Wind Erosion in Western Queensland Australia

Chapter 5 – Land Erodibility Model Developmenttree and stone cover effects on wind erosion, provide a more appropriate parameterisation ofthe erodibility continuum and further improve the model skill in assessing land susceptibilityto wind erosion.Vegetation EffectsVegetation effects on land erodibility are considered by separating the effects of woody(shrub/tree) and herbaceous (grass) cover. Herbaceous vegetation cover in arid and semi-aridenvironments tends to increase and decrease at rates faster than shrub- and tree cover inresponse to climate and management (Specht and Specht, 1999). While regional tree covercan be considered static over short periods, and its effect on wind erosion can be consideredby a simple threshold, the effects of temporally dynamic grass cover are better modelled by acontinuous function. The effects of grass cover (E gc ) on land erodibility are modelled by anegative exponential relationship:E gc(% gc)= exp(5.2)where α and β are regression coefficients (55.873; -0.0938) denoting the equation interceptand rate of change in erodibility given a change in percentage cover (Chapter 2, Figure 2.7).The relationship was determined by wind tunnel experimentation at wind speed 18 ms -1(Leys, 1991a) and is similar to that determined by Chepil (1944), and the Soil Loss Ratio(SLR) relating soil loss from a soil with cover to that of a bare soil (Fryrear, 1985; Findlateret al., 1990; Chapter 2, Section 2.2.8). While the expression was developed for prostrate (flatlying) wheat stubble, similar exponential relationships were determined by Siddoway et al.(1965) and Lyles and Allison (1981) for standing stubble. By increasing surface roughness,low cover levels of standing vegetation can reduce wind erosion to rates experienced athigher levels of prostrate cover. The mixture of prostrate and standing grass cover in thestudy region can be accounted for by adjusting the regression coefficients in Equation 5.2 toincrease or decrease the sensitivity of land erodibility to grass cover.Marshall (1972) determined a threshold for wind erosion control based on tree and shrubcover. The threshold exists where tree/shrub spacing is approximately 3.5 times the averagemaximum height of the vegetation, corresponding to roughly 20% lateral cover in the drylandshrub communities in Australia. Tree cover effects on land erodibility are modelled in138