Uniqueness of Soil Hydraulic Parameters Determined by a Combined Wooding Inverse Approach

Knowledge of soil hydraulic properties is essential for a proper understanding and evaluation of physical and chemical processes within the vadose zone involved in variably saturated water flow and transport of water-dissolved salts and pollutants. Soil hydraulic properties are often expressed using functional relationships between the soil hydraulic conductivity (K), water content (θ), and matric potential Ψ). Our objectives were (i) to combine the Wooding's analytical solution for steady-state infiltration from a circular pond and the inverse determination of parameters from transient infiltration events into a coupled method for the in situ estimation of soil hydraulic properties, and (ii) to develop a simple semiautomatic device for the in situ estimation of the soil hydraulic functions. The experimental method consists of measurements of transient (short-term) and steady-state (long-term) infiltration flow rates from a set of rings having different radii, each positioned sequentially at the same location on the soil surface. A shallow water depth is maintained within a 1- to 2-mm range over the soil surface with an electrode set. The flow rate is determined by continuous weighing of a water reservoir. The flow is monitored and controlled by a laptop computer, which also automatically calculates the soil hydraulic properties from collected data. The coupled method starts with the application of the Wooding's analytical solution to obtain estimates of the soil hydraulic properties using steady-state fluxes. These estimates are then finalized using numerical inversion of the transient data. The coupled method was evaluated using numerically generated data. Unique and fast reproduction of soil hydraulic properties for generated data was obtained. The method's applicability was also tested using field experiments for two soils. The technique was found to be sound and the device simple to operate.