Wind Erosion in Western Queensland Australia

Chapter 4 –Modelling Soil Erodibility DynamicsPhysical and biological crusts both influence soil erodibility (Chapter 2, Section 2.2.6).Physical crusts are identified by their mechanisms of formation: structural crusts form as aresult of water droplet impact (e.g. during rainfall) and in situ particle rearrangement,whereas depositional crusts form by the settling of fine particles transported by surface water(Valentin and Bresson, 1992). Microbiotic crusts are characterized by the biologicalorganisms driving crust formation, e.g. mosses, liverworts, lichens, algae and cyanobacteria.These bind soil particles with structural filaments or through the secretion of gels, andinfluence the availability of loose erodible material, soil micro-topography and z 0 (Eldridgeand Greene, 1994; Johansen, 1993). Additional crust characteristics that influence soilerodibility include: lateral cover; structure (arrangement of particles by grain size); thickness;strength; and modulus of rupture, the resistance to breaking by saltating particles (Rice et al.,1999; Figure 2.6).Moisture is the principal agent driving the initiation of inter-particle bonding in aggregate andcrust formation (Harris et al., 1966, Section 2.2.5). The processes are therefore particularlysensitive to climate variability. The amount, intensity and frequency of precipitation, soiltexture and site stability determine the type of crust formation and productivity of soilmicrobiota (Belnap and Elderidge, 2003). Precipitation efficiency is regulated by solarradiation intensity and potential evaporation. Field studies of freeze-thaw induced changes inthe aggregate size distribution of soils in North America have demonstrated the addedimportance of temperature in regulating soil aggregation (Chepil, 1954; Bisal and Ferguson,1968; Merrill et al., 1999; Bullock et al., 2001).Mechanical disturbance of the soil surface by machinery and livestock trampling is afundamental control on DASD and crust characteristics. In cultivated landscapes soilaggregation is affected by tillage practices, while in the rangelands aggregation is related tocrust disturbance levels (Eldridge and Leys, 2003). Additional factors affecting aggregate andcrust breakdown include: photo-degradation during long periods of exposure (i.e. duringdrought); clay lattice expansion-contraction during wet-dry cycles (e.g. in self-mulchingsoils); freeze-thaw cycles; fire; and salt efflorescence (Harris et al., 1966; Breuninger et al.,1989). Aeolian abrasion associated with saltation bombardment during wind erosion furtheraffects DASD and crust integrity (McKenna-Neuman et al., 2005). These disturbancemechanisms are superimposed on the factors driving aggregate and crust formation, resulting103