Preferential flow in soil may occur by one or more of three possible mechanisms: macropore flow, finger flow or wetting front instability, and sub-surface layering and textural interfaces (funnel flow). These are described briefly below, along with the soil textures which they typically occur.

Macropore flow in structured soils

Macropores flow is the result of a variety of soil forming factors such as flow through non-capillary cracks or channels within a profile.

Macropores and sub-surface channels result from either biological activity (eg., root channels, worm holes, etc), geological forces (eg., subsurface erosion, desiccation and synaerisis cracks and fractures) or agrotechnical practices (e.g., plowing, bores and wells). Surface cracks and channels that bypass the root zone are also responsible for rapid transport of moisture and chemicals through the unsaturated zone.

Highly conductive macropore pathways form and persist in structured soils. These macropores are the result of a variety of soil forming factors such as flow through non-capillary cracks or channels within a profile. In clay and loam soils, for example, areas of relatively low permeability are riddled with channels consisting of cracks partially filled with sand and small stones, as well as passages formed by roots and earthworms. When it rains, water infiltrating the ground follows these channels in preference to the surrounding matrix, whose small pores are penetrated comparatively slowly (Figure 1). Flow is often fairly uniform through the uppermost soil horizon (typically the plow layer for current or former agricultural soils), which functions as a "distribution zone", distributing the flow to macropores in the subsoil. Different channels sometimes cross each other, with a resultant mixing of water and solutes.

Figure 1a. Macropore flowpaths in a structured soil near Ithaca, NY, indicated by blue dye. Note more uniform flow through the plow layer which acts as the distribution zone, followed by differentiation into macropores in the subsoil.

Figure 1b. The dye, a nontoxic food coloring, infiltrated the soil from the surface (simply move your mouse over the picture to see the preferential flow paths) .

Figure 1c. Earth worm, the "pore generator".

Figure 2 illustrates an example of a 2 cm wide and 40 cm deep surface channel. Many field studies have shown preferential flow to be a wide spread phenomena. Water and chemicals that travel in macropores often bypass the bulk of the soil matrix. For example, a single, continuous 0.3 mm diameter macropore can conduct more water than surrounding soil of 100 mm diameter . Flow in macropores and channels can occur with little or no interaction with the surrounding soil-matrix. On the other hand, matrix flow is the comparatively slower movement of water and solutes percolating though soil pores. The relative importance of the two forms of percolation; preferential and matrix flow is dependent on the soil type and rainfall intensity. For example, well-structured soils consisting of clay and loam mixes typically experience low permeability rates. In such soils, less than 1% of the pore-volume consists of cracks and subsurface channels. However, during rain events, water infiltrating from the soil surface, often flows through these channels in reference to the surrounding soil-matrix, whose small pores are penetrated comparatively slowly. Even though these channels consist of a relatively small percentage of the total pore volume, they may be responsible for the bulk of moisture and solute transport after an infiltration event. Preferential flow may be initiated well below soil-water saturation.

Figure 2. A surface crack (2 cm wide, 1 m long, 40 cm average depth). Surface channels may act as direct conduits transporting surface-applied chemicals to groundwater, bypassing the soil matrix. Photograph taken on the Warrnambool campus of Deakin University, 1995.

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