A simple treatment of physical nonequilibrium water flow in soils

Water now in seas with sufficiently large aggregate size or pore-class hete rogeneity 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. M odels of this process usually divide the soil into two or more exchanging f low regions. A simpler treatment is possible by combining Richards' equatio n with a dynamic description of the approach to equilibrium. We present a f irst-order time constant equilibration model of infiltration into a hypothe tical structured soil and use the model to describe published outflow respo nses to constant rate rainfall on six large, undisturbed cores. Using measu red hydraulic properties and varying only the A and B horizon time constant s, 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, respectivel y, and this also gave good agreement with TDR measured water contents. Cumu lative outflow and runoff from a further four of the six cores was describe d using the same A and B horizon time constants and varying only the macrop ore hydraulic conductivity. The remaining core contained a decayed root, wh ich conducted water rapidly with little opportunity for lateral exchange. A description of cumulative outflow required both the macropore hydraulic co nductivity and the time constant to be altered.