Universality of Preferential Flow in Field Soils: Some Theoretical Perspectives.

Preferential flow (PF) is a fundamentally important soil hydrologic process that controls a variety of soil physical, chemical, and biological processes.  However, existing conceptual and technological bottlenecks continue to hinder the modeling and prediction of PF in the field.  Based on three connected theories and extensive published experimental evidence, this paper attempts to justify the universality of PF in natural soils -- meaning that PF can occur in any soil anywhere in nature.  First, we examine non-equilibrium thermodynamics as applied to the open dissipative system of field soils with continuous energy inputs.  This provides a theoretical foundation for explaining the genesis and evolution of ubiquitous structured heterogeneity in all soils that leads to widespread potential for PF occurrence.  A dual-partitioning of pedogenesis results in ΔS_soil = ΔS_matrix + ΔS_structure, where ΔS_matrix is the entropy change related to dissipative processes and soil matrix formation, while ΔS_structure is the entropy exchange with the surrounding that is associated with organizing processes and soil structure formation.  Second, we explore constructal theory to explain universal dual-flow regimes in natural soils -- one with high resistivity (Darcy flow) and the other with low resistivity (PF) -- together, they form natural PF configuration that provides the least global resistance to flow.  While limited chronologic data suggest reduction in subsoil saturated hydraulic conductivity as soil ages, constructal theory appears to partially explain some general characteristics of weathering process.  Third, the theory of evolving networks sheds light on diverse flow networks in soils that increase the efficiency or effectiveness of matter or energy transfer in the subsurface, since networks are part of the organization resulting from minimum energy dissipation and far-from-equilibrium thermodynamics.  Percolation theory is one of the common approaches used for modeling flow networks that can be linked to the dynamics of hydrologic connectivity and threshold behavior in the subsurface.  All the three theories discussed support the notion that PF is universal in natural soils.