Preferential flow mechanisms identified from staining experiments in forested hillslopes

Abstract

Abstract:Field staining experiments in five different plots at two sites in Japan (Okaya in Nagano Prefecture and Konohara in Mie Prefecture) were undertaken to improve understanding of subsurface stormflow runoff within organic layers of natural forested hillslopes. This type of shallow lateral subsurface flow, specifically referred to as biomat flow, was observed only at the Okaya site based on staining experiments conducted under controlled water application rates. When the same irrigation rate (50-100 mm h À1 ) was applied to the Konohara site, overland flow without a significant shallow subsurface component was the dominant flow mechanism. Even in gently sloping (15-20°) forest soils at the Okaya site, biomat flow was responsible for lateral dye transport over much longer distances than subsurface flow in the matrix of mineral soil layers. Based on analysis of staining pattern images observed in the Okaya site, we conclude that (i) the organic biomat layer could be divided into two sub-layers of different structure, (ii) biomat flow transported the dye tracer longer distances than subsurface flow in the matrix and (iii) the biomat layer topography affected biomat flow by generating preferential flowpaths and subsequent percolation into the deeper soil. Based on field experimental results and pore-scale consideration of water infiltration into pores of soils with varying wettability properties, we hypothesized and developed a conceptual model for two key biomat flow mechanisms. The first mechanism considers lateral subsurface flow because of a permeability contrast between the much more porous and hence permeable biomat layer and the underlying mineral soil. The second mechanism involves a hydrophobic soil layer between the biomat and the underlying mineral soil. Flow across the hydrophobic layer is believed to occur when a threshold pore pressure is exceeded, i.e. as a result of a perched water table within the biomat layer. Lower pore pressures are needed to initiate flow when preferential flow paths exist, which are less hydrophobic than the surrounding organic layers. Numerical models of catchment hydrology should include lateral biomat flow when such layers are present in hillslope soils, in addition to typical subsurface flow within the soil matrix.

Phosphorus fractions in preferential flow pathways and soil matrix in hillslope soils in the Thuringian Forest (Central Germany)

“…This is confirmed by the (visual) observation of lateral translocations of tracer solution along the hillslope in the upper soil during excavation of the soil pits. The high importance of lateral subsurface flow in the O-layer and adjacent mineral soil, known as biomat flow (Sidle et al, 2007), was shown recently by Gerke et al (2015) for forested hillslopes in Japan. They divided two key flow mechanisms:…”

Spatial Variability of Preferential Flow and Infiltration Redistribution along a Rocky-Mountain Hillslope, Northern China

“…The preferential flow fraction, PF-fr, is the fraction of the total infiltration that flows through preferential flow paths [48]. The calculation method for the PF-fr was widely used in previous studies that only relied on the UniID as the boundary between the matrix flow area and the priority flow area [20,21,26,48,49]. However, the traditional calculation can lead to the inability to truly and completely distinguish the priority flow from the matrix flow.…”

Pore-scale permeability prediction for Newtonian and non-Newtonian fluids

“…CC BY 4.0 License. for laboratory measurements on pore scale are: (i) change of the sample's microstructure and therefore its physical properties through cracking and self-filtration (Zeinijahromi et al, 2016;Dikinya et al, 2008) (ii) pressure changes due to the influence of wall effects (Ferland et al, 1996) and finally (iii) difficulties to measure on irregular grain shapes and small grain sizes of the porous medium (Cui et al, 2009;Gerke et al, 2015).…”