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.

The intensification of agricultural production is one of the factors determining economic development. Increasing mechanisation and use of fertilisers in agriculture lead to higher yields, but at the same time they can pose a threat to the environment. The overuse of chemical fertilisers contributes to increased concentration of nutrients in agricultural runoff. One of such areas is the Szreniawa River catchment, the study area located in the southern part of Poland. In this catchment, intensive mostly mechanical ploughing is applied in, for instance, vegetable production. The area has loess soils, which with intensive ploughing are susceptible to erosion. The study aims to determine changes in the quality of flowing waters against the background of agricultural production and land-use characteristics. Surface waters were classified as class II and occurred at all analysed points. The highest concentrations of N-NO3, N-NH4 and P-PO4 were found at a point in the middle of the catchment (lower part of research area). There, the lowest concentrations were recorded in 2018, which was related to the amount of precipitation during the growing season. On the other hand, the volume of plant and animal production closely correlated with the quality of surface water in the area. This was also confirmed by the land use structure. In conclusion, intensive agricultural production, mainly in terms of plough tillage causes significant hazards associated with soil erosion especially on agriculturally sensitive soils, although it provides good yields.