The type and distribution of vegetation in arid environments play a critical role in controlling the initiation of the wind-driven sediment transport system as well as the magnitude of the flux of sand and dust. Despite this knowledge, the role of natural vegetation is not well accounted for in present regional wind erosion models. Previous research indicates that the change in sand transport rate due to large solid non-erodible roughness elements is a function of both the size and the distribution of the elements. For vegetation its properties (e.g., porosity, flexibility, drag coefficients, distribution, etc.) create uncertainty as to how similar their effects are as compared to solid element roughness, which is typically assumed in current wind erosion models. This project will investigate through field-based experimentation, how plant community structure as characterized by its distribution pattern and associated aerodynamic properties affects sand transport by wind. The data needed to answer these questions will be collected at four sites within the USDA Jornada Experimental Range (JER), NM, where the landscape has changed from one dominated by grasslands to one that is becoming dominated by shrubs, with a resultant increase in wind erosion and land degradation. The four sites represent different stages of mesquite invasion at the JER. At each of the sites instruments will measure regional and local wind speeds, surface shear stresses, sand movement activity, and the vegetation distribution characteristics. This research will aid in the identification of key characteristics of vegetation that can be used to indicate either stability or the approach of a critical threshold that subsequently leads to an increase in wind erosion and environmental degradation. The results from this project will provide a means to more accurately quantify the effect of sparse vegetation on sediment transport thresholds and sand fluxes, which can be incorporated into regional wind erosion models.
This research is designed to increase our understanding of the links between wind-driven sediment transport processes and vegetation in desert environments in general. Desert environments worldwide are under pressure from climate change as well as expanding populations and their need for resources. Vegetation plays a critical role in the stability of desert environments, specifically as a buffer against wind erosion and the often-associated dust emission process. However, as evident in parts of the Chihuahuan desert in the U.S. southwest some desert plant communities, by virtue of their survival mechanisms, can exacerbate wind erosion, leading to environmental decline. Vegetation can therefore both positively and negatively influence wind erosion processes and the results of this project will aid in determining the nature and degree of these influences under a variety of environmental conditions. Such knowledge can subsequently be used to aid in evaluating how climate change and human-induced pressures could exacerbate degradation in these sensitive areas.
This research was designed to increase our understanding of the links between wind-driven sediment transport processes and vegetation in desert environments in general. Desert environments worldwide are under pressure from climate change and expanding populations and their need for resources. Vegetation plays a critical role in the stability of desert environments, either as a buffer against wind erosion and the often-associated dust emission process. Vegetation both positively and negatively influences wind erosion processes and the results of this project provide a means to assess vulnerability of sparsely vegetated communities in desert and semi-arid environments, which can subsequently be used to aid in evaluating how climate change and human-induced pressures could exacerbate degradation in these sensitive areas. Specific findings of our field-based project were that two simple parameters, the wind speed ratio and the shelter index could provide a means to characterize the complex wind field of the vegetation community and the areas among the vegetation that experienced the greatest exposure to winds where wind erosion could occur. The wind speed ratio is the ratio of wind speed near the ground/wind speed at 14 m. Shelter index is the distance downwind to that position divided by the height of the roughness element directly upwind of that position. The shelter index of a surface changes as a function of wind direction so it can be mapped for an event or for an accumulation of wind and transport events. By repeatedly mapping an area and estimating the distribution of shelter index that characterizes the wind climatology for an area it should be possible to evaluate if the vegetation is changing towards a state where it may be theatened by increased erosion, or maintaining its resilience to resist increased erosion. Our research also revealed some fundamental physics of wind flow and sediment transport for the vegetation-covered dunes called nebkhas found at our study site in New Mexico. The results show that flow fields and sediment flux responses between solid bluff body forms and the vegetion-covered dunes are not equivalent. Because of these differences it is expected that roughness arrays of similar sized elements, but different porous covers, would result in different amounts and patterns of sand transport. We also observed that the wind speed recovered to its upwind speed in eight element heights, which appears to be a fundamental physical constraint on wind speed recovery in the lee of a bluff?body in a multi?element array. This corroborated several other studies, but those studies did not acknowledge this fundamental length scale. Regardless of the form of the roughness (solid or porous) a dominant control on the amount of sand transported by the wind is the supply of available sand. Sediment availability will, to a high degree, control the net amount of sand moved by the wind as the wind continually interacts with the vegetation changing the distribution of the shelter index. We are confident that our results indicate that there is high merit for application of the wind speed ratio and shelter index characterization for evaluating the susceptibility of vegetated communities to degradation by wind erosion, or conversely their resilience to this type of environmental degradation. The results of our research are broadly applicable to understanding how vegetation in deserts or dylands is both affected by wind erosion pressures and how the vegetation itself influences the threshold for sand transport and the sand transport pathways. This project also offered the opportunity to aid in the professional development of graduate and undergraduate students who participated in the field measurement campaigns. These students worked alongside senior faculty members who provided guidance to improve their technical skills with instruments, data collection, data analyses and writing. Two students used data from this project to produce master's theses and two have moved on to pursue Ph.D. degrees.
