EN
The nature of changes in conditions of runoff and the related issues depend upon the degree of human impact. The paper presents the influence exerted by a medium sized town on the runoff in a small lowland basin (47.21 sq. km). The selected object of the study is constituted by the uncontrolled catchment area of Srebrna (a right-hand tributary of Mienia river). Its upper part (29.6 sq. km) has a quasi-natural character and is subject to an impact from farming. The course of the hydrological processes in the lower part (17.61 sq. km) is influenced by the town of Mińsk Mazowiecki, of 35,631 inhabitants (in 1999). Given the background of the spatial urban structure of the town, analysis was carried out of the changes in conditions of surface runoff. These changes were expressed through the comparison of runoff coefficients corresponding to the existing land cover with urban structures, and the conditions corresponding to lack of such structures. Independently of this analysis, the transformations of the hydrological structure of the basin were characterised, with special consideration of the natural, transformed, and artificial drainage network. A separate space was devoted to a brief characterisation of the water and sewage economy of the town as of 1999. Besides, an attempt was made of assessing the influence of the town on the runoff of Srebrna with the use of results of synoptic measurements of discharge. This allowed for the setting up of the daily runoff balance for the various hydro-meteorological conditions in the urbanised part of the basin.