1. Introduction
In recent years, land degradation studies regarding soil and water quality have been systematically expanded rapidly throughout the world, with enormous health, social, and economic consequences (Kanownik 2005; Pawlowski 2011; Frąk and Baryła 2012; Gao et al., 2014; Chiwa 2021). Several factors influence the physicochemical quality of surface water, namely the concentrations of nitrate, phosphate, and sulfate anions (Akan et al., 2012), as well as sodium, potassium, calcium, and magnesium cations (Purandara et al., 2012). In surface waters, agricultural activity is the major cause of poor physicochemical quality (Bedla and Misztal 2014; Halecki et al., 2019). Thus, a key objective of water erosion assessment is to determine the physicochemical quality of the surface water that flows out of a catchment (Robson 2014). Specifically, soil protection research focuses on managing the terrain such that excess rainwater can be drained to limit the amount of intense washout in erosion-prone regions. The purpose of these procedures is to limit the spread of surface runoff and infiltration of water into the substratum. Furthermore, the granular composition of the soil affects the suitability of the area for farming (Seneviratne et al., 2010; Brocca et al., 2012). Cross-slope cultivation lessens the erosion of soil, but the amount of annual soil loss increase with increasing slope steepness (Vermeulen and Nieuwenhuis 2005; Kowalczyk and Twardy 2012). Thus, various environmental engineering methods are used to manage slopes in the Eastern Carpathian catchment (Mostowik et al., 2021).
Technical and ecological solutions are commonly sought in mountain catchments for the design of forest roads that slow down the flushing of soil, including polders with control devices that regulate outflow and inflow, blockage traps, and transverse barriers (Łapuszek and Witkowska 2005). In agricultural areas on the northern slopes of the West Carpathian, slope length is a major controlling factor for surface water erosion in different soil and agrotechnical systems. Moreover, the intensity of rainfall can affect the rate of soil erosion (Obi and Salako 1995), which depends strongly on the moisture content of the soil surface layer. Notably, runoff is one of the major determining factors of soil degradation (Zhao et al., 2021). The valley bottom usually contains silt in high concentrations due to surface drainage. Grazing pastures in mountainous terrain are designed for the retention of water-soaked clastic material (rill erosion or interrill erosion). For example, cultivated land on the Beskid flysch slope enriches the surface eroded material and soil surface water (Gil et al., 2021).
SWAT+ is a model used to simulate the quality of surface water in small watersheds and river basins. The current research examined the flow of nitrates and phosphorus in mountain streams. The study aimed to determine the factors that affect the inorganic nutrient accumulation and distribution based on the total suspended solids (TSS) found upstream and downstream in the flysch catchment. Therefore, we established the following objectives:
  1. Apply a new SWAT+ hydrological model in flysch watersheds affected by human activities.
  2. Assess inorganic nutrient distributions in surface waters by statistical autoregression.
  3. Model selected inorganic nutrients and TSS fluxes between the main watercourse and tributaries.
  4. Estimate hydraulic parameters in the main stream and their effects on inorganic nutrient concentrations in the watercourse.