John Leys

Sub-basin scale dust source geomorphology detected using MODIS

[1]xa0The spatial and temporal variability of dust emissions from different surfaces in the Lake Eyre Basin, Australia is determined using MODIS data. For 2003–6 the sources of 529 dust plumes were classified: overall 37% of plumes originated in areas of aeolian deposits, 30% from alluvial deposits and floodplains and 29% from ephemeral lakes or playas. At this sub-basin scale, the relative importance of different dust source geomorphologies varied primarily in response to sediment supply and availability and was not related to aeolian transport capacity, suggesting the Lake Eyre Basin is a supply-limited system.

INFLUENCE OF CRYPTOGAMIC CRUST DISTURBANCE TO WIND EROSION ON SAND AND LOAM RANGELAND SOILS

A portable field wind tunnel was used to assess the sediment flux rates of loam and sand textured soils in the Mallee region of southeastern Australia. Three levels of crust disturbance (nil, moderate and severe) simulating stock trampling were investigated. The results demonstrated the importance of cryptogamic crusts in binding the soil surface and providing roughness after the soil was moderately disturbed. On the loamy soil, the crust helped maintain sediment flux rates below the erosion control target to 5 g m -1 s -1 for a 65 km h -1 wind measured at 10 m height. Once the crust was severely disturbed, sediment fluxes increased to 1·6 times the erosion target. On the sandy soil, even with no crust disturbance the sediment flux was 1·6 times the erosion control target. Disturbing the crust increased sediment fluxes to a maximum of 6·7 times the erosion control target. Removal of the crust also decreased the threshold wind velocity that resulted in an increase to the ris k of erosion from <5 per cent to 20 per cent.

Effects of soil moisture and dried raindroplet crust on saltation and dust emission

[1]xa0Field experiments using a sand particle counter and an optical particle counter clarified the links between saltation and dust emission under wet and weakly crusted conditions in a fallow field previously cultivated with wheat in Australia. A crust was formed by the impact of raindroplets after small precipitation events. A little soil moisture enhanced the strengths of crust and aggregation even though the soil was dried. Dust concentration was proportional to friction wind velocity, but the proportionality was dependent on ground surface conditions, such as the minimally dispersed particle size distribution of parent soil and the presence or absence of crust.

Parameterization of size‐resolved dust emission and validation with measurements

[1]xa0A critical problem in dust research is to estimate size-resolved dust emission rates. Several dust schemes have been proposed but are yet to be rigorously tested against observed data. In the recent Japan-Australia Dust Experiment (JADE), size-resolved dust fluxes were measured. In this study, the JADE data are used to test a size-resolved dust scheme. Our aim is to examine whether the scheme has the capability to predict size-resolved dust fluxes, what the ranges of the scheme parameters are, and whether the scheme is sensitive to the parameters. The JADE data show that dust emission depends linearly on saltation flux and thus confirm the basic assumption of the scheme. The magnitudes of the scheme parameters are found to be consistent with those reported in earlier studies. The estimated size-resolved dust fluxes are in satisfactory agreement with the measurements, although considerable discrepancies remain and are difficult to rectify without speculative tuning of the scheme input parameters. The discrepancies have been traced back to the uncertainties in the parent soil particle size analyses and in the dust flux observations. Ensemble tests showed both model physics uncertainties and parameter uncertainties. It is proposed that the dust scheme under consideration is not as sensitive as previously suspected and is likely to perform well if the parameters are specified within a reasonably correct range.

SEDIMENT FLUXES AND PARTICLE GRAIN-SIZE CHARACTERISTICS OF WIND-ERODED SEDIMENTS IN SOUTHEASTERN AUSTRALIA

Grain-size characteristics and the flux of sediment transported by wind from a cultivated paddock in a Quaternary relict dune field are described. Sediments were collected at seven heights between 0.7 and 2.0u2009m. The distribution of sediment mass with height is explained by a power function (of the order of −1), which is highly skewed towards the bed. The distribution of <90u2009μm sediment mass is explained by a log function of height and is less skewed towards the bed because these finer particles are influenced by the vertical velocity component of the wind. The particle-size distribution (PSD) of the eroded sediments is strongly influenced by the PSD of the parent material. Enrichment of the suspended sediment (PSA<90u2009μm) was in the order of 2.3 times. Sediment flux measurements show that 93 per cent of the erosion occurred in 3 per cent of the time.

Numerical simulation of the October 2002 dust event in Australia

[1]xa0In comparison to the major dust sources in the Northern Hemisphere, Australia is a relatively minor contributor to the global dust budget. However, severe dust storms do occur in Australia, especially in drought years. In this study, we simulate the 22–23 October 2002 dust storm using an integrated dust model, which is probably the most severe dust storm in Australia in at least the past 40 years. The model results are compared with synoptic visibility data and satellite images and for several stations, with high-volume sampler measurements. The model simulations are then used to estimate dust load, emission, and deposition, both for over the continent and for over the ocean. The main dust sources and sinks are identified. Dust sources include the desert areas in northern South Australia, the grazing lands in western New South Wales (NSW), and the farm lands in NSW, Victoria, and Western Australia, as well as areas in Queensland and Northern Territory. The desert areas appear to be the strongest source. The maximum dust emission is around 2000 μg m−2 s−1, and the maximum net dust emission is around 500 μg m−2 s−1. The total amount of dust eroded from the Australian continent during this dust event is around 95.8 Mt, of which 93.67 Mt is deposited on the continent and 2.13 Mt in the ocean. The maximum total dust load over the simulation domain is around 5 Mt. The magnitude of this Australian dust storm corresponds to a northeast Asian dust storm of moderate size.

Simulation of the spatiotemporal aspects of land erodibility in the northeast Lake Eyre Basin, Australia, 1980-2006

This paper explores spatiotemporal patterns in land erodibility in the northeast portion of the Lake Eyre Basin, Australia, using the Australian Land Erodibility Model (AUSLEM) in simulations from 1980 to 2006. First, spatial patterns in land erodibility are examined. We then present an analysis of seasonal and interannual variations in land erodibility. Patterns in land erodibility change are compared to rainfall variability, the El Nino–Southern Oscillation (ENSO) and Pacific (inter-) Decadal Oscillation (PDO). Land erodibility is found to peak in the study area between early spring (September, October, November) and late summer (January, February, March), and reach a minimum over winter (June, July, August). Weak correlations are found between modeled land erodibility, rainfall, ENSO, and the PDO. The results indicate a complex landscape response to climate variability, with land erodibility dynamics being affected by antecedent rainfall and vegetation conditions which generate lag responses in land erodibility change. The research highlights the importance of developing methods for monitoring conditions driving variations in wind erosion at the landscape scale to enhance land management policy in arid and semiarid landscapes at a time of uncertain future climate changes.

A model to predict land susceptibility to wind erosion in western Queensland, Australia

This paper describes the development and validation of the Australian Land Erodibility Model (AUSLEM), designed to predict land susceptibility to wind erosion in western Queensland, Australia. The model operates at a 5x5km spatial resolution on a daily time-step with inputs of grass and tree cover, soil moisture, soil texture and surficial stone cover. The system was implemented to predict land erodibility, i.e. susceptibility to wind erosion, for the period 1980-1990. Model performance was evaluated using cross-correlation analyses to compare trajectories of mean annual land erodibility at selected locations with trends in wind speed and observational records of dust events and a Dust Storm Index (DSI). The validation was conducted at four spatial length scales from 25 to 150km using windows to represent potential dust source areas centered on and positioned around eight meteorological stations within the study area. The predicted land erodibility had strong correlations with dust-event frequencies at half of the stations. Poor correlations at the other stations were linked to the inability of the model to account for temporal changes in soil erodibility, and comparing trends in the land erodibility of regions with dust events whose source areas lie outside the regions of interest. The model agreement with dust-event frequency trends was found to vary across spatial scales and was highly dependent on land type characteristics around the stations and on the types of dust events used for validation.

Further development of a parameterization for convective turbulent dust emission and evaluation based on field observations

Further developments of a parameterization scheme for convective turbulent dust emission (CTDE) are presented. The scheme is advanced by including (1) a new statistical description of instantaneous momentum flux, (2) a correction function for cohesive force to account for the effect of soil moisture, and (3) a correction function for lifting force to consider the effect of vegetation roughness elements. The probability density function describing instantaneous momentum flux is now derived from large-eddy simulations for different atmospheric stabilities. The vegetation correction function is based on a drag partition theory. Additional improvements on the representations of interparticle cohesive force and particle size distribution are introduced. The new CTDE scheme is tested against the field data obtained at a sand storm monitoring station in the Horqin Sandy Land in China in 2011 and during the Japan-Australia Dust Experiment in Australia in 2006.

Soil organic carbon dust emission: an omitted global source of atmospheric CO2

Soil erosion redistributes soil organic carbon (SOC) within terrestrial ecosystems, to the atmosphere and oceans. Dust export is an essential component of the carbon (C) and carbon dioxide (CO(2)) budget because wind erosion contributes to the C cycle by removing selectively SOC from vast areas and transporting C dust quickly offshore; augmenting the net loss of C from terrestrial systems. However, the contribution of wind erosion to rates of C release and sequestration is poorly understood. Here, we describe how SOC dust emission is omitted from national C accounting, is an underestimated source of CO(2) and may accelerate SOC decomposition. Similarly, long dust residence times in the unshielded atmospheric environment may considerably increase CO(2) emission. We developed a first approximation to SOC enrichment for a well-established dust emission model and quantified SOC dust emission for Australia (5.83 Tg CO(2)-e yr(-1)) and Australian agricultural soils (0.4 Tg CO(2)-e yr(-1)). These amount to underestimates for CO(2) emissions of ≈10% from combined C pools in Australia (year = 2000), ≈5% from Australian Rangelands and ≈3% of Australian Agricultural Soils by Kyoto Accounting. Northern hemisphere countries with greater dust emission than Australia are also likely to have much larger SOC dust emission. Therefore, omission of SOC dust emission likely represents a considerable underestimate from those nations C accounts. We suggest that the omission of SOC dust emission from C cycling and C accounting is a significant global source of uncertainty. Tracing the fate of wind-eroded SOC in the dust cycle is therefore essential to quantify the release of CO(2) from SOC dust to the atmosphere and the contribution of SOC deposition to downwind C sinks.