Wind Erosion in Western Queensland Australia

Chapter 4 –Modelling Soil Erodibility Dynamics2.34(%)Q = 737.43 clay(4.6)An assumption made in selecting this limit is that the crust conditions on the soils measuredby Leys et al. (1996) are representative of the minimum erodibility of those soils, i.e. lowestfraction of loose erodible material and dry soil surface. This model operates under theassumption that soil moisture content is equal to zero, with soil erodibility being controlledby the soil aggregate size distribution.The maximum erodibility limit now needs to be defined. The highest values for Q obtainedby Leys et al. (1996) for sandy soils were around 150 gm -1 s -1 at a wind velocity of 65 kmh -1 .For the current study this value will be considered representative of Q max for all soil textures.An assumption made in setting this limit is that given the right climate and land managementconditions the dry aggregate size distribution of any soil could potentially match that of thesandy soil producing Q at 150 gm -1 s -1 . In practice Q max will not be constant for all soiltextures. While sandy soils display high erodibility and a relatively small range of variationdue to aggregation, soils with high clay content may experience a wide range of aggregation.The extent to which erodibility can increase for soils with high clay content is dependent onthe nature and severity of disturbance mechanisms. If a hypothetical maximum disturbancewere reached on clay textured soils, a decrease in surface roughness may result, causing anincrease in u *t and decrease in erodibility by the nature of that measured by Bagnold (1941) -relating to increased inter-particle cohesion in the very fine clay grains (< 0.08 mm).Furthermore, the minima in measured soil erodibility at ~27% clay due to strong grainbinding and aggregation in some clay loam soils (e.g. Chepil, 1953) would likely cause a dipin Q at around this point and moving up the continuum.A three-dimensional plot can be produced from Equations (4.4) and (4.5) to conceptualise thephysical dimensions of the soil erodibility continuum (Figure 4.3). Values of Q computedwith inputs of clay content from 0 to 100% can be matched to values of Q computed usingEquation (4.5) for a full range of soil aggregate size distributions (%DA >0.84 mm). Here thecoefficient a = 150 based on Q max , and -b = -0.078 (r 2 = 0.99, p < 0.01) based on Leys et al.(1996). The lower limit for Q is defined by values computed from Equation (4.6). The modeloperates on the premise that the percentage of dry aggregates >0.84 mm, the non-erodiblefraction, determines soil erodibility. The plot illustrates the potential erosion rates108