COMPUTED-TOMOGRAPHY DATA ON SOIL STRUCTURAL AND HYDRAULIC PARAMETERS ASSESSED FOR SPATIAL CONTINUITY BY SEMIVARIANCE GEOSTATISTICS

Visual observations on the spatial distribution, at 1-cm intervals, of bulk density (rho), porosity (epsilon), fractal dimension (D), water content (theta), and unsaturated hydraulic conductivity (K-us) in unif ormly packed soil columns showed randomness. We explore the use of sem ivariance geostatistics to clarify the issue of randomness and continu ity on the spatial distribution of rho, epsilon, D, theta, and K-us da ta obtained using a custom-built gamma scanner and computed tomography technique. Semivariance increased with increasing lag distance and pl ots of semivariance v. lag distance produced spherical semivariograms for most of the soil parameters investigated. This indicated that even though randomness existed in the spatial distribution of the soil par ameters, there existed specific trends in their spatial continuity. Hi gher spatial continuity, in water stable aggregates, was characterised by smaller values of semi-, sill-, and nugget-variances and larger va lues of span. Opposite trends were observed for unstable aggregates. W etting in unstable aggregates produced further reductions in span incr eases for other geostatistical parameters, indicating that wetting dec reased spatial continuity. The results indicate that geostatistical an alysis is useful to clarify the issue of randomness at very small scal es and to quantify and discriminate the influence of differences in st ructural stability and wetting-induced changes in the spatial continui ty of soil parameters, particularly epsilon.