Wind Erosion in Western Queensland Australia

Chapter 2 – Land Erodibility Controlsdryland environments. These areas include sub-humid, semi-arid, arid and hyper-arid lands.Conditions driving aridity in drylands include (after Thomas, 2000:7):• Atmospheric stability: drylands are common in the sub-tropical high-pressure belts. Theseare zones of stable air beneath the descending arm of the Hadley Cell.• Continentiality: increased distance from oceans reduces the frequencies of moisturebearingweather systems and may enhance atmospheric stability.• Topography: rain shadows may develop in the lee of mountain ranges, increasing aridity.• Cold Ocean Temperatures: ocean currents affect atmospheric temperatures, humidity andprecipitation, and the effects of this may be continual (e.g. along the west coast of SouthAfrica) or dynamic (e.g. related to the El Niño/Southern Oscillation).Dryland environments are characterised by low precipitation and high annualevapotranspiration rates. Thornthwaite (1931) developed a precipitation-evaporation (P-E)ratio to provide an indicator of regional scale aridity:( E)1.1112 P P = (2.8)i=1 t + 12. 2 where (P-E) is the precipitation-evaporation ratio, P is the monthly mean precipitation (mm),t is the monthly mean temperature (°C) and i is the month. Chepil (1956) usedThornthwaite’s (1931) index of aridity to provide a climatic indicator (C) of wind erosionbased on windiness and moisture availability, a driver of vegetation cover and soil particlecohesion:3VC = (2.9)( P E) 2where V is the mean annual wind velocity (ms -1 ) at 9 m, and (P-E) is the index of aridity.Analyses of dust-storm frequencies in relation to rainfall indicate a non-linear relationshipbetween decreasing rainfall and increasing dust-event frequencies (e.g. Middleton, 1984;McTainsh et al. 1989). The effects of rainfall on land erodibility are seen at multiple spatialand temporal scales. At short time scales (days), rainfall affects the soil moisture content,37