The impact of a single shrub on the distribution of surface shear velocity in a semi-arid rangeland

Wind erosion is a major driver shaping landscape patterns in drylands. Vegetation is known to provide protection from wind erosion by sheltering the ground surface, extracting wind momentum, and trapping sediment. Significant progress has been made in parameterizing the protective effect of vegetation elements as a function of vegetation cover for wind erosion model development. However, the influence of single vegetation elements on shear stress patterns at the surface in two dimensions, particularly in the spanwise direction where increases in surface shear stress and subsequent soil erosion have commonly been observed, has not been included in drag partition schemes to date. Studies on wind speed around a shrub (or other obstacles) in the spanwise direction underscore the potential role of vegetation in intensifying erosion issues, e.g., dust emissions, desertification and challenges in land management. Here, we quantify wind erosivity patterns around a single shrub within a semi-arid rangeland environment for a range of wind velocity and plant phenology. Field data were collected 2/7/2023-7/19/2023 and 2/12/2024-6/20/2024 at the National Wind Erosion Research Network site at the USDA Jornada Experimental Range in south-central New Mexico, USA. A total of 18 Irwin sensors were installed around a honey mesquite shrub (Prosopis glandulosa Torr.), mainly perpendicular to the dominant wind direction, to quantify surface shear velocity (u_*s, m/s). A sonic anemometer, installed at 1 m above the surface 3 m upwind of the shrub measured turbulent shear velocity (u_*, m/s). We use the shear stress ratio (SSR= u_*s/u_*) to delineate and quantify shelter and acceleration zones around the shrub. Our objectives were to: 1) compare the spatial patterns of erosivity; 2) examine the shelter and acceleration effects around the shrub at different wind speeds; and 3) examine the plant phenological influence on shelter and acceleration effects around the shrub. Results show that the shrub's impact on erosivity patterns varied depending on its phenological phase. On average, the largest SSR, approximately 1.5 times greater than that at the upwind location, occurred during the dormant and green-up phases, at a spanwise distance of 2 meters (about twice the shrub height) from the shrub center. The smallest SSR, about 39% of that at the upwind location, occurred closest to the shrub during the green-up phase in the spanwise direction and during the leaf-on phase at the downwind location. In the dormant phase, the shape of the shelter zones became more streamlined as wind speed increased. In the green-up phase, the size of the shelter area increased with wind speed, reaching its maximum extent. In the leaf on phase, the acceleration zone expanded as wind speed increased. Our results suggest that incorporating the 2-D surface shear velocity impacts around vegetation, i.e., including both acceleration and shelter effects, is needed to increase the accuracy of wind erosion models in vegetated dryland ecosystems.