Wind Erosion in Western Queensland Australia

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Chapter 4 –Modelling Soil Erodibility DynamicsWind tunnel experimentation has shown that sandy soils are typically highly erodible under arange of conditions. The model suggests that even on crusted sandy soils the availability ofloose erodible material will be sufficient to initiate saltation. This is supported by wind tunnelmeasurements of Q for crusted sandy soils (e.g. Belnap and Gillette, 1997). Soils with claycontent greater than 50% may experience a greater range of variability in susceptibility towind erosion (Chepil, 1954; Skidmore, 1994). This variability is driven by the temporalevolution of the surface particle size distribution and therefore loose erodible material thatresults from crust and aggregate formation and breakdown.4.4 Modelling Temporal Changes in Soil Erodibility4.4.1 ApproachTemporal changes in soil erodibility can be modelled by: 1) predicting soil aggregation levels(e.g. DASD or %DA >0.84 mm) then using that as input to Equation (4.5), or; 2) predictingthe lateral cover of surface crusts and using that as input to Equation (4.3), which can then beinput to Equation (4.5). These approaches are similar to those used in the WEPS model(Hagen, 2001; Visser et al., 2005; Chapter 3, Section 3.2.3). However, development andapplication of these approaches is restricted by the lack of quantitative data on factorscontrolling crust cover and dry aggregate size distributions in rangeland environments. Theapproach presented here seeks to characterise the form and rate of temporal changes in soilsurface conditions between the states of minimum and maximum erodibility.4.4.2 Temporal Model FrameworkFigure 4.1 illustrates the relationships between mechanisms controlling soil erodibility. Themechanisms can be placed into three groups. The first group includes climatic factors that inthe first instance may act in reducing soil erodibility. The dominant factor in this group israinfall, the effects of which are moderated by solar radiation intensity, air temperature,evaporation rates and windiness. The second group are related to management, and includefactors that may increase the susceptibility of a soil to wind erosion. In rangelandenvironments the dominant factor in this group is the stocking rate (animals per unit area)which drives disturbance (trampling) intensity and crust/aggregate breakdown. Climatic110
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
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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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Chapter 1 - Introduction• The abi
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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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Wind Erosion in Western Queensland Australia

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