The study covered water resources of two mountain streams in the Polish Carpathians. These were the Biała Woda and Czarna Woda streams, the catchments of which are adjacent to each other. Water flows in both streams were measured during the hydrological years from 2006 to 2020. Next, water outflows from the catchments were calculated. The study aimed to determine differences in the water resources of those catchments in a very small mountainous area. The study showed quantitative similarity in water resources in the entire multi-annual period but at the same time large differences in shorter periods. Instantaneous and daily outflows showed the largest differences, but differences in annual outflows of up to 20% were also recorded. Therefore, hydrological data from operational cross-sections to assess water resources of neighbouring uncontrolled watercourses should cover multi-annual mean values. It was found that during periods of increased runoff (from melting snow or precipitation), the outflow from the Biała Woda catchment was much larger, while during rain-free periods, the outflow from the Czarna Woda catchment prevailed. All short- term flood like outflows were at least several tens of per cent higher in the Biała Woda catchment. The higher retention capacity of the Czarna Woda catchment can be attributed to the land use (mainly forest areas). The results can be used for modelling catchments of similar parameters and determining their retention capacity.

Water erosion in mountainous areas is a major problem, especially on steep slopes exposed to intense precipitation. This paper presents the analysis of the topsoil loss using the SWAT (Soil and Water Assessment Tool) model. The SWAT model is a deterministic catchment model with a daily time step. It was designed to anticipate changes taking place in the catchment area, such as climate change and changes in land use and development, including the quantity and quality of water resources, soil erosion and agricultural production. In addition to hydrological and environmental aspects, the SWAT model is used to address socio-economic and demographic issues, such as water supply and food production. This program is integrated with QGIS software. The results were evaluated using the following statistical coefficients: determination (R2), Nash–Sutcliff model efficiency ( NS), and percentage deviation index ( PBIAS). An assessment of modelling results was made in terms of their variation according to different land cover scenarios. In the case of the scenario with no change in use, the average annual loss of topsoil (average upland sediment yield) was found to be 14.3 Mg∙ha ^–1. The maximum upland sediment yield was 94.6 Mg∙ha ^–1. On the other hand, there is an accumulation of soil material in the lower part of the catchment (in-stream sediment change), on average 13.27 Mg∙ha ^–1 per year.