Selection of a Parameter Describing Soil Surface Roughness

The microrelief of the soil surface, termed soil surface roughness, affects water movement into a soil profile as well as seedling germination in the seedbed. When analyzing surface roughness, the selection of a measurable, physically significant parameter describing roughness is critical. An evaluation was conducted on eight roughness parameters, including maximum peak height, a microrelief index (the area per unit transect length between the measured surface profile and the least-squares regression line through all measured positions of the transect), peak frequency, and MIF (the product of the microrelief index and peak frequency). The objective of the study was to select the parameter being the best descriptor of soil surface roughness. An automated, noncontact profiler was used to obtain surface profiles along transects, 5-cm apart, of 1-m by 1m plots after a cultivation and a simulated rainfall application at each of three different stages of soybean [Glychae max (L.)I development. For each cultivation, surface profiles were obtained on bare plots before rainfall and on adjacent vegetated plots after rainfall. The common logarithm of the MIF parameter was selected as the best descriptor of surface roughness because of its sensitivity to simulated rainfall as a source of variation, and its consistent response to such rainfall. MIF can also account for spatial dependency and can be measured relatively precisely. S OIL SURFACE ROUGHNESS is an important soil characteristic which affects hydrologic and hydraulic properties of the soil surface. Some recent studies have demonstrated relationships between surface roughness and infiltration (Moore et al., 1980), erosion (Johnson et al., 1979), runoff (Foster et al., 1984; Onstad, 1984), and radiant energy transfer (Linden, 1982). In spite of its importance, surface roughness remains a characteristic not well defined. The attention that roughness has received is perhaps not enough when viewed in relation to its importance. This may be, in large part, due to the lack of a quantitative, physically significant parameter that can adequately describe soil surface roughness. Several parameters have been proposed to obtain exact descriptions of surface roughness. Kuipers (1957) proposed an expression R 100 log s [ l ] where s was the standard deviation (a measure of sample variation) of surface elevations measured along 20 2-m transects. While Kuipers' R parameter was adjusted for slope, it was not adjusted for tillage tool marks. Later, Burwell et al. (1963) utilized a parameter which was calculated as the standard deviation of the natural logarithms of 400 height measurements G.A. Lehrsch, USDA-ARS, Soil & Water Management Res. Unit, Rt. 1, 3793 N. 3600 East, Kimberly, ID 83341; F.D. Whisler, Dep. of Agronomy, Mississippi State Univ., P.O. Box 5248, Mississippi State, MS 39762; and M.J.M. ROmkens, USDA Natl. Sedimentation Lab. , P.O. Box 1157, Oxford, MS 38655. Part of a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree at Mississippi State Univ. Contribution from the Mississippi Agric. For. Exp. Stn. , Mississippi State, MS, and the USDA Natl. Sedimentation Lab. , Oxford, MS. Journal no. 6472. Received 8 Dec. 1986. •Corresponding author. Published in Soil Sci. Soc. Am. J. 52:1439-1445 (1988). taken with a point gage on a 1-m by 1-m plot with a 5.1-cm by 5.1-cm grid spacing. Their parameter was adjusted for the effects of tillage tools but not, it appears, for field slope. Subsequently, Alimaras et al. (1966) used an index termed random roughness which was given as