Wind Erosion in Western Queensland Australia

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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
Chapter 4 –Modelling Soil Erodibility Dynamicsexperienced for soils under a range of potential aggregation levels. The model suggests thatunder heavy disturbance soils with high clay content may have erodibility levels similar tosoils with low clay content that are not disturbed. This is supported by Chepil’s (1953) data inwhich soils with 60% clay had the same erosion rates (~12 tons/acre) as soils containing 8%clay. It is important to note that as soil moisture content increases, it will also affect theposition of a soil on the erodibility continuum. When the soil moisture content is sufficient toinduce an increase in u *t soil erodibility will decrease further toward (and will reach) a valueof zero.Figure 4.3 3D plot of the soil erodibility continuum as defined by the soil erodibility (indicated by Qgm -1 s -1 ) relationship with soil clay content (percentage clay) and aggregate size distribution thatcontrols the quantity of non-erodible soil aggregates (%DA > 0.84 mm)The soil erodibility continuum model can be used to describe changes in soil erodibilitythrough time. A sandy soil with less than 7% clay may become resistant to wind erosion withstabilization due to biological crust formation. Crusts on sandy soils are, however, weak,abrade easily under saltation bombardment, and so offer a small reduction in soil erodibility.109
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Modelling Land Susceptibility toWin
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AUSLEM was developed as a Geographi
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who joined me on many trips, shared
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Conference Presentations by the Aut
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2.2.5 Soil Moisture Effects........
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Chapter 6: Assessing Land Susceptib
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List of FiguresChapter 1: Introduct
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Figure 2.11 Conceptual model of lan
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hand column) presents trajectories
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List of TablesChapter 1: Introducti
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Chapter 1 - Introduction• The abi
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Chapter 8 - Conclusionsland use cha
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Chapter 8 - Conclusionsthe opportun
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Beadle, N.C.W., 1945. Dust storms.
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Bryan, R.B., Govers, G., Poesen, J.
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Chepil, W.S., 1965. Transport of so
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Fryrear, D.W., Sutherland, P.L., Da
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Gregory, J.M., 1984. Prediction of
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Hugenholtz, C.H., Wolfe, S.A., 2005
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Littleboy, M., McKeon, G.M., 1997.
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Marshall, J.K., 1973. Drought, land
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Assessment Report of the Intergover
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Penner, J.E., et al. , 2001. Climat
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Rickert, K.G., McKeon, G.M., 1982.
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Stokes, C., J., McAllister, R.R.J.,
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Williams, W.J., Eldridge, D.J., Alc
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Wind Erosion in Western Queensland Australia

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