Wind Erosion in Western Queensland Australia

Chapter 4 –Modelling Soil Erodibility Dynamicsunderstanding of factors driving temporal changes in soil erodibility to wind. It is hoped thatthis will incite discussion and research action that is required to quantify relationshipsbetween controls on soil erodibility so that robust predictive models can be developed.4.2 Aggregation, Soil Crusts and Soil ErodibilityBagnold (1941) demonstrated by laboratory wind tunnel experimentation that as graindiameter decreases, u *t decreases toward a minimum where grain diameter is ~0.08 mm.Further reductions in grain size result in an increase in u *t . As described in Section 2.2.3,Iversen and White (1982) attributed the increase in u *t for small grain sizes to enhanced interparticlecohesion between fine (clay) grains. In field situations the effect of inter-particlecohesion is seen across all soil textures, with particle-bonding driving soil aggregation andsurface crusting. Chepil (1950a) demonstrated that soil aggregate size is directly related toerodibility, with aggregates 0.84 mm non-erodible. The dry aggregate size distribution (DASD) thusdrives the availability of loose erodible material on a soil surface. The physical mechanismbehind the aggregation-erodibility relationship can be drawn back to grain (aggregate) sizeeffects on u *t , with increased aggregation resulting in an increase in surface roughness (z 0 )and energy required for grain mobilisation. Once grain mobilisation has occurred, DASD andsurface crusting will in turn affect the saltation load, abrasion efficiency, and potential dustproduction.DASD characteristics are driven by processes of aggregate formation and breakdown(Breuninger et al., 1989). A primary control on aggregate formation is soil texture, with soilparticle size and mineralogy affecting inter-particle bonding (Harris et al., 1966). Bondstrength is subject to the nature of particle contacts. The plate-like structure of fine clayparticles provides large inter-particle contact surfaces, and so a greater potential for bondingthan in soils with sub-rounded or angular particles, e.g. sands (Smalley, 1970). As sandy soilsare least susceptible to aggregate formation, they are also the most consistently erodible(Chapter 2, Section 2.2.4). As clay soils have the most potential for aggregate formation, theydisplay the greatest range of variability in aggregation and therefore erodibility (Chepil,1954).102