Water flow in natural soils is fundamental to terrestrial hydrology, biogeochemical cycles, ecosystem services, climate change, land use, contaminant transport, natural hazard mitigations, and many other applications. However, our ability to realistically model and predict water flow in diverse real-world soils remains quite limited. This paper synthesizes three principles of water flow in natural soils and suggests a mosaic theory that explains evolving flow networks embedded in the land surface and subsurface. The first principle came to light in the 19th century, known as the Darcy’s law, which was later modified by
E. Buckingham to describe unsaturated water flow in soils. This principle is essentially a macroscopic view of steady-state water flux being linearly proportional to hydraulic gradient and hydraulic conductivity. The second principle, proposed by L.A. Richards in the 20th century, describes the minimum pressure gradient needed to initiate water flow through the soil-air interface. This principle is extended here to provide a more cohesive explanation to a number of soil hydrologic phenomena related to various interfaces and microscopic features (such as hysteresis, hydrophobicity, and flow through layered soils). The third principle is emerging in the 21st century, where a combined macroscopic and microscopic consideration portrays mosaic-like complex flow regimes in heterogeneous soils. This principle is summa-
rized as: Water seldom moves uniformly in natural soils but always displays a dual-flow regime, i.e., it follows the least-resistant or preferred paths when “pushed” (e.g., by storms) or “attracted” (e.g., by plants), and moves diffusively into the matrix when “relaxed” (e.g., at rest) or “touched” (e.g., adsorption). The dynamic interaction between preferential flow and matrix flow domains under changing conditions results in complex and evolving flow networks that are embedded in the matrix of land surface and subsurface. This leads to a mosaic theory that can be further tested and refined across scales and geographic regions
from a growing number of observatories in the world. The implications of these principles and the proposed theory are discussed, including flow measurements and modeling. Further quantification and integration of these flow principles and the mosaic theory in natural soils and landscapes can lead to improved prediction and enhanced management of soil and water resources in the real-world.
Lin, H. (2012): Three principles and a mosaic theory of water flow in real-world soils. 2nd International Conference on Hydropedology, Leipzig, Germany, July 22-27, 2012..
This Paper/Book acknowledges NSF CZO grant support.