Accounting for surface roughness on measurements conducted with PI-SWERL: Evaluation of a subjective visual approach and a photogrammetric technique

Abstract

Abstract The effect of small-scale surface roughness on measurements with PI-SWERL (The Portable In-Situ Wind Erosion Laboratory) was investigated using a viscometer-type device. The relationship between shear stress at the soil surface and PI-SWERL rate of rotation (RPM) was determined empirically with test surfaces of varying roughness including fine to coarse sandpaper, small domed structures (H = 2.5–2.9 mm) arranged in arrays, small (H ≈ 2 mm) and large pebbles (H ≈ 5–7 mm), thin cylinders representing blades of grass (D ≈ 2 mm), and wave-like surfaces intended to represent small ripples. An empirical equation that uses a single parameter α was developed to relate RPM to the shear stress experienced by a test surface. Two methods were presented for estimating α. The first relies on a user’s ability to estimate α by comparing to a catalog of standards of roughness. The second uses a digital photogrammetry technique in combination with image processing to parameterize roughness. The photographic method offers a factor of two improvement in estimation of α compared to the first method. Although a limited number of surfaces was tested, the photogrammetry technique appears to warrant further development. Additional findings relate to the optimal effective area for use in calculating wind erosion parameters from field tests and an estimate of errors for both dust emissions measurements and the associated friction velocity (u∗) when using the PI-SWERL. Overall, this work serves to reduce the uncertainties associated with using the PI-SWERL as a tool for estimating windblown dust emissions.