The effect of surface moisture on the entrainment of dune sand by wind: an evaluation of selected models

Abstract

Reliable predictions of wind erosion depend on the accuracy of determining whether erosion occurs or not. Among the several factors that govern the initiation of soil movement by wind, surface moisture is one of the most significant. Some widely used models that predict the threshold shear velocity for particle detachment of wet soils by wind were critically reviewed and evaluated. Wind‐tunnel experiments were conducted on pre‐wetted dune sand with moisture contents ranging from 0·00 to 0·04 kg kg−1. Sand samples were exposed to different wind speeds for 2 min. Moisture content was determined gravimetrically before and after each experiment, and the saltation of sand particles was recorded electronically with a saltiphone. Shear velocities were deduced from the wind speed profiles. For each moisture content, the experiments were repeated at different shear velocities, with the threshold shear velocity being determined by least‐squares analysis of the relationships between particle number rates and shear velocity. Within the 2‐min test runs, temporal changes in particle number rates and moisture contents were detected. A steep increase in the threshold shear velocity with moisture content was observed. When comparing the models, large differences between the predicted results became apparent. At a moisture content of 0·007 kg kg−1, which is half the moisture content retained to the soil matrix at a water tension (or matric potential) of −1·5 MPa, the increase in ‘wet’ threshold shear velocity predicted with the different models relative to the dry threshold shear velocity ranged from 117% to 171%. The highest care should therefore be taken when using current models to predict the threshold shear velocity of wet sediment. Nevertheless, the models of Chepil (1956; Proc. Soil Sci. Soc. Am., 20, 288–292) and Saleh & Fryrear (1995; Soil Sci., 160, 304–309) are the best alternatives available.

How important are atmospheric depressions and mobile cyclones for emitting mineral dust aerosol in North Africa?

“…An increase of soil moisture strengthens the bonding forces between soil particles constraining higher wind speeds for dust emission (Cornelis and Gabriels, 2003;Fecan et al, 1999). While precipitation amounts and therefore soil moisture are generally small in large areas of the Sahara, cyclones are an important source for rainfall in North Africa (Hannachi et al, 2011) and may be able to moisten the soil sufficiently to increase the threshold of dust emission onset.…”

The effects of surface moisture on aeolian sediment transport threshold and mass flux on a beach

“…When the beach is dry the distribution would probably reflect in some way the particle size distribution. When the beach surface is quite moist the whole distribution will shift towards much higher threshold wind speeds and the shape of the distribution may change to include the influence of a different set of properties related to the binding forces exerted by moisture that control entrainment (McKenna Neuman and Nickling, 1987;Cornelis and Gabriels, 2003). The shape of the distribution for the moist surface ( Figure 13) should also reflect the potential for surface drying, which would have the effect of increasing the range of the threshold wind speed distribution (Wiggs et al, 2004a;Davidson-Arnott et al, 2005).…”

“…It has been recognized for a long time that moisture content is an important control on aeolian sediment transport (Belly, 1964;Nickling and Davidson-Arnott, 1990;Namikas and Sherman, 1995;Cornelis and Gabriels, 2003;Cornelis et al, 2004;Ravi et al, 2006). The early work, carried out primarily in wind tunnels, showed that surface moisture increased the threshold wind speed or shear velocity (u*) at which sediment began to move (Belly, 1964;Azizov, 1977;Logie, 1982;Hotta et al, 1984;McKenna Neuman and Nickling, 1989).…”

Measurement of water content as a control of particle entrainment by wind

“…Svasek and Terwindt, 1974;Sarre, 1988;Arens, 1996;Jackson and Nordstrom, 1997;Wiggs et al, 2004a,b). Even in highly controlled settings established in wind tunnel experiments, researchers observe that spatial variation is an intrinsic aspect of the phenomenon of pore water constraint on particle entrainment (Cornelis and Gabriels, 2003;Cornelis et al, 2004d;McKenna Neuman and Maljaars Scott, 1998).…”