The intensive agricultural use of the land affects both quantity and quality of river water in the catchment area. Such impact is visible also in the Szreniawa River catchment in the Małopolskie Voivodeship. The combination of intensive plowing and soil susceptibility to water erosion are the main causes of soil and nutrients depletion during the heavy rainfall. The aim of the study is to determine changes in the water quality in the Szreniawa River catchment compared to the agri-culture use and precipitation level.

The quality of surface water has been analysed in the river catchment area in three sampling points. The concentration of the total suspended soils in the samples collected after heavy rainfall in August 2017 reached a value of 837 mg·dm–3. The average concentrations of N-NO3 in the years of 2016–2019 ranged from 0.16 to 13.46 mg·dm–3, with the highest val-ues in the summer (up to 13.46 mg·dm–3). The concentration of N-NH4 and P-PO4 in the Szreniawa water was affected by precipitation. The highest value of average concentration of N-NH4 3.00 mg·dm–3 was recorded in the autumn of 2019 in the middle section of the river. The highest value of P-PO4 0.90 mg·dm–3 was recorded in the autumn of 2019 mostly due to water erosion of the loess areas. Erosion has been caused by the short-term heavy rainfall. As a result, suspended solids, soluble and insoluble phosphorus compounds leaked to the river.

Changing atmospheric conditions, including above all the deepening extreme weather phenomena, are increasing from year to year. This, in consequence, causes an increase in the incidence of low outflows.
The study compares low water levels for two catchments: Biała Woda and Czarna Woda, and phosphorus and nitrogen load using the Nutrient Delivery Ratio (NDR) model in InVEST software. The objective of the NDR is to map nutrient sources from catchment area and transfer to the river bed. The nutrient loads (nitrogen and phosphorus) spread across the landscape are determined based on a land use (LULC) map and associated loading rates described in literature. The studies have shown that low water levels have been more common recently and pose the greatest threat to the biological life in the aquatic ecosystems. The structure of land use is also of great importance, with a significant impact on the runoff and nitrogen and phosphorus load. Phosphorus and runoff from surface sources to the water of Biała Woda and Czarna Woda catchments area has been reduced in forested areas. Only higher run-offs are observed in the residential buildings zone. The nitrogen load was also greater in the lower (estuary) parts of both catchments, where residential buildings dominate.