Wind Erosion in Western Queensland Australia

Chapter 3 – Modelling Land Erodibility ReviewInput variables for TEAM include wind speed, relative humidity, soil particle sizedistribution, clay content, residue and vegetative cover, soil aggregate cover, field length, andwindbreak height and porosity. The threshold friction velocity is calculated for the averageparticle size d (not including the clay fraction) and surface moisture conditions of the fieldundergoing simulation. This computation is based on wind tunnel research by Gregory andDarwish (1990), Darwish (1991), Gregory (1991) and Puri et al. (1925). The static thresholdis computed by:0.5 0.0045 1.2 0.1W/ W 0.11821.2d1+0.01W+ + e ( W W)2c Wu*t= (3.9) d dwhere d is the diameter of the average soil particle size not including clay, W is the watercontent of the soil in percentage weight, W W is the wilting point, and W C is the percentage ofwater on the surface needed to fill inter-particle spaces. The moisture content factor accountsfor the change that occurs in u *t once water content is above a threshold (described in Chapter2, Section 2.2.5). The water content is based on atmospheric humidity (%). W c is thethreshold water content at which an increase in u *t is initiated. If W is less than W c set Wequal to W c so the last term does not apply (Singh et al., 1999).The dynamic threshold friction velocity model accounts for the effects of soil surfacecondition, vegetation cover, and wind gustiness (Gregory et al., 2004). The dynamicthreshold is computed by first adjusting u *t to compensate for wind gustiness:u /* d= 0 .8u*tGf(3.10)where G f is a gust factor. The effects of vegetation cover (S) are applied to determine themaximum transport rate M, which is also a function of soil particle diameter at differentstages of the particle distribution (D 50 , D 75 , D g ), B d an empirical coefficient, and the dynamicthreshold and u * :2 1 D D75 0.08 502 2M = 0.004B1 125d( Su* u*d) u *u +(3.11)* dDRD R 78