Characterizing soil preferential flow using iodine–starch staining experiments and the active region model

Abstract

Thirteen iodine-starch staining experiments with different boundary conditions and measurement scales were conducted at two sites to study preferential flow processes in natural unsaturated soils. Digital imaging analyses were implemented to obtain the corresponding preferential flow patterns. The test results are used to evaluate a recently proposed active region model in terms of its usefulness and robustness for characterizing unsaturated flow processes at field scale.Test results provide useful insights into flow patterns in unsaturated soils. They show that flow pattern depends on the top boundary condition. As the total infiltrating-water depth increased form 20 mm to 80 mm for the 100×100 cm 2 plots, the corresponding flow pattern changed from few preferential flow paths associated with a relatively small degree of stained coverage and a small infiltration depth, to a pattern characterized by a higher stained coverage and a larger infiltration depth, and to Although further studies are needed to confirm this finding, it seems to be consistent with a consideration that some fractal parameters (e.g., fractal dimension)are universal for a large range of scales.

Modeling preferential water flow and solute transport in unsaturated soil using the active region model

“…However, the mobile region in the dualdomain model is fixed in the flow process, whereas the active region in the ARM is fractal and changeable, and the relative portion of the active region is expressed as a function of the soil water saturation in the flow region. The treatment of flow region in the ARM is consistent with the field observations (Larsson et al 1999;Liu et al 2005;van Dam et al 1996;Sheng et al 2009). While the previous studies on the ARM mainly focus on its constitutive relationship (Liu et al 2005;Sheng et al 2009), in this study, derivation and evaluation of ARM governing equations are presented for preferential water flow and solute transport in the unsaturated soil.…”

“…The ARM is an extension of the active fracture model (AFM) developed for modeling unsaturated flow and transport in the fractured rock (Liu et al 1998(Liu et al , 2003. Sheng et al (2009) demonstrate that the ARM is able to capture the major features of the observed flow patterns under different infiltration conditions, and revealed that ARM parameter (  ) used for describing the flow heterogeneity may be scale independent within their experiment scales.…”

“…(1) or (4). The parameter  is a positive value between zero and one and increases with the flow heterogeneity (Liu et al 2005;Sheng et al 2009). According to Liu et al (2005), the ARM parameter can be expressed as a function of the fractal dimension of the flow region as follows:…”

“…By using the horizontally averaged soil water content (Liu et al 2005;Sheng et al 2009), it is assumed that the soil water and solute move in the z-direction only (Fig. 1).…”

“…As the climate was arid and the soil was almost pure sand, it was assumed that the initial soil water contents within the stained and unstained regions were not significantly different after long-time soil water redistribution (Sheng et al 2009). In addition, the upper 30 cm soil layer was tilled before experiment, which uniformed the initial soil water contents between the stained and unstained regions and changed the preferential flow paths between the upper and deeper soil layers.…”