Submitted to: Dust Aerosol, Loess Soils and Climate Change Meeting
Publication Type: Proceedings
Publication Acceptance Date: October 11, 1998
Publication Date: N/A
Interpretive Summary: Soil surface roughness is one of the most important factors determining the wind or water erodibility of a soil. The chain technique, whereby the foreshortening of a roller chain laid on the surface measures its roughness, is a recent and highly practical tool for helping farmers and conservationists estimate soil erosion risks on the farm. A leading soil erosion scientist has questioned use of the chain method because of the notion that a fine chain would give the same measurement for a surface with many small roughness elements (clods, aggregates, etc.) as for one with a smaller number of larger roughness elements. A computer simulation study conducted by the senior author of the current paper showed that use of a set of roller chains, where each chain had a progressively greater linkage length (i.e., 2, 4, 8 units, for example), would completely overcome this problem and give better information than a single chain. This paper reviews the simulation study and gives results of research using an actual set chain of roller chains with linkage lengths from 0.5 to 30 cm (0.2 to 12 inch). Measurements made on soils covered with clods and aggregates confirmed the findings of the simulation study. Comparisons of chain set measurements with those of a laser scanner (a device that measures fine details of soil topography) showed that chain measurements would correlate well with wind erosion indicators calculated from laser scanner data.
Technical Abstract: The chain method for measuring soil surface roughness is practical for wind and water erodibility assessment. Chain roughness is: CR = (1 - L2/L1)*100, where L2 is horizontal distance between ends of a roller chain and L1 is chain length. Concerns arise because a single chain of sufficient fineness could yield similar values for a soil with many uniformly small roughness elements (RE's) versus one with a smaller number of large RE's. Aerodynamic parameters are sensitive to larger RE's. Surface roughness was simulated by random sequences of hemispherical RE's. Calculated CR values for a chain with 0.89-cm links were 20% to 18% for sequences of uniform RE's with 45 to 2.0 cm diameters. CR values were 19% to 3% for a 22.4-cm linked chain on same simulated surfaces, with intermediate results obtained for 2, 5, and 10-cm link lengths. Regressions of CR values (Y) vs. log of chain link length (X) showed decreasing slope with increasing RE size variance (greater "fractal character"). Measurements with an actual 7 member chain set with 0.5 to 30 cm link lengths on randomly rough soil surfaces confirmed results of simulation study: the aerodynamically dominant (larger) RE sizes could be distinguished and soils with differences in fractal character produced distinct chain set results. Comparisons of chain set and laser microtopographic scanner measurements indicated that chain set readings correlated significantly with a wind erosion-relevant index, shelter angle distribution.