STATISTICAL AND FRACTAL EVALUATION OF THE SPATIAL CHARACTERISTICS OF SOIL SURFACE STRENGTH

In order to understand processes causing soil crusting and to optimize sampling schemes, improved understanding of crust spatial characteris tics is needed. For this purpose, statistical and fractal analyses of soil surface strength, as measured by a 1.59-mm-diam. flat-tipped micr openetrometer, were carried out for a variety of soils and under a var iety of sampling schemes. Specifically, this work considered maximum p enetration forces sampled at intervals of 0.005, 0.01, 0.05 and 0.5 m along four parallel transects 0.25 m apart, for nine different sites w ithin California's Central Valley. Force measurements were non-homogen eous in space. For eight of the nine sites, both the mean and the vari ance were non constant; and for all sites, the range and nugget varian ce of fitted semivariograms were scale dependent. Fractal analysis of the measurements allowed qualitative discrimination among soils, despi te small variations across scales. For all sites, fractal dimensions o f sampled series at alternative scales were similar, but the smallest deviations across scales were observed on high-strength silt loam, and the largest on low-strength loamy sand soils. Multifractal spectra (f or data sets normalized and interpreted as probability measures) gave similar entropy dimensions for all sampling schemes, with very small d eviations across scales for silt loam soils and largest again for loam y sands. Results suggest that the fractal attributes (fractal and entr opy dimensions), being scale-independent attributes, may be relevant q ualifiers of the intrinsic variability of penetration measurements and that this variability can be modeled as a fractal process.