HYDROLOGICAL CONDITIONS IN SMALL-SCALE RETENTION BASINS RESTORED IN RURAL AREAS

This paper discusses the results of a hydrological survey of small-scale retention basins, including three lakes in a cascade system which were created by a dam on the Setal Stream and three smaller basins which were filled due to a rise in groundwater levels after the construction of weirs. All basins occupy an agricultural catchment in the municipality of Dywity (Olsztyn Lakeland). They are characterized by a small surface area (1-41 ha) and a shallow depth (1.6-3.6 m). The objective of this study was to analyze water level variability and determine the retention capacities of the examined basins. The survey covered three hydrological years between 2005 and 2007. Water levels were monitored on a weekly basis. Retention capacities were determined from bathygraphic curves for average water levels (based on bathymetric maps which were developed for the needs of the study). An analysis of the retention capacities of the studied basins during winter and summer points to higher levels of water accumulation in winter in all reservoirs. Lower retention in summer can be attributed to high ambient temperatures which speed up water evaporation and actual evapotranspiration from aquatic vegetation. The differences observed between the summer seasons in the analyzed hydrological years (other than evaporation which lowers retention) indicate that summer precipitation levels significantly affect retention capacity. The difference in the overall volume of water retained in all basins between the "dry" year of 2005 and the "wet" year of 2007 reached 104,600 m(3). Total winter retention during the three-year survey was determined at 1,511,600 m(3), and it was 69,700 m(3) higher in comparison with summer retention. The greatest fluctuations in the volume of retained water were noted in closed basins. In one of the studied reservoirs, a nine-fold difference was observed between the minimum and maximum retention levels.