Principles of Water Flow in Real-World Soils and Related Imbalances

Heterogeneities, structures, interfaces, roughness, and organisms in multiphase soil systems make the real-world soil deviated significantly from the continuum assumption. The first principle of water flow in porous media 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 was proposed by L.A. Richards in the 20th century, which describes the minimum pressure gradient needed to initiate water flow through the soil-air interface. This principle can be extended 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 view portrays mosaic-like complex flow regimes in heterogeneous soils in which imbalance seems to be common leading to widespread preferential flow. The dynamic interaction between preferential flow and matrix flow under changing conditions results in complex, evolving flow networks that are embedded in the matrix of land surface and subsurface. Quantification and integration of these flow principles can lead to improved prediction of water flow in real-world soils and landscapes.