Spatial variability of the hydraulic properties of a volcanic soil.

The spatial variability of soil properties in a typical volcanic Vesuvian soil was investigated by collecting undisturbed soil samples at equal intervals of 1.5 m along a 135-m transect at a depth of 0.45 m. In this study, the spatial structure of soil hydraulic properties was examined using both statistical and geostatistical concepts. Soil hydraulic properties were determined in the laboratory from an evaporation experiment by a parameter estimation method developed in view of reducing experimental efforts involved in such studies without sacrificing the accuracy of the estimated soil parameters. The laboratory experiment was applied to a 0.10-m-length soil sample with simultaneous measurements of sample weight and matric potentials at only two soil depths during the transient. Van Genuchten's water retention relation and an exponential expression relating unsaturated hydraulic conductivity to water content were assumed to describe soil hydraulic properties. By measuring independently the saturated water content and setting the value of the residual water content equal to zero, unknown parameters in the constitutive closed-form hydraulic relations were estimated by means of a non-linear ordinary least-squares procedure which minimizes the deviations between the numerical solution of the one-dimensional transient evaporation flow process and the real system response measured during the experiment. The method also provides information concerning parameter uncertainty. In addition, particle-size distribution, dry bulk density, and saturated hydraulic conductivity were measured for each sample. The large number of data made reasonable comparisons possible between measured and optimized hydraulic conductivity at saturation. It was found that on average the optimization procedure tends to underestimate the measured value of saturated hydraulic conductivity. The results further indicate that observations are correlated over space with a range of about 6 m, and reveal the presence of periodic phenomena. It is suggested that employing cost-effective and accurate methods to determine soil hydraulic properties and their spatial variability may be extremely important to detect correlations between adjacent measurements, especially in a fairly homogeneous soil like that considered here