Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility ControlsThe effects of climate, soil properties, cohesion agents, management and vegetation on winderosion can be physically defined through the computation of u *t for a land area. Thisthreshold computation can be seen as a function employing adjustments to the principalformulation presented by Bagnold (1941) in Equation 2.5. Shao (2000) presented a pragmaticapproach to defining u *t based on a standard threshold for sand particles of size d s where thesoil is dry, bare and free of crust and salt:u( d ,,sc,cr,...) u ( d ) f ( ) f ( ) f ( sc) f ( )...* t s; * t s w sc crcr= (2.34)where u *t (d s ) is the threshold friction velocity for sand particles of size d s where the soil isdry, bare and free of crust and salt. The formula is a function of particle size only, whichcould be determined by wind tunnel experimentation with loose sand. Adjustments are madeto this base threshold by incorporating functions for λ, the frontal area index for surfaceroughness elements, θ, soil moisture content, sc the soil salt content, and cr, a surface crustfactor. Additional adjustments can be made to the formulation as required.Conceptually, erodibility can be considered the inverse of u *t . A decrease in erodibilityimplies an increase in u *t , as more wind energy is required to mobilise particles on the soilsurface. While u *t has some dimension and can be physically measured, erodibility is aconcept that can only be inferred through proxies like u *t , or through empirical relationshipsbetween environmental conditions and erosion rates for a given wind speed.The systems analysis presented in this chapter has demonstrated that a number of models areavailable that can be used to represent the effects of these conditions on soil and landerodibility. The models vary considerably in form and complexity. Approaches to integratingthese models to assess land erodibility and simulate wind erosion processes are also diverse,but in general are based on the framework of Equation 2.34.Figure 2.11 presents a conceptual model of the land erodibility continuum. The conceptualmodel simplifies theories of landscape dynamics (e.g. Noy-Meir, 1973; Westoby et al., 1989),presenting the range of potential vegetation and soil erodibility conditions within individualand separate continuums. In doing this, the model suggests that the state of the erodibility of aland area can be defined by the response of its fundamental controls (vegetation cover and65