Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controlsincreasing inter-particle cohesion and physical crusting. At intermediate time scales (weeks),elevated soil moisture may induce biological crust growth and plant growth. Together thesereduce the availability of loose erodible sediment on the soil surface (reducing soilerodibility) and increase the surface roughness and therefore u *t . At longer times scales(months – seasons), rainfall effects on land erodibility are controlled by soil-vegetationfeedbacks, with moisture retention in the soil, soil crust condition and surface roughnesscontrolled by vegetation cover levels. In turn, these are controlled by rainfall amounts andfrequency. The feedbacks and the capacity of the landscape to retain moisture and sustainvegetation are regulated by climate through evaporation rates. Where potential evaporationexceeds rainfall, the plant available soil moisture will be low, vegetation cover will be low,soil aggregation and crusting will be low, and land erodibility will be elevated.2.2.3 Soil Texture and Grain Size EffectsBagnold (1941) used wind tunnel experiments to find values of u *t for a range of soil particlesizes. His results show a decreasing threshold velocity is required for entrainment given adecrease in particle diameter (Figure 2.3).Figure 2.3 Graph of the threshold friction velocities (u *t ) for a range of grain/particle sizes (afterBagnold, 1941). The two curves illustrate the difference between fluid and impact thresholds.38