Instability in the wetting front leads water to find its way down through coarse soils in a number of channels called fingers.

In contrast to macropore flow, unstable wetting front or finger flow occurs in unstructured coarse soils.This fingers phenomenon also occurs in homogeneous soils. In 1972, David Hill and Parlange documented, for the first time, preferential flow in homogeneous sand at low infiltration rates. The main experimental apparatus is a column that consists of a one-centimeter-thick layer of sand between two glass plates. A strong, uniform light is located at the back of the column, and water is allowed to infiltrate the sand between the plates. Since the amount of light transmitted through the sand increases with wetness, quantitative observations of the low pattern are possible. The image is picked up by a video camera and computer software assigns different colors to different intensities of light, making differences in water content immediately apparent (Figure 1).

Figure 1. The way water forms fingers in homogeneous sandy soils. These images are produced by passing light through a "sand sandwich" and converting the different intensities to different colors by a computer program. The black represents regions of low moisture content and red represents soil-water saturation. The range of colors between black and red represent degree of moisture saturation.

Other experiments make use of a chamber that allows observation in three dimensions (Figure 2). Since the light-intensity method cannot be employed in this situation, flow paths are marked by water containing a blue dye. After the water has had a chance to penetrate the soil, the sample is frozen, and when the loose dry sand has fallen out, the congealed flow paths can be examined.

Figure 2. A three-dimensional study. Frozen fingers of blue-dyed water, exposed by removing dry sand.

The above research has shown that water finds its way down through sandy soils in a number of channels that called fingers. A poorly conducting layer of topsoil at the surface produces a wetting-front instability. Gravity drives the instability and surface tension has a contrary, stabilizing effect. What happens is analogous to the dripping of water from a sponge; the pull of gravity is opposed by surface tension, which makes the drops increase in size before they fall. In the case of soils, this balance of forces determines the diameter of the fingers.

Some fingers do not carry enough water to keep growing, and the number of fingers diminishes with increasing depth. In sand that is quite homogeneous, the fingers are nearly vertical and do not merge with one another. If the sand is less homogeneous, however, the fingers deviate from a strictly vertical path, and can merge (Figure 3). When this happens, they do not come together on equal terms, like the arms of a Y. Instead, one finger continues on it course, while the other donates water as a tributary entering from the side. This results from a difference in the water content of the fingers, with capillary action pulling water form the wetter of the two and delivering it to the dryer one, which stays on course.

Figure 3. Fingers in layered sand, shows several instances of fingers merging, as well as a case of splitting.

After a merger, the continuing finger carries considerably more water than either of the contributors, as color-coded images such as Figure 3 clearly shows. Since the continuing finger is only slightly larger in diameter, the extra water increases its conductivity and, in accordance with the conservation of mass, the speed with which it grows.

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