Soil, the source of dust
Soils are formed by the weathering of rocks, and are also influenced by climate, organisms, landforms and the effect of time. Soils are classified into groups, but the exact combination of the above factors makes each soil unique. As the source of dust, knowing about soils is helpful in learning whereabouts dust comes from – transported dust matches most of the same characteristics as the dust component of the soil it came from. Scientists examine the different soils across Australia and use this information to work out whereabouts particular dust events originated. Soil erodibility by wind can be measured by testing soil strength, crust cover, the amount of erodible material on the soil surface, aggregation levels, particle-size characteristics and soil chemistry. Each soil type has a different susceptibility to wind erosion. Sandy soils common to the Mallee region of south western Victoria are highly vulnerable to wind erosion. This is attributed to their sandy texture, low aggregation and variable soil moisture. In contrast, the highly aggregated soils have a lower susceptibility.
Wind erosion and sediment loss
Wind is the transport agent in wind erosion. The speed of the wind must surpass a threshold in order to move sediments. Large particles (>90µm) are usually the first particles to move, hopping across the soil surface. Each time a large particle impacts the surface, more sediment is disturbed and released into the airstream, a bit like rainsplash. The smaller particles are lighter and therefore can be held a lot higher and longer aloft. It is this fine fraction which is called ‘dust’ that can travel a great distance in the air. Australian dust has been tracked to New Zealand and Antarctica. Dust also contains a lot of soil nutrients (stored in organic matter and clay) thus depleting the nutrients at the source (where the dust originated) and enriching nutrients at the sink (where the dust is deposited). For example, oceanic dust deposits are an important source of nutrient iron for marine organisms in the Southern Ocean.
Measuring sediment loss is problematic. A dust event could encompass one paddock, or the entire eastern Australia. Measurement of how ‘deep’ (high) the dust plume extends is difficult, relying on meteorological measurements and observations from airplane pilots.
The role of water in wind erosion
The greater the rainfall, the less wind erosion. This is due to an increase in soil moisture which in turn promotes vegetation growth – two factors that decrease a soils susceptibility to wind erosion. Soil moisture also increases the binding capacity of soil particles and encourages the growth of microorganisms which produce protective soil crusts. Continental scale wind erosion frequency can be related to the ENSO cycle following highs and lows of rainfall patterning. Through geological history, high periods of dust activity correlate with arid phases. On a smaller time scale, new research suggests that a little rain may in fact increase wind erosion rates. The disruption of soil particles from raindrop impact, without their redistribution (sealing effect) as with larger amounts of water, results in a soil with more loose particles. The loose particles may later be picked up by the wind. Another role that water plays in wind erosion is sediment supply. Water erosion in the internally draining river systems of the Lake Eyre Basin carries vast quantities of sediment downstream. The shallow floodplains of central Australia receive a supply of very fine sediments which, when dry, are often picked up by the wind and redistributed back across the continent.
The role of vegetation
Plants protect the soil against wind. Vegetation slows the wind velocity close to the soil surface by creating surface roughness. Another measurement is vegetation resistance to wind- a dense bush will produce a turbulent flow behind the bush, whilst a porous bush will allow wind through but slow its velocity. In the Mesquite (small prickly bush) country of southwest USA, the mesquite bushes are dense and regularly spaced. Despite the overall high vegetation cover, wind erosion occurs from the accelerated wind velocities between the bushes. Monitoring vegetation can be done in the field (using quadrat or transect surveying methods) or by remote sensing (using satellite imagery). Remote sensing of vegetation cover is frequently used for a large number of natural resource management issues. Attempts have been made to relate these measures to frequency of dust events, but desert vegetation (where a lot of wind erosion occurs) is difficult to monitor with these techniques. This is because the satellite technology uses colour (green) to detect vegetation; but, unfortunately, arid vegetation is often grey, silver, or brown, which means they escape detection by these instruments.
How does wind erosion affect our activities?
Wind erosion over time degrades soil resources. The nutrient rich components of soil are blown away by the wind, depleting the remaining soil of nutrients, organic matter and fine sediments. The loss of soil nutrients means increased need for fertilisers, and can therefore lead to increased costs. Wind erosion also affects the growth of vegetation - poor soils are less able to support plants; this means crops, pasture, or even the stubble. Severe wind erosion can undermine the built environment e.g. eroding the soil away from fence posts, or depositing huge quantities of sand along fence lines and across roads. Dust build-up on powerlines can cause major disturbances to power networks. Large dust storms are a huge cost to both the land and people; schools and businesses cancel work; asthma cases increase; transport systems are delayed. Dust is a health hazard to both people and livestock. A dusty environment also causes domestic disturbances and costs; keeping homes and businesses clean is much harder.