Wind Erosion in Western Queensland Australia

Chapter 4 –Modelling Soil Erodibility Dynamicsfor crusted soils to a higher position for disturbed soils (raw data shown in Figure 4.4). Thisis physically supported by changes (decrease) in u *t in response to surface disturbancemeasured for a range of agricultural and desert soils in the United States (Gillette, 1980;Gillette et al., 1982; Gillette, 1988).Mathematically, movements of the erodibility curve for changes in aggregation and crustingare defined by a range of values for the power b in Equation (4.2). In terms of soil claycontent then the soil erodibility continuum can be defined by the range:bminbmaxQ = a.(%clay)→maxa.(%clay)minQ = (4.4)where b max defines the erodibility maximum and b min defines the erodibility minimum.The availability of loose erodible sediment ultimately drives changes in the position of thecurve between those at b max and b min . The directional form of the change is thereforecontrolled by the relationship between soil aggregation (namely %DA > 0.84 mm) andstreamwise sediment flux Q. Chepil (1953) and Leys et al. (1996) demonstrated that thisrelationship is consistent for all soil textures (i.e. 0 – 100 % clay) and can be expressed as:Qb%DA= aexp (4.5)where a is the equation intercept and -b defines the sensitivity of Q to %DA, the percentagemass of non-erodible dry aggregates (>0.84 mm). The applicability of Equation (4.5) acrossthe soil texture range links back to the energy requirements for grain mobilization and theelementary relationship between grain (aggregate) size and u *t . Progressively increasing soilsurface disturbance results in weakened inter-particle bonds and an increase in the fraction ofloose erodible material. Soils can therefore exist with some range of ‘grain’ sizes determinedby inter-particle cohesion as a result of aggregation or crust disturbance (Breuninger et al.,1989). As soil aggregation changes through time in response to climate and managementfactors the position of a soil on the curve defined by Equation (4.5) will also change. Figure4.2 illustrates a hypothetical vertical displacement of the Q-%clay curve (4.2b) in response toa change in %DA (4.2a). Differences in the regression coefficients a and -b (Equation 4.5) aspresented by Chepil (1953) and Leys et al. (1996) are attributed to site specific variations in106