Non-equilibrium water flow characterized by means of upward infiltration experiments

Upward infiltration experiments under tension were used to demonstrate the presence of non‐equilibrium flow in soils, the phenomenon that has important implications for the accelerated movement of fertilizers, pesticides, non‐aqueous liquids, and other pollutants. Data obtained from these experiments were analysed using the single‐porosity Richards equation, as well as a variably saturated, dual‐porosity model and a dual‐permeability model for characterizing non‐equilibrium water flow. The laboratory experiments were carried out on 0.10‐m‐long soil cores having an internal diameter of 0.10 m. Constant pressure heads of ?0.10 and ?0.01 m were used as the lower boundary condition. Each infiltration was followed by a single‐rate evaporation experiment to re‐establish initial conditions, and to obtain the drying soil hydraulic properties. Pressure heads inside the cores were measured using five tensiometers, while evaporative water loss from the top was determined by weighing the soil samples. The data were analysed to estimate parameters using a technique that combined a numerical solution of the governing flow equation (as implemented in a modified version of the Hydrus‐1D software) with a Marquardt–Levenberg optimization. The objective function for the parameter estimation was defined in terms of pressure head readings, the cumulative infiltration rate, and the final total water volume in the core during upward infiltration. The final total water volume was used, as well as the pressure head readings during the evaporation part. Analysis of flow responses obtained during the infiltration experiment demonstrated significant non‐equilibrium flow. This behaviour could be well characterized using a model of physical non‐equilibrium that divides the medium into inter‐ and intra‐aggregate pores with first‐order transfer of water between the two systems. The analysis also demonstrated the importance of hysteresis.