Dust Production Mechanisms - Mineral Dust: A Key Player in the Earth System - page 100

forces can develop. This conceptual view of water partitioning in the soil is reflected
in the parameterisation of the increase of the erosion threshold due to soil moisture
depending on the soil clay content proposed by Fécan et al. ( 1999 ).

for w < w 0 W U tw =U td

D 1 (5.4a)

h 1

w '/ 0:68 i 0:5 for w > w 0 W U tw =U td

D C 1:21. w (5.4b)

with U tw the threshold wind friction in wet conditions, U td , the threshold wind
friction in dry conditions, w the gravimetric soil moisture content and w' the
minimal soil moisture for which the erosion threshold increases, which depends
on soil texture and more specifically on soil clay content:

0:0014.%Clay/ 2

w 0 D

C 0:17.%Clay/ (5.5)

Cornelis et al. ( 2004 ) proposed a different approach based on a complete analysis of
the forces acting to maintain the particles at the surface, including capillary forces
due to soil moisture. They proposed a single expression for the dependence of the
erosion threshold as a function of the size, particle density and the soil moisture
content. This expression was adjusted on wind-tunnel measurements performed over
dry and wet soils, but including mainly sandy and loamy soils.
There are very few field measurements of erosion thresholds over wet soils in
the literature. The increase of the erosion thresholds for wet soils compared to dry
conditions measured in the Taklamakan Desert (Ishizuka et al. 2005 ) was found
to be in agreement with the parameterisation of Fécan et al. ( 1999 ). Experimental
results of Wiggs et al. ( 2004 ) for field measurements of threshold erosion velocities
for sand showed that moisture contents of less than 2 % had little or no effect on
sand flux. Gillette et al. ( 2001 ) observed vigorous wind erosion 10-30 minutes after
a rainstorm on sandy soil. The reason for such a short time reduction of wind erosion
in the field is that the eroding soil layer is thin and can dry rapidly under conditions
of low humidity, high temperature and high winds. Consequently, soil moisture may
have a limited influence on the erosion threshold in sandy deserts under a hot and
dry climate, such as the Sahara, while it can significantly inhibit the dust emissions
in more temperate climate for soils with higher silt and clay soil content such as
Asian deserts (e.g. Laurent et al. 2006 , 2008 ).

5.3.3 Influence of Surface Roughness

U t values measured over rough surfaces are higher than those measured over smooth
surfaces (e.g. Gillette et al. 1982 ). Non-erodible roughness elements absorb part of
the wind momentum that would otherwise be available to initiate particle motion.
To reach the erosion threshold, the shear stress component acting on the erodible