@article {JRN55068, title = {Soil Organic Carbon Erosion is Critical for Land Degradation Neutrality}, journal = {Environmental Science and Policy}, volume = {93}, year = {2019}, month = {03/2019}, pages = {43-52}, chapter = {43}, abstract = {

The Land Degradation-Neutrality (LDN) framework of the United Nations Convention to Combat Desertification (UNCCD) is underpinned by three complementary interactive indicators (metrics: vegetation cover, net primary productivity; NPP and soil organic carbon; SOC) as proxies for change in the land-based natural capital. The LDN framework assumes that SOC changes slowly primarily by decomposition and respiration of CO2 to the atmosphere. However, we show there is growing evidence that SOC erosion also reduces SOC stocks and rapidly after land use / cover change. Here we use a physically-based model to quantify global SOC erosion by wind (2001-2010) and identify global dryland regions where SOC erosion may be a significant problem for achieving LDN. Selected sites in global drylands show SOC erosion accelerating over time. Without targeting and reducing SOC erosion, management practices will fail to sequester SOC and reduce land degradation, thereby undermining the LDN framework. We describe the interrelated nature of the LDN indicators, the importance of including SOC erosion and how land cover can be replaced by erosion that better represents the physical effects of land cover on land degradation processes. Therefore, we call for SOC erosion to become an LDN indicator. Furthermore, our results suggest that fluxes of SOC and soil nutrients by soil erosion should be emphasised in the forthcoming Intergovernmental Panel on Climate Change (IPCC) Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems.

}, keywords = {land cover, land degradation neutrality, sequestration, soil organic carbon, wind erosion}, doi = {doi.org/10.1016/j.envsci.2018.12.020}, url = {files/bibliography/19-009.pdf}, author = {A. Chappell and Nicholas Webb and John Leys and Waters, Cathy and Orgill, Susan and Eyres, Michael} } @article {JRN54888, title = {Improving ground cover monitoring for wind erosion assessment using MODIS BRDF parameters}, journal = {Remote Sensing of Environment}, volume = {204:756-768.}, year = {2018}, month = {09/29/2017}, abstract = {

Measuring and monitoring controls on wind erosion can facilitate detection and prediction of soil degradation important for food security. Ground cover is widely recognised as an important factor for controlling soil erosion by wind and water. Consequently, maintaining ground cover (e.g., vegetation, crop canopy, crop residue) is a recommended management practice which is widely adopted by farmers and land owners. Wind erosion is a lateral or horizontal process and the amount of ground cover needed to maintain lateral cover (Lc =nbh/S where n roughness elements occupy ground area S and have b and h mean breadth and height, respectively) is not wellestablished. Soil may be removed from beneath or between crop and natural vegetation canopies depending on the width, height and distribution of cover types relative to wind direction and strength. Monitoring by repeated measurement or estimation of ground cover provides information to develop an understanding of its spatial and temporal variation. Fractional cover (fc) retrieved from optical satellite remote sensing (e.g., Moderate Resolution Imaging Spectroradiometer; MODIS) provides a consistent and repeatable measure of ground cover when viewed from above. Therefore, fc provides an areal assessment of components of ground cover. Fractional cover is consequently not the most appropriate approximation of the protection of the soil from wind erosion. Extant wind erosion model parameterisations of Lc already benefit from the use of satellite-derived cover data (Lfc). However, the parameterisations are not well developed. Here, we address the need for a dynamic (multitemporal), moderate resolution and global metric for wind erosion assessment and modelling. We demonstrate the benefits of using Lc within the context of monitoring ground cover for the assessment of wind erosion and review the basis for estimating ground cover using Lc. We describe a new method for an albedo-based approximation of aerodynamic sheltering (L{\textquoteright}). We use ray-casting of rough surfaces from an existing wind tunnel study to establish a relation between measured Lc and directional hemispherical reflectance {\textquoteright}dir(0{\textdegree},{\textquoteright}), the socalled {\textquoteleft}black-sky albedo{\textquoteright} and its inverse to estimate shadow. The relation is confirmed to be dependent on the solar zenith angle ({\textquoteright}) and spectral ({\textquoteright}) confounding factors (e.g., soil moisture, soil mineralogy). We reduced the {\textquoteright} dependency of {\textquoteright}dir(0{\textdegree},{\textquoteright}) by normalising with the MODIS (MCD43A1) BRDF parameter fiso to estimate albedobased lateral cover (L{\textquoteright}) globally over space (500 m pixels) and time (every 8 days). We compared L{\textquoteright} with fc and Lfc over time for selected locations in Australia and examined L{\textquoteright} across Australia and the USA using national biogeographic regions. Consistent with current approaches to estimating Lc, our results were not field validated due to the dearth of ground-based measurements. However, our results demonstrate that L{\textquoteright} will improve wind erosion models particularly over large areas and L{\textquoteright} is likely to be a valuable source of decision-support information to guide policy makers and land managers on where, when and how to reduce wind erosion.

}, doi = {10.1016/j.rse.2017.09.026}, url = {files/bibliography/18-005.pdf}, author = {A. Chappell and Nicholas Webb and Guerschman, J.P. and Dean Thomas and G. Mata and Handcock, R. and John Leys} } @conference {JRN55060, title = {Significance for dust emission modelling of omitting changing aerodynamic roughness}, booktitle = {International Conference on Aeolian Research}, year = {2018}, month = {06/2018}, address = {Bordeaux, France}, abstract = {

Experiments of dust emission processes have informed the development and evolution of dust emission schemes that underpin global dust emission models (GDEMs) [1, 2]. Despite these improvements, many GDEMs retain only crude approximations of the emission processes because they do not adequately balance upscaling, parameterisation parsimony and process fidelity [3]. The GDEMs focus on static and homogeneous land surface aerodynamic roughness (and soil characteristics, not tackled here). This approach omits the spatio-temporal dynamics of soil surface shear stress due to land use / land management change, CO2-fertilisation and drought amplification. Consequently, observed reduction in dust emission in e.g., the west African Sahel cannot be replicated by GDEMs [4, 5]. The same omission very likely causes modelled wind speed outputs to be much more uncertain than currently recognized. For example, observed wind stilling is absent in modelled wind speed outputs [6]. To establish the significance of these omissions, we improve shear stress dynamics using an albedo-based parameterization of the aerodynamic roughness normalized by wind speed [7]. We use MODIS albedo (MCD43A1 8-daily, 500 m) and GLDAS modelled wind speed data and other global datasets to investigate the spatio-temporal dynamics of global and dust emission. Global drylands show distinct and opposing trends. Modelled winds dominate dust emission inconsistent with trends. Omitting changing aerodynamic roughness in wind data and dust emission will render GDEM outputs ineffective and interpretations unreliable. A solution is to use u_S*/U_f and embed GDEMs in to LSMs coupled to GCMs. Improved modelling of dust emission spatio-temporal dynamics will likely reduce uncertainty in radiative forcing and biogeochemical cycling for climate change projections.

}, author = {A. Chappell and Nicholas Webb} } @conference {JRN54636, title = {Using shadow to reformulate wind erosion modelling, mapping and monitoring}, booktitle = {International Conference on Aeolian Research}, year = {2016}, month = {07/2016}, publisher = {International Conference on Aeolian Research}, organization = {International Conference on Aeolian Research}, address = {Mildura, Australia}, abstract = {
Wind erosion and dust emission models are required to assess the impacts of dust in the Earth system. We describe a need in aeolian research to adequately represent the spatial variability and particularly the area average of the key aerodynamic properties which influence these models and our understanding of the processes. The models are underpinned by a geometric property (lateral cover; L) used to characterise the aerodynamic roughness (sheltered area or wakes) of the Earth{\textquoteright}s surface and calibrate the momentum it extracts from the wind. We reveal an error in the calculation of L and demonstrate that significant aerodynamic interactions between roughness elements and their sheltered areas have been omitted particularly under sparse surface roughness. We describe a solution which develops earlier work to establish a relation between sheltered area and the proportion of shadow over a given area; the inverse of direct beam directional hemispherical reflectance (black sky albedo; BSA). We show direct relations between BSA and wind tunnel measurements and thereby provide direct calibrations of key aerodynamic properties. We demonstrate how those properties can be retrieved from BSA from any platform (mast, aircraft, satellite). MODIS BSA across Australia is used to illustrate how areal estimates of aerodynamic properties improve wind erosion modelling, mapping and monitoring frequently in time (8 days) and space (500 m) across scales of variation (grain to landscape). We show that this approach provides new opportunities to investigate the dynamics of wind erosion in space and time and elucidate aeolian processes across scales.
}, author = {A. Chappell and Nicholas Webb} } @article {JRN54095, title = {Australian net (1950s-1990) soil organic carbon erosion: implications for CO2 emission and land-atmosphere modelling}, journal = {Biogeosciences}, volume = {11}, year = {2014}, month = {09/2014}, pages = {5235-5244}, chapter = {5235}, abstract = {

The debate remains unresolved about soil erosion substantially offsetting fossil fuel emissions and acting as an important source or sink of CO2. There is little historical land use and management context to this debate, which is central to Australia\&$\#$39;s recent past of European settlement, agricultural expansion and agriculturally induced soil erosion. We use \"catchmen\" scale (25 km2) estimates of 137Cs-derived net (1950s\–1990) soil redistribution of all processes (wind, water and tillage) to calculate the net soil organic carbon (SOC) redistribution across Australia.\  We approximate the selective removal of SOC at net eroding locations and SOC enrichment of transported sediment and net depositional locations. We map net (1950s\–1990) SOC redistribution across Australia and estimate erosion by all processes to be 4 Tg SOC yr-1, which represents a loss of 2\% of the total carbon stock (0\–10 cm) of Australia. Assuming this net SOC loss is mineralised, the flux (15 TgCO2-equivalents yr-1) represents an omitted 12\% of CO2-equivalent emissions from all carbon pools in Australia. Although a small source of uncertainty in the Australian carbon budget, the mass flux interacts with energy and water fluxes, and its omission from land surface models likely creates more uncertainty than has been previously recognized.

}, doi = {10.5194/bg-11-5235-2014}, url = {/files/bibliography/14-030.pdf}, author = {A. Chappell and Nicholas Webb and Raphael A. Viscarr Rossel and Bui, Elisabeth} } @proceedings {JRN54046, title = {Retrieving land surface aerodynamic properties using MODIS albedo }, year = {2014}, month = {07/2014}, pages = {p. 8}, publisher = {International Society of Aeolian Research (ISAR)}, address = {Lanzhou, China}, abstract = {

Wind erosion and dust emission models are essential to estimate the lateral and vertical fluxes of carbon and nutrients between terrestrial and marine ecosystems and reduce uncertainty about dust radiative forcing of climate models. \ Wind erosion models approximate the turbulent transfer of momentum by surface roughness elements using roughness density (lateral cover or the frontal area index; Lc). \ Instead of using the aeordynamic roughness length (z0) Raupach et al. (1993) demonstrated using Marshall\’s (1971) wind tunnel data how the threshold friction velocity ratio\  Rt could be estimated as Rt(Lc). However,\  estimation of Lc over large areas (e.g., continents) is difficult and often approximated using classifications of cover (bare surfaces and vegetation type).\ \  When rough surfaces are exposed to the wind, wakes or areas of flow separation are created downwind of obstacles. These sheltered areas reduce the area of exposed substrate and protect the erodible surface \ and some of the roughness elements from the wind (depending on their size and spacing). These turbulent wakes have been conceptualised using ssheltered areas (Raupach, 1992) and shown to be proportional to shadow (Chappell et al., 2010). We made virtual reconstructions of Marshall\’s (1971) wind tunnel surfaces and applied a ray-tracer to approximate their directional (at-nadir) hemispherical \‘black-sky\’ albedo (\ωdiff(\θ, \λ)) as a function of waveband (\λ) and solar zenith angle (\θ) and their diffuse \ bi-hemispherical \‘white-sky\’ albedo \ωdiff(\θ). We normalised the albedo (\ωn=dir(\θ) / \ωdiff) such that it would not depend on (or reduce the influence of) the solar spectrum and estimated Rt(\ωn) and the friction velocity U*(\ωn) both without the use of Lc or z0. The Moderate Resolution Imaging Spectroradiometer (MODIS) includes a bi-directional reflectance model of surface roughness which provides a direct beam (local solar noon) \ωMdir and a diffuse \ωMdiff products. Normalising these products (\ωMn = \ωMdir / \ωMdiff) was equivalent to the ray-traced estimates and enabled a global estimate\  of Rt(\ωMn) and U*(\ωMn) every 500 m and every 16 days consistently using a single holistic framework of albedo. This approach also unifies the erosivity and erodibility components of wind erosion and dust emission models. We illustrate the information content of these new estimates using several spatial and temporal scales at various locations around Australia prone to wind erosion and dust emission.

}, author = {A. Chappell and Guerschman, J.P. and John Leys and Handcock, R. and Dean Thomas and G. Mata and Harry Butler and Nicholas Webb} } @conference {JRN53999, title = {The role of carbon dust emission as a global source of atmospheric CO2}, booktitle = {Soil Carbon Sequestration for Climate, Food Security and Ecosystem Services International Conference}, year = {2013}, month = {05/2013}, address = {Reykjavid, Iceland}, abstract = {

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 (CO2) budget, because wind erosion contributes to the C cycle by selectively removing SOC from vast areas and transports C dust quickly offshore; augmenting the net loss of carbon from terrestrial systems. However, the contribution of wind erosion to rates of carbon release and sequestration is poorly understood. Here we show that SOC dust emission is omitted from national carbon accounting, is an under-estimated source of CO2 and a largely unaccounted-for, likely accelerant of SOC decomposition. We developed a first approximation to SOC enrichment for a dust emission model and quantified SOC dust emission for Australia (5.83 Tg CO2-e y-1) and Australian agricultural soils (0.4 Tg CO2-e y-1). These amount to under-estimates for CO2 emissions of ~10\% from combined C pools in Australia (yr=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 also likely represents a considerable underestimate from those nation\’s carbon accounts.

}, author = {A. Chappell and Nicholas Webb and Harry Butler and Craig Strong and Grant McTainish and Lleys, John and Raphael A. Viscarr Rossel} } @article {49842, title = {Soil organic carbon dust emission: an omitted global source of atmospheric CO2}, journal = {Global Change Biology}, volume = {19}, year = {2013}, month = {07/2013}, pages = {3238-3244}, chapter = {3238}, abstract = {

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 (CO2) 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 a neglected source of CO2 and a likely accelerant of SOC decomposition. Similarly, long dust residence times in the unshielded atmospheric environment may considerably increase CO2 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 CO2-e y-1) and Australian agricultural soils (0.4 Tg CO2-e y-1). These amount to under-estimates for CO2 emissions of ~10\% from combined C pools in Australia (yr=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{\textquoteright} C accounts. We suggest that the omission of SOC dust emission from C cycling and C accounting is a significant source of uncertainty. Tracing the fate of wind-eroded SOC in the dust cycle is therefore essential to quantifying the release of CO2 from SOC dust to the atmosphere and the contribution of SOC deposition to downwind C sinks.

}, keywords = {Australia, carbon accounting, carbon budgets, carbon dioxide, dust emission, soil organic carbon}, doi = {10.1111/gcb.12305}, url = {/files/bibliography/13-023.pdf}, author = {A. Chappell and Nicholas Webb and Harry Butler and Craig Strong and Grant McTainish and John Leys and Raphael A. Viscarr Rossel} } @proceedings {JRN54542, title = {Australian carbon dust emission: a carbon accounting omission?}, volume = {Sequestration - Accounting}, year = {2012}, month = {12/2012}, publisher = {Australian Society of Soil Science Conference}, address = {Hobart Tasmania}, abstract = {Erosion preferentially removes the finest carbon- and nutrient-rich soil fractions, and consequently its role may be significant within terrestrial carbon (C) cycles. However, the impacts of wind erosion on soil organic carbon redistribution are not considered in most SOC models, or within the Australian national carbon accounting scheme. Although SOC can be redistributed locally by water and wind erosion, dust emission can remove surface SOC from vast areas of inland and agricultural areas of Australia and transport it quickly offshore; representing a net loss of SOC from terrestrial systems. Estimates of the carbon dust emission magnitude require information on the spatial and temporal variation of SOC enrichment in dust emissions (P). We developed a process-based approximation of P within the Computational Environmental Management System (CEMSYS v5) national wind erosion model. It enabled the prediction of carbon dust emissions at a 50 km spatial resolution across Australia every month from 2000{\textendash}2010. Carbon dust emissions were summed for all months in the study period and across Australia to generate a time series of national carbon dust emissions. The magnitude, frequency and spatial variation of carbon dust emissions and their implications for Australian national carbon accounts will be discussed.}, author = {A. Chappell and Nicholas Webb and Harry Butler and Craig Strong and Grant McTainish and John Leys} }