NUMERICAL MODELING OF UNSATURATED FLOW TO PROVIDE GUIDELINES FOR HYDRAULIC CONDUCTIVITY ESTIMATION

Recent research has shown that Ground Penetrating Radar (GPR) techniques can be used to estimate the hydraulic conductivity of soil in the shallow subsurface at the field scale. GPR data can be acquired quickly and with high resolution, so this technique could offer substantial improvements in understanding variability in near-surface hydraulic conductivity. However, to apply GPR techniques for hydraulic conductivity estimation, data must be acquired before and after a wetting event such as irrigation, and to meet the mathematical requirements of estimation, steady-state vertical flow conditions must be established in the soil during the wetting event. This project seeks to define the soil and wetting characteristics needed to establish steady-state vertical flow over a defined depth so that GPR techniques could be used to estimate hydraulic conductivity.

The establishment of steady-state vertical flow depends upon soil conditions such as initial soil moisture and unsaturated hydraulic conductivity, as well as wetting parameters such as the rate and duration of infiltration. To quantify the impacts of each of these factors, the unsaturated zone modeling program Hydrus-1D was used to model infiltration for different soil textures and wetting conditions. Unsaturated hydraulic conductivity and water content retention curve parameters were estimated based upon soil texture, and different rates and durations of infiltration were applied to simulate typical irrigation or rainfall events. Using the results of these simulations, the combination of soil and wetting characteristics that would result in steady-state flow conditions over a pre-defined depth interval were determined. These data can be used to determine when GPR techniques might be applicable for hydraulic conductivity estimation by indicating what irrigation rates are appropriate to establish steady-state flow or whether a natural precipitation event was suitable to establish these conditions; these data can also suggest what depth range is appropriate for hydraulic conductivity estimation.