An evaluation of a fragmentation fractal dimension technique to determine soil erodibility

The fragmentation fractal dimension, D-f, was used to characterise aggregate-size distribution, as an index of soil erodibility, of five soils with contrasting properties from semiarid and temperate areas. The D-f was the slope of the regression of aggregate number vs. log of aggregate diameter/diameter of the largest aggregates using scale-variant bulk density and assuming a constant shape for all the aggregates sizes. Six aggregate sizes, ranging from 0.25 to 16 mm, were obtained for each soil. Aggregate bulk density (ABD) ranged from 1.21 to 1.81 mg m(-3) for semiarid soils and from 0.83 to 1.55 mg m-3 for temperate soils. Significant correlations (from r = -0.72 to r = -0.98, p < 0.05) between ABD and size indicated scale-variance in ABD. D-f values ranged from 2.92 to 3.25 for semiarid and from 2.15 to 2.24 for temperate soils. Results from aggregate stability test (using a laboratory rainfall simulator) and soil loss rates (from erosion plots) for each soil were compared with D-f to assess the suitability of D-f as an erodibility index. Values of D-f equal to or less than 2.24 were associated with stable soil, with a predominant percentage of large aggregates (43% to 53% of aggregates 8-16 mm), and negligible soil loss rates. in contrast, D-f values greater than or equal to 2.84 were associated with a less stable soil, with a low percentage of large aggregates size (7% to 16% of aggregates 8-16 mm), and notable soil loss rates. The values of D-f in describing mass-size distribution were found to be a useful and effective index to discriminate soil erodibility in the range of soil included in this study. (C) 1999 Elsevier Science B.V. All rights reserved.