Summary:

Wielkopolska region covers the area of North European Plain with particularly unfavorable water conditions. These conditions are determined by the climate prevailing in the region, namely relatively low annual rainfall and high evapotranspiration with usually unfavorable distribution of these meteorological elements over a year. Important modulating influence can be attributed also to some non-climatic physiographical characteristics typical in vast parts of the region, mainly soil properties. In addition, evapotranspiration may be stimulated locally by vegetation. The paper comprehends initial results of hydrologic balancing of lowland catchment typical for the region of Wielkopolska. The balancing was carried out in Trojnaka Stream catchment placed in Puszcza Zielonka forest (the central Wielkopolska). High forest cover proportion, deep layer of sandy soils and high evapotranspiration shape the storage capabilities in the tested catchment. The balancing covered the average annual rainfall year (2001/2002) following average long-term rainfall period. The Period of the negative retention changes occurred from the end of April until the mid of September. The maximum decrease and increase of storage change depth occurred respectively in June (-27,7 mm) and February (50,2 mm). Average specific discharge in investigated catchment equaled 2,5 dm³∙s^-1∙km^-2 with maximum and minimum runoff depth respectively in January (8,8 mm) and September (3,6 mm). Extreme amounts of evapotranspiration occurred respectively in December (6,7 mm) and May (90,1 mm). Annual change of storage equaled 52 mm. Both storage and runoff rebuilding abilities of the catchment, if favorable meteorological conditions would occur, are relatively high. On the other hand, while the favorable meteorological conditions for the catchment alimentation stopped, the significant decrease of storage and runoff followed. In addition, the long-term storage decrease did not occur during negative climatic balance period in the catchment. Thus, it may be fairly assumed that more stable forms of retention in the catchment, active over an average annual rainfall year, are related to zone of saturation and the part of aeration zone, which is not susceptible to direct evaporation from soil and vegetation uptake. Forest cover along with upper soil layers in investigated catchment may cause the reduction of runoff level by approximately 20 to 40% in comparison with natural catchments of the region.