Dust Production Mechanisms - Mineral Dust: A Key Player in the Earth System - page 107

particles and the fluid and the intermittency of the saltation induced by turbulent
flows or surface heterogeneity (e.g. Shao and Li 1999 ; Kok and Renno 2009 ; Dupont
et al. 2013 ). Another limitation of these simple saltation flux models is that they
are not designed to estimate the erosion fluxes properly for supply limited sources.
Supply limitation is often encountered in semi-arid areas and is often associated to
areas or time periods when soil crusts protect the surface soil against erosion. In
such cases, the measured horizontal flux is lower than the expected potential flux
a non-supply limited soil would have for the same wind conditions (Gillette et al.
2001 ) and can evolve with time as a function of sediment depletion. Accounting for
sediment supply is also an issue when addressing climatic timescales (e.g. Kocurek
and Lancaster 1999 ). Similarly, classical saltation models fail to reproduce the
erosion fluxes over complex surfaces such as vegetated or cultivates surfaces, where
trapping can occur on surfaces roughened by vegetation or ridged soil. Trapping
is especially important for agricultural soils where ploughing causes both non-
erodible surface clods and furrows. As an example, the reduction of the erosion flux
measured in wind tunnels over sandy ridged surfaces ranges between 65 and 85 %
under high wind conditions (>14 m s 1 ) regardless of the height and spacing of the
ridges (Kardous et al. 2005 ). Despite these limitations, which mainly concern semi-
arid areas, saltation can be considered as the best understood and well-described
stage of the dust emissions processes.

5.5 Dust Emission

Dust production by sandblasting results from the balance between the kinetic energy
of the saltating particles and the cohesion of the dust particles to the soil aggregates.
The efficiency of this process is thus dependent on the intensity of the saltation flux
and of the cohesive properties of the dust particles or the resistance of the soil. While
the description of the saltation bombardment can be derived from the size-dependent
computation of the saltation flux, an explicit description of dust entrainment by the
impact of saltating grains is not straightforward. In addition, the estimation of the
amount of dust particles available in a soil that can be released by sandblasting or
disaggregation is also a major difficulty.

5.5.1 Empirical Approaches

Up to now, the most complete and comprehensive dataset available for dust mass
flux is that from Gillette ( 1977 ), which includes synchronous measurements of
the saltation and dust emission fluxes as a function of U * for several soils in the
USA with different textures. This dataset shows different behaviour of the ratio
of dust and saltation fluxes, the so-called sandblasting efficiency, depending on
the soil: for some soils, the efficiency tends to vary with U * , while for others, it