Conservative Solute Transport from Soil to Runoff Flow in a Steep Slope Area

Solute transport through soil is the major source of nonpoint pollution. Overland flow development in a steep slope area is a complex and nonlinear system that can be influenced by infiltration excess, saturation excess, and subsurface flow. Variations in rainfall intensity, slope, and land cover can also affect the soil moisture dynamics leading to overland flow formation. This study aims to define the dominant mechanism of runoff generation in a steep slope area. A computational model is developed to estimate the quantities of runoff flow and salinity concentration in soil layers at different depths. For this purpose, field research was conducted with several rainfall heights and land cover types in an area of 57 m² with average slope of 46.7%. Field experiments indicated that the surface and subsurface flows in a soil depth of up to 30 cm were not dominant mechanisms in clay soil with steep slope even under high rainfall intensity. The output of the flow quantitative model showed that overland flow generation in the field plot was dominated by the saturation excess mechanism. The natural grass plot showed the lowest overland flow percentage; by contrast, removed grass and without grass plots showed percentages of 19.49% and 19.18% for rainfall height of 24.9 and 21.8 mm, respectively. Land cover was identified as an important factor affecting runoff generation. The output of the solute transport model for rainfall height of 24.9 mm with salt addition indicated that natural grass and removed grass plots had the lowest salinity concentrations of 55.45 and 33.62 ppm, respectively. Salinity transport was slowest on the natural grass plot, and it started only 45–50 min after artificial rain was applied.

Solute transport through soil is the major source of nonpoint pollution. Overland flow development in a steep slope area is a complex and nonlinear system that can be influenced by infiltration excess, saturation excess, and subsurface flow. Variations in rainfall intensity, slope, and land cover can also affect the soil moisture dynamics leading to overland flow formation. This study aims to define the dominant mechanism of runoff generation in a steep slope area. A computational model is developed to estimate the quantities of runoff flow and salinity concentration in soil layers at different depths. For this purpose, field research was conducted with several rainfall heights and land cover types in an area of 57 m² with average slope of 46.7%. Field experiments indicated that the surface and subsurface flows in a soil depth of up to 30 cm were not dominant mechanisms in clay soil with steep slope even under high rainfall intensity. The output of the flow quantitative model showed that overland flow generation in the field plot was dominated by the saturation excess mechanism. The natural grass plot showed the lowest overland flow percentage; by contrast, removed grass and without grass plots showed percentages of 19.49% and 19.18% for rainfall height of 24.9 and 21.8 mm, respectively. Land cover was identified as an important factor affecting runoff generation. The output of the solute transport model for rainfall height of 24.9 mm with salt addition indicated that natural grass and removed grass plots had the lowest salinity concentrations of 55.45 and 33.62 ppm, respectively. Salinity transport was slowest on the natural grass plot, and it started only 45–50 min after artificial rain was applied.