Wind Erosion in Western Queensland Australia

Chapter 4 –Modelling Soil Erodibility Dynamicsfactors may also play a significant role in soil aggregate destruction, for example drivingphoto-degradation of biological crusts and through freeze-thaw process. The third groupinclude soil textural properties and cohesion agents. This group is influenced by the climateand management conditions, but also contains factors that are particular to specific soil types,e.g. moisture holding capacity. Movement of a soil through the erodibility continuum can bemodelled through an expression of the behaviour of the soil in response to these forcingmechanisms (Chapter 2, Section 2.4).Figure 4.4 illustrates the movement of a soil through the erodibility continuum. Themovement can be considered to follow three phases, labelled i, ii and iii. The mechanismscontrolling the position of a soil within the continuum will vary for each phase.The first phase (i) defines a condition of minimum erodibility. Sufficient rainfall to inducesoil surface sealing will result in a breakdown of dry aggregates and the consolidation ofsurface material in a saturated matrix (Kemper et al., 1987, Maulem et al., 1990). At thispoint reorganisation of the grains may take place, forming structural or depositional crustsand potentially rejuvenating biological crust growth (Valentin and Bresson, 1992). In termsof the erodibility continuum defined by Equation (4.4), this phase positions a soil at Q min ,defined by b min , and controlled primarily by climatic factors (Figure 4.1). At the cessation ofrainfall the control on erodibility will shift to being dominated by soil moisture, and the soilwill remain at around the position of Q min until the moisture content lowers to a positiondefined by Fécan et al. (1999) as w’, the minimum moisture content required to induce anincrease in u *t . Here the soil textural properties become important. Erodibility will remainconstant during phase (i) for soils that seal and form physical crusts. For sandy soils anincrease in erodibility will occur during this phase. This increase in erodibility can beexpressed by a power function that defines a decrease in the ratio of u *tw for the wet soil tou *td for the soil in a dry condition with decreasing soil moisture content (Fécan et al., 1999).The period of time that a soil remains in this phase is therefore determined by its physicalproperties and cohesion agents; this is likely to be shorter for well drained sandy soils thanfor soils with higher clay content that have higher water holding capacity (Cornelis andGabriels, 2003).111