Surface initial characteristics and rainfall influence on fractal dimension of soil microrelief

[Abstract] The complex behavior of soil surface microrelief and its evolution is difficult to quantify. Several empirical roughness indices and geostatistical procedures currently used have been found to be sensitive to describe roughness differences between surfaces and changes within a surface due to rainfall. Such indices are simple, quantitative and synthetic descriptors of the complex soil surface organization, thought spatial indices provide some information about the association of main elements (biggest clods and aggregates) determining microrelief variations. Fractals are mathematical objects that show the same structure when examined at all possibles scales. Fractal dimension, the basic parameter characterizing a fractal object, is a potential index to quantify soil micorelief characteristics and changes induced by rainfall energy. A high resolution non-contact laser profile meter was used to measure microtopography on two artificial soil surfaces, before and after simulated rainfall. The experimental surfaces were reconstructed from aggregates of a plough layer and were thought to simulate two types of natural seedbeds, rough and fine. The calculation of the fractal dimension was performed through a variational method, by a numerical algorithm based on the roughness around the local root mean square (RMS), which develops a straight line roughnes (SLR). Plotting SLR values versus distance along a profile in a log-log scale results in a straight trend line over a limited range of distance, the slope of which is designed as Hurst exponent, related to the fractal dimension. Thus, the spatial organisation of the soil surface can be considered as a fractal structure over a finite range of scales. Mean values of the surface fractal dimension were 2.51 for the rough surface and the 2.72 for the fine one. The slow decrease of microrelief caused by surface sealing under rainfall was also described by the fractal index. This study showed that fractal analysis provides a relevant quantification of seedbed type and an assessment of microrelief changes in relation to rainfall amount