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.