Soil morphology and preferential flow along macropores

Abstract Preferential flow of water along macropores can only be characterized in quantitative terms when the flow system is functionally characterized by using tracers. Standard descriptions of soil structure do not provide adequate information. Expensive micromorphometric techniques allow specific measurements of macropores in terms of size, type, shape and continuity. In expensive macromorphometric techniques are attractive for field use. Preferential flow in saturated soil involves rapid displacement of water from macropores (hydrodynamic dispersion). In unsaturated soil, flow into air-filled macropores (short-circuiting) occurs, which is followed by lateral absorption. Examples are discussed which illustrate the use of soil morphology to characterize preferential flow along macropores: (i) empirical extrapolation of measured data on the basis of macrostructure descriptions. So far, this procedure could only be applied to breakthrough curves of soils with identical textures but very different macrostructures; (ii) using staining techniques which provide essential boundary conditions for newly developed physical flow models. The latter describe macropores in terms of size, type and shape, rather than in terms of relative volumes; and (iii) developing physical interpretations of pedological features, such as mottling patterns. The examples are based on four case studies which deal with practical problems of soil water management. In these studies, morphological methods provided essential data, which could not have been obtained by physical methods.

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