions, underground water occurs close to the surface of the river valley bottoms. On lower parts of slopes the dry layer is several meters thick, and it reaches 30—50 m in watershed areas. Gap systems existing in tectonic zones play an important role in the drainage of underground waters, which are joined to a network of river valleys. These are one-river areas in which hydrogeological conditions are most favourable for underground water intake. The high output of wells of over 150 m3/h which is obtained in smali depressions confirms suitable hydrogeological conditions for building reservoirs in tectonic zones.
The available waters of the Bystrzyca river basin are 55 million m3/year in a moderately-dry year and 88 million m3/year in a moderately-humid one (Michalczyk 1986, Michalczyk et al. 1983). Similar calculation results for available waters were given by Wilgat (1980), and distinctly higher ones by Krajewski and Hassa (1973). The resources of the Bystrzyca river basin were approved for exploitation at 220 thousand m3/day, i.e. about 80 million m3/year (Jałowiec 1989).
The supply conditions of the rivers are satisfactory, owing to large resources of underground waters. Nevertheless, the total unitary outflow was 4.01/s km2 in the years 1951-1990 (Michalczyk, Turczyński 1992). The average share of the underground supply in the outflow is estimated at 75%; in 1951—1990 it varied from 60% to over 90%. Of significance are considerable differences in the water availability of the rivers in dry and moist periods (Wilgat 1980, Michalczyk 1986). In dry years, average unitary outflows decrease to 21/s km2, and minimal temporal ones do not even reach 1 1/s km2.
THE EXPLOITATION OF WATER RESOURCES IN OLD LUBLIN
Lublin was located in the fork of the Bystrzyca and Czechówka rivers; the waterlogged valley bottoms of these rivers warranted good defence conditions. As early as in the 14th century both rivers appeared to be of great economic importance besides having defence advantages. In the 16th and 17th century 45 water mills were working in the Bystrzyca river basin, thus 29.2 km2 were served by one mili (Łoś 1986). Special attention is deserved by the ’big’ royal mili on the Bystrzyca river, which used ac. 4 m high fali of water collected in a pond of about 130 ha. Two other mills can be mentioned: one in Wrotków on the Bystrzyca with a pond of 120 ha dammed up about 2.5 m, the other — a ‘smali’ royal mili built on the Czechówka river at Lublin Castle. In the 16th and 17th c. there were 8 ponds and 10 mills, a paper-mill, a fullery and an ironworks in the neighbourhood of Lublin (Sierpiński 1843; Gawarecki 1974; Łoś 1986). The mechanisms damming up water (Fig. 2) were destroyed during catastrophic water rising or wars. They were usually reconstructed in their original places.
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