A Simple Treatment of Physical Nonequilibrium Water Flow in Soils

Abstract

Water flow in soils with sufficiently large aggregate size or pore‐class heterogeneity may exhibit a nonequilibrium between the actual water content and that given by the water retention curve. The result is deeper penetration of infiltrating water than predicted using classical infiltration theory. Models of this process usually divide the soil into two or more exchanging flow regions. A simpler treatment is possible by combining Richards' equation with a dynamic description of the approach to equilibrium. We present a first‐order time constant equilibration model of infiltration into a hypothetical structured soil and use the model to describe published outflow responses to constant rate rainfall on six large, undisturbed cores. Using measured hydraulic properties and varying only the A and B horizon time constants, the model was fitted to cumulative outflow from one particular soil core that had measured time domain reflectometry (TDR) water contents at the 0.05‐ and 0.5‐m depths recorded during the experiment. Cumulative outflow was fitted using time constants of 4 and 5 h for A and B horizons, respectively, and this also gave good agreement with TDR measured water contents. Cumulative outflow and runoff from a further four of the six cores was described using the same A and B horizon time constants and varying only the macropore hydraulic conductivity. The remaining core contained a decayed root, which conducted water rapidly with little opportunity for lateral exchange. A description of cumulative outflow required both the macropore hydraulic conductivity and the time constant to be altered.

Modeling dynamic non-equilibrium water flow observations under various boundary conditions

“…Water flow for the equilibrium fraction is described by the RE (1) and water flow for the non-equilibrium fraction is modeled using the approach of Ross and Smettem (2000). If we assume that the equilibration of the pressure heads between the two regions is instantaneous (at least relative to the movement of water in the main flow direction), the main equation describing water flow becomes :…”

A review of model applications for structured soils: a) Water flow and tracer transport

“…A one-domain PNE concept of infiltration (Fig. 1b) was realized by assuming a rate-limited equilibration between the soil water content and the pressure head in the Richards equation (Ross and Smettem, 2000).…”

“…The uniform flow model failed to describe this behaviour. By contrast, the PF model could describe both cumulative outflow and water contents for five out of six other soil cores, when only the macropore hydraulic conductivity was individually calibrated (Ross and Smettem, 2000). Šimůnek et al (2001) implemented the MarquardtLevenberg parameter optimization in HYDRUS-1D for inverse DPM, MIM and SPM simulations of upward infiltration experiments performed on undisturbed sandy loam soil cores (0.10 m height and diam.…”

Dual‐permeability model for flow in shrinking soil with dominant horizontal deformation

“…Relatively small interactions are assumed in twodomain mobile-immobile flow models [e.g., Zurmühl and Durner, 1996;Ross and Smettem, 2000] assuming mobileimmobile pore domains for flow that allow for local nonequilibrium or kinetic nonequilibrium in water contents.…”