ANALYSIS OF SELECTED HYDROLOGICAL CHARACTERISTICS OF TWO SMALL LOWLAND CATCHMENTS

Aim of the study: i. Comparison of characteristic discharges (mean and mean of annual minimum, i.e. mean low discharge) of small lowland river between two periods (1963-1990 and 1991-2020) estimated on the base of field measurements. ii. Comparison of specific discharges of two small catchments of various size in the period 1991-2020. iii. Evaluation of local indirect methods for estimation of mean and mean low discharge applied for the two small catchments. Material and methods: Since 1962, hydrological investigations have been conducted in a small lowland catchment of the Zagożdżonka River. The catchment area is characterized by agro-forested land use, and is located in the south part of the Masovia region, Poland. Continued monitoring of water stages and discharge measurement at the river gauge Płachty Stare were conducted for estimating daily and characteristic discharges for two periods: 1963-1990 and 1991-2020. Other methods were also used for both of the hydrological characteristics. They were based on maps of specific discharges from the Hydrological Atlas of Poland. Results and conclusions: Mean discharge of the Zagożdżonka River at the gauge Płachty Stare between the two periods decreased by 24%. Mean low flow decreased by 43%. Higher evapotranspiration, caused by climate change, i.e. temperature increase and higher ratio of forested area in the catchment, is indicated as the main reason of the changes. Mean specific discharge from the smaller catchment (at Wygoda gauge) was 36% less than from the larger catchment. The application of the indirect methods for estimation of mean and mean low discharges for the larger catchment seems to give satisfactory results.