SOIL SURFACE ROUGHNESS AS AFFECTED BY RAINFALL, CULTIVATION, AND CROP CANOPY DEVELOPMENT

The roughness of the soil surface affects infiltration, water storage on the soil surface, runoff velocities, erosion, and plant establishment and growth. It is important to know how roughness responds to tillage and rainfall through developing crop canopies. Surface roughness is seldom studied because it is difficult to quantify. Thus, the objectives were (1) to identify and describe the spatial variation of a measurable, physically-significant parameter describing surface roughness, (2) to determine specific soil properties that influence roughness measured after three cultivations in a soybean Glycine max (L.) growing season, and (3) to measure and describe the response of roughness to simulated rainfall occurring at different stages of soybean development.;The common logarithm of an MIF parameter, calculated for individual transects as the product of 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) and peak frequency, was most promising for describing roughness. Its spatial dependence (zone of influence) varied from 10 to 38 cm on individual plots and averaged 20 cm for plots on which it was spatially dependent. Dry and wet bulk density measured at the soil surface prior to cultivation individually accounted for 64 and 52%, respectively, of the variation in MIF. Water content at the soil surface measured prior to cultivation accounted for 21% of the variation in surface roughness. On plots with no soybean canopies present and canopies covering 63% of the plot surface, rainfall applied at 5 cm hr('-1) for 1 hour at an energy rate of 2,000 kJ (ha cm)('-1) decreased soil surface roughness (the logarithm of the MIF parameter) by 11.9 and 3.7% respectively.;An automated, noncontact profiler measured surface profiles along transects 5 cm apart of 1 x 1 m plots after a cultivation and a rainfall application at each of three soybean growth stages. For each cultivation, surface profiles were obtained perpendicular to the soybean rows on bare plots before rainfall and on adjacent vegetated plots after rainfall. Soil properties at the 10.8- and 30.5-cm depths were measured prior to primary tillage and at the surface immediately before the cultivations. Ground cover was determined using photographs.