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.

Overland flow; Rainfall intensity; Soil characteristics; Solute transport; Steep slope area

Solute transport from soil through overland flow is a major source of nonpoint pollutants in receiving surface water bodies. Salinity is the main problem that occurs in almost all irrigated areas and even in some non-irrigated areas such as grasslands and open fields (Szabolcs, 2011). Studies found that up to 20 million hectares of soil in Southeast Asia is affected by salinity. Salinity problems in tropical countries cause soil characteristics to deteriorate, increase soil erodibility, and inhibit plant growth (Kovda & Szabolcs, 1979; Szabolcs, 2011). The Food and Agriculture Organization (FAO) and the United Nations Educational, Scientific, and Cultural Organization (UNESCO) estimate that many existing irrigation systems worldwide are affected by secondary salinization, alkalization, and waterlogging. These phenomena are seen in both old and newly irrigated areas. Salinization is a global environmental phenomenon that significantly decreases the quality of water resources; nonetheless, it has not yet attracted much global attention (Gibbs et al., 2011).

The movement of overland flow directly affects the soil salinity. Xu and Shao (2002) stated that salinization is closely related to hydrological processes that occur on the soil surface and in ground water, because the movement of water is a very important factor in the transfer of salt as a conservative material. In steep-slope areas, the theory of overland flow is based on differentiation between overland flow generation from excessive infiltration and excessive saturation and subsurface flow (Ameli et al., 2015). Overland flow is harder to define precisely than in-channel flow, and the use of hydraulic procedures in predicting overland flow and its characterization has limitations (Kirkby, 1978). It is unstable and varies spatially as it arises from rainwater and decreases due to infiltration; these two processes are not constant over time and space. Studies thus far have mainly focused on a basis for quantifying the transformation from rainfall to overland flow.

Alaoui et al. (2011) indicated that hillslopes with higher clay content produced higher overland flow volumes than hillslopes in forest areas. Land cover also influences overland flow formation in steep slope areas, where soil with less dense vegetation tended to show a higher overland flow formation rate compared to soil with denser natural vegetation cover (Qing-Xue et al., 2013). Zhao et al. (2015) found that rainfall intensity, slope, and land cover types can affect soil moisture dynamics and lead to overland flow formation. A detailed study of the overland flow formation mechanism in steep slope areas is required to establish a runoff model that researchers can use to determine appropriate management practices to reduce the amount of discharge runoff and the relocation of solutes via surface runoff, especially from steep slopes to water bodies.

Therefore, field research was conducted in a steep-slope catchment having uniform slope of 46.7%. In order to define the runoff generation mechanism based on the distribution of conservative matter, results of several scenarios on rain intensity, soil characteristics, and vegetation density were evaluated.

The proposed field experiment design, although difficult to perform, provides a general structure for analyzing runoff flow and a better understanding of the main flow mechanism in steep-slope areas. In clay soil in a steep slope area, surface or subsurface flows were not observed below depths of 0–30 cm in soil even under high artificial rainfall intensity. This indicates that surface and subsurface flows at depths of 0–30 cm were not the dominant mechanism in soil with high water-holding capacity. Rainwater can be held in soil or can infiltrate the deeper soil layer. Land cover might be a more important factor than rainfall intensity in clay soil for runoff flow generation. This study proposes a suitable computational model to estimate runoff generation and salinity concentration based on moisture dynamics if runoff cannot be observed in the discharge. The output of the flow generation model for each plot indicates that overland flow generation on a steep slope plot might be dominated by saturation excess, whereas a Natural grass plot shows the lowest overland flow generation percentage compared to Removed grass and Without grass plots. For higher rainfall height of 24.9 mm with salt addition, the salinity concentration in the WG plot was the highest at 124.43 ppm compared to 56.04 and 33.05 ppm for the Natural grass and Removed grass plots, respectively. Furthermore, salinity was detected only after 45 and 50 min of low and high rainfall, respectively, in the NG plot.

The authors are grateful to the Bandung Institute of Technology and wish to acknowledge the Financial Support from AUN-Seed Net Scholarship 2016. We would like to thank everyone else who has supported this research.

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