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 importan t implications for the accelerated movement of fertilizers, pesticides, non -aqueous liquids, and other pollutants. Data obtained from these experiment s were analysed using the single-porosity Richards equation, as well as a v ariably saturated, dual-porosity model and a dual-permeability model for ch aracterizing non-equilibrium water flow. The laboratory experiments were ca rried 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 boundar y condition. Each infiltration was followed by a single-rate evaporation ex periment to re-establish initial conditions, and to obtain the drying soil hydraulic properties. Pressure heads inside the cores were measured using f ive 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 par ameter estimation was defined in terms of pressure head readings, the cumul ative infiltration rate, and the final total water volume in the core durin g upward infiltration. The final total water volume was used, as well as th e pressure head readings during the evaporation part. Analysis of flow resp onses obtained during the infiltration experiment demonstrated significant non-equilibrium flow. This behaviour could be well characterized using a mo del of physical non-equilibrium that divides the medium into inter- and int ra-aggregate pores with first-order transfer of water between the two syste ms. The analysis also demonstrated the importance of hysteresis.