Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controlsgeometry for particle shapes than cones. The outcome was a model that could be applied todetermine the threshold friction velocity of both dry and wet sediment:s fu* tw= A gd(2.25)fwhere u *tw is the threshold friction velocity for entrainment as affected by near surface soilmoisture, ρ s is the particle density (Mgm -3 ), ρ f is the fluid density (Mg m -3 ), g is thegravitational acceleration (ms -2 ), d is the particle diameter (m). A defines the effect ofmoisture on the threshold by the expression:A =A11+ w +A21( )sfgd21 +A3md2 dexpw 6.5w1.5(2.26)where A 1 , and A 2 are coefficients associated with aerodynamic and inter-particle forcesbetween dry particles, σ is the surface tension of the liquid (Nm -1 ), ψ md is the matric potentialat oven dryness, w is the gravimetric water content (kgkg -1 ), and w 1.5 is the gravimetric watercontent at -1.5 MPa (kgkg -1 ). The coefficient A 3 is associated with the effects of soil moisturethrough capillary and adhesive forces, and is determined by curve fitting to experimental datadepending on soil particle shape and dimensions. Cornelis et al. (2004b) calibrated the modelusing wind tunnel measurements of u *t at a range of soil water contents then compared modelpredictions with those of Chepil (1956). Good agreement was found between the models,although discrepancies were found between these models and those of Azzizov (1977), Hottaet al. (1985) and Chen et al. (1996).Limitations of the models are that they have been developed for limited particle size and soiltexture ranges. Model accuracy also tends to decline outside the ranges of data from whichthey were developed. Cornelis and Gabriels (2004a) reported that inconsistencies in themethods and timing used in previous research to identify the moment of particle mobilizationhas created differences in the model performance. Further, under moist conditions surfaceparticles may rapidly dry under high wind velocities, dropping below the entrainmentthreshold and mobilizing for a moment until particles are removed and the moist surface49