A wind tunnel investigation to examine the role of air humidity in controlling the threshold shear velocity of a surface and in controlling the mass flux of material from a surface
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Aeolian erosion, the wind-forced movement of soil particles, is a dominant process in arid regions. The impacts of wind-driven dust emissions are numerous and can have both localized effects and far reaching impacts. The susceptibility of a soil surface to wind erosion is gauged using the threshold friction velocity (u*t), which is related to the wind speed needed for the erosion of surface material to commence. This threshold varies for every surface and depends on numerous variables, including surface soil moisture. In arid regions, which contain air-dry soils, soil moisture is the driving factor controlling changes in the threshold shear velocity of a surface at short, diurnal timescales (Ravi and D’Odorico, 2005). Therefore, an understanding of these varying moisture conditions, and the associated changes in soil erodibility, is vital in understanding and modeling aeolian soil erosion (Ravi et al., 2006). In such regions, where rainless periods are common and there is a lack of moisture provided by precipitation or capillary rise, the humidity of the overlying air is particularly influential in controlling soil moisture content at the surface (Ravi and D’Odorico, 2005). Through wind tunnel tests, this study analysed the control relative humidity had on the threshold shear velocity for three soils: a loamy fine sand, a clay loam and a sandy muck. The loamy fine sand and the clay loam material were previously studied and the results found here agreed with those from other studies (e.g., Ravi et al., 2006). The relationship found is made up of a high and low section of humidity, where the threshold shear velocity increases with humidity, and a mid-range section of humidity, where the threshold shear velocity decreases with increased humidity. The previously unstudied sandy muck also showed this relationship. The results were explained using the theory of equilibrium between the surface soil moisture and the humidity of the overlying atmosphere. In such studies, an accurate measurement of the threshold shear velocity is vital and this study compared the sensitivity of two particles sensors (Wenglor YH03PCT8 and Sensit model H10) in determining the threshold shear velocity of a soil surface. It was found that the newer Wenglor sensor recorded the threshold at considerably lower wind speeds throughout the test and for each soil. This, however, did not significantly change the relationship seen between the threshold shear velocity and relative humidity. For the first time, this study aimed to investigate the effect humidity has on the erodibility of a surface over a sustained period of time. This was done by analysing the mass flux of PM10 from each soil at a range of humidities. It was found that the initial threshold shear velocity of the surface had no control over the average mass flux of material from the surface. For the clay loam and sandy muck, the humidity also had no control over the average mass flux of material from the surface. The loamy fine sand, however, exhibited a strong, linear relationship between the humidity and the total mass removed. This was explained using a theory suggesting that the higher percentage of larger grains in this soil preferentially absorbed water from the atmosphere when compared to smaller grains, meaning smaller grains are less affected by the threshold. Finally, this study analysed the control of humidity on the time it takes for mass flux to reach its peak value. It was found that the threshold shear velocity had a substantial control over the time to mass flux. This showed that the overlying humidity of the air only affected the grains at the surface (≈top 2mm) and, under prolonged wind erosion, the initial threshold of the surface was no longer apparent at the surface and a new threshold was developed.