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Preferential flow

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The picture shows the infiltration pattern of a stained tracer solution, which infiltrated into a layered sandy soil at different water contents.

On the left hand side of the profile the upper layer was initially very dry and slightly hydrophobic. Therefore the tracer penetrated only through a few local paths and reached the second layer quite fast.

On the right hand side, the upper layer was irrigated before the application of the tracer. The soil matrix lost its hydrophobicity, leading to a more homogeneous infiltration of the staining tracer solution.

Under natural conditions soil formation almost always produces quite a heterogeneous medium. Layers of differing texture and skeleton content, macropores, fissures and wide cracks, as well as zones of differing water contents or even hydrophobic regions, and last but not least a network of roots and burrows make up such heterogeneities. Therefore, water transport in field soils is rarely ever as uniform as in a sand column in the laboratory. Even in sand columns local mixing is heterogeneous. It is all a matter of scale. In most of the cases rain (or melt water) infiltrates through preferential flow paths. Often but not always it is the macropores or cracks that form the main conducts of preferential flow. But textural boundaries – especially when inclined – differently wetted zones in a hydrophobic soil, or differently rooted areas also exhibit very pronounced preferential flow even in absence of an obvious macroporosity. Depending on the infiltration conditions at the soil surface of apparently homogeneous soils preferential flow occurs Due to microrelief preferential ports of entry form even in small depressions. Such preferential ports of entry also occur at layer boundaries, e.g. on plough pans.

Preferential flow leads to bypassing of large parts of the soil matrix and therefore to a much faster displacement of chemical substances towards the groundwater than one would expect considering the soil as a homogeneous medium. Thus, it is very important to include the phenomenon of preferential flow for predicting the leaching of fertilizer, pesticides, and other auxiliary or toxic compounds in field soils.

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