Simplified Multistep Outflow Method to Estimate Unsaturated Hydraulic Functions for Coarse-Textured Soils

Although the multistep outfl ow (MSO) method is well suited for the estimation of soil hydraulic properties by inverse solution techniques, this method has not been widely adopted because it requires advanced instrumentation and is time consuming. Th e objective of this study was to develop a modifi ed version of the multistep outfl ow technique that largely simplifi es laboratory procedures and reduces costs and time. Th e numerical inversion procedures require applying user-friendly HYDRUS soft ware to estimate fi tting parameters for soil water retention and unsaturated hydraulic conductivity curves. Whereas values of saturated water content and saturated hydraulic conductivity must be measured independently, the remaining functional parameters are estimated using an inverse solution of a transient drainage experiment using multiple suction steps and a hanging water column, with drainage outfl ows measured during drainage. A comparison test showed that the simplifi ed experiment without tensiometric measurements provided suffi cient information in the parameter identifi cation compared with a traditional pressure outfl ow experiment with tensiometric measurements for an Oso Flaco sand and a loamy sand fi eld soil in the suction range of 0 to 17 kPa

Ground-penetrating radar reflection data sensitivity to van Genuchten variations.

International audienc

Ground Penetrating Radar: Water Table Detection Sensitivity to Soil Water Retention Properties

International audienceWe are interested in Ground Penetrating Radar (GPR) as a geophysical tool useful for determining the depth of the ground water table (GWT) and for monitoring shallow water infiltration in sandy soils. At hydrostatic equilibrium, the water content distribution in a homogeneous unsaturated soil down to the water saturated zone depends on the soil water retention function. A classical way to fit retention curve data is to use the van Genuchten continuous model. Using Finite Difference Time Domain simulations, we study the sensitivity of the GPR signal reflected by a van Genuchten type transition to the hydraulic parameters. We show a power type relationship between the reflected signal amplitude and the slope of the soil retention curve. Furthermore, for simulating GPR reflection data acquired above a transition from unsaturated to saturated soil, geophysicists often approximate the soil water retention curve by a piece-wise linear model. We test the validity of such an approximation depending on the frequency of the radar signal and the abruptness of the retention curve. We illustrate our results with high resolution GPR data (1600 MHz) acquired above a fluctuating water table in a sand column at the laboratory scale