دانلود رایگان مقاله دینامیک آستانه سرعت باد به عنوان محرک شار جرمی رسوب بادی

abstract

Horizontal (saltation) mass flux is a key driver of aeolian dust emission. Estimates of the horizontal mass flux underpin assessments of the global dust budget and influence our understanding of the dust cycle and its interactions. Current equations for predicting horizontal mass flux are based on limited field data and are constrained to representing transport-limited equilibrium saltation, driven by the wind momentum flux in excess of an entrainment threshold. This can result in large overestimation of the sediment mass flux. Here we compare measurements of the soil entrainment threshold, horizontal mass flux, and their temporal variability for five undisturbed dryland soils to explore the role of threshold in controlling the magnitude of mass flux. Average and median entrainment threshold showed relatively small variability among sites and relatively small variability between seasons, despite significant differences in soil surface conditions. Physical and biological soil crusts had little effect on the threshold value, and threshold appeared to play a minor role in determining the magnitude of sediment transport. Our results suggest that horizontal mass flux was controlled more by the supply limitation and abrasion efficiency of saltators present as loose erodible material or originating from neighboring soil sources. The omission of sediment supply and explicit representation of saltation bombardment from horizontal flux equations is inconsistent with the process representation in dust emission schemes and contributes to uncertainty in model predictions. This uncertainty can be reduced by developing greater process fidelity in models to predict horizontal mass flux under both supply- and transport-limited conditions. Publish

4. Discussion and conclusions

The difference between wind shear velocity and the entrainment shear velocity threshold is represented as the dominant control on horizontal mass flux in aeolian sediment transport models (Darmenova et al., 2009; Kok et al., 2014). The significance of the entrainment threshold as a control on the magnitude of the mass flux reflects theory derived for the ideal case of equilibrium saltation, in which the entrainment threshold represents the main land surface control on sediment transport (Bagnold, 1941; Owen, 1964; Creyssels et al., 2009; Kok and Renno, 2009). Field and wind tunnel measurements in a range of agricultural and dryland settings have been evaluated in this context (e.g., Gillette et al., 1980; Gillette, 1988). However, interpretation of the role of the entrainment threshold, and of threshold dynamics, is both location-specific and dependent on how threshold is defined (Webb and Strong, 2011). Our results suggest that for undisturbed dryland soils the entrainment threshold may not always play a dominant role in determining the magnitude of horizontal mass flux.