4.2. Transport of inorganic nutrients and TSS floating in the watercourse
The mechanisms of transport from sedimentary materials are also related to the clastic weathering of the soil (Łach 2012). Consequently, denudation mechanisms are common in mountain areas (Baryła 2004) and have been evaluated for N-NO3- (Wu et al., 2021). Small catchments are subject to slow chemical denudation and less weathered drainage than channel processes, where the sedimentation rate and nitrate flux of fluvial material change with the flow of water (Richards et al., 2021). In the current study, we demonstrated that N-NO3- leaches from influent streams into the main stream, especially at its outlet, where it reaches a value of 16–23 kg/ha for the Smugawka stream and 6.5–7.8 kg/ha for the Mszanka stream (Fig. 6). Other forms of nitrogen were distributed similarly – the highest flow rate results in greater leaching into the main watercourses (Fig. 7). To reconstruct the geomorphology of river valleys, the denudation balance has been calculated in a previous report based on the amount of mechanical degradation of the catchment area (da Silva 2004). These results are also the basis for the development of erosion hazard maps with different denudation intensities.
In the main watercourse, we observed a gradual decrease in TSS concentration closer to the outlet (Figs. 4 and 5). This may be due to the hydromorphological parameters and the reduction of the flow velocity, which favors the sedimentation and hydrodynamic processes (Wu et al., 2022). The hydrological model showed that N-NH4+ is concentrated mainly in outlet drainage areas. The SWAT+ model also showed that N-NO3- accumulated in the main stream (Fig. 6). A method must be developed to depict the full transport path of clastic materials to visualize the relationship between the TSS content and the characteristics of the surface water quality. The sources of pollution and the scale of the risk of water and surface erosion also need to be considered. Specifically, hazard assessment of transported clastic weathering is necessary for providing surface water protection plans (Haritash et al., 2016; Krasowska 2017) and environmental requirements related to soil conservation (Barbayiannis et al., 2011). For example, in the Beskidy Mountains, such systems are used for forecasting landslide risks. From the SWAT+ model, we observed that sediment in the catchment outlet was highly concentrated. In a future study, the daily concentration of TSS should be compared with the monthly precipitation totals in a given year to determine the intensity of erosion. It has also been indirectly shown that, regardless of the scale of the flysch catchment, the leaching tendency of inorganic nutrients is maintained. The key factors are the land use of the catchment area and the hydrographic network density (Fig. 7).
Applications of SWAT+ software for precipitation
Beskidy streams undergo periodic weathering, but the amount of material discharged and accumulated is not proportional to the flow (Starkel, 2011). Erosive rainwater is actively degrading soil because it washes away the soil’s material as a result of its intensity (Kroczak et al., 2022; Meusburger 2012). According to Martínez-Mena et al., (2020), soils are also susceptible to erosion and surface runoff based on their granulometry, water absorption (water accumulation), permeability, and hydrogeology. As a result of a severe rainfall events accompanied by at least 100 mm/h of intensity, a targeted and intense washout occurs, but 20 mm of daily rainfall is a threshold value that triggers initial soil washing in a small catchment (Kruk 2017). The heavy rainfall, continuing for several days and reaching a precipitation sum of 50 to 150 m, intensifies hydrological and geomorphological processes, leading to fine-scale surface erosion (Starkel 2006). Part of the rainfall infiltrates the ground (percolation) in the form of an aqueous solution, while the rest flows along the stream, washing away soil particles. As a result, an increased accumulation of soluble forms of inorganic nutrients in the main watercourse is observed.
The roughness of the surfaces due to intensive farming partially determines water storage on the soil surface and may indirectly affect the ability of water to infiltrate and cause nitrate leaching (Lin et al. 2015; Wang et al., 2014). Rough surfaces also increase hydraulic resistance, which allows weathered soil to be carried away by rainwater flowing down an incline (Zheng and He 2012). A solution mechanism propels aggregates with diameters less than 0.05 mm. As the intensity of water erosion increases, the relationship between erosion forces and sediment load changes (Hao et al., 2019). The change in the roughness of the soil surface occurs at the precipitation intensity of 0.68 mm·min-1. Then, soil particles migrate along the slope as the size of splash erosion increases (Liao et al., 2019).
For each catchment, TSS was spatially distributed in tributaries differently. There was no accumulation of TSS in the main watercourse following its outflow (Fig. 4). Similarly, the forested part of catchments influences the TSS leaching process (Fig. 5). This suggests that management in rich-relief flysch catchments should be geared toward the forests. Accordingly, the river valleys do not possess the capacity to retain inorganic nutrients, and in the case of TSS, this may result in earlier sedimentation in the river channel. One of the key mechanisms affecting this process is the washing away of soil particles. In this research, a hydrodynamic model was developed to understand how phosphorus and nitrate concentrations are distributed in mountain streams. With the help of passive and active remote sensing, it is possible to address knowledge gaps in the surface water sector, including rivers, lakes, reservoirs, and wetlands.