Field evidence for the upwind velocity shift at the crest of low dunes

Wind topographically forced by hills and sand dunes accelerates on the upwind (stoss) slopes and reduces on the downwind (lee) slopes. This secondary wind regime, however, possesses a subtle effect, reported here for the first time from field measurements of near-surface wind velocity over a low dune: the wind velocity close to the surface reaches its maximum upwind of the crest. Our field-measured data show that this upwind phase shift of velocity with respect to topography is found to be in quantitative agreement with the prediction of hydrodynamical linear analysis for turbulent flows with first order closures. This effect, together with sand transport spatial relaxation, is at the origin of the mechanisms of dune initiation, instability and growth.Comment: 13 pages, 6 figures. Version accepted for publication in Boundary-Layer Meteorolog

International demands for austerity: examining the impact of the IMF on the public sector

What effects do International Monetary Fund (IMF) loans have on borrow-ing countries? Even after decades of research, no consensus exists. We offer a straight-forward explanation for the seemingly mixed effects of IMF loans. We argue thatdifferent loans have different effects because of the varied conditions attached to IMFfinancing. To demonstrate this point, we investigate IMF loans with and withoutconditions that require public sector reforms in exchange for financing. We find thatthe addition of a public sector reform condition to a country’s IMF program signifi-cantly reduces government spending on the public sector wage bill. This evidencesuggest that conditions are a key mechanism linking IMF lending to policy outcomes.Although IMF loans with public sector conditions prompt cuts to the wage bill in theshort-term, these cuts do not persist in the longer-term. Borrowers backslide oninternationally mandated spending cuts in response to domestic political pressures

Early‐stage aeolian protodunes: Bedform development and sand transport dynamics

Early‐stage aeolian bedforms, or protodunes, are elemental in the continuum of dune development and act as essential precursors to mature dunes. Despite this, we know very little about the processes and feedback mechanisms that shape these nascent bedforms. Whilst theory and conceptual models have offered some explanation for protodune existence and development, until now, we have lacked the technical capability to measure such small bedforms in aeolian settings. Here, we employ terrestrial laser scanning to measure morphological change at the high frequency and spatial resolution required to gain new insights into protodune behaviour. On a 0.06 m high protodune, we observe vertical growth of the crest by 0.005 m in two hours. Our direct measurements of sand transport on the protodune account for such growth, with a reduction in time‐averaged sediment flux of 18% observed over the crestal region. Detailed measurements of form also establish key points of morphological change on the protodune. The position on the stoss slope where erosion switches to deposition is found at a point 0.07 m upwind of the crest. This finding supports recent models that explain vertical dune growth through an upwind shift of this switching point. Observations also show characteristic changes in the asymmetric cross‐section of the protodune. Flow‐form feedbacks result in a steepening of the lee slope and a decline in lower stoss slope steepness (by 3°), constituting a reshaping of protodune form towards more mature dune morphology. The approaches and findings applied here, (a) demonstrate an ability to quantify processes at requisite spatial and temporal scales for monitoring early‐stage dune evolution, (b) highlight the crucial role of form‐flow feedbacks in enabling early‐stage bedform growth, alluding to a fluctuation in feedbacks that require better representation in dune models, and (c) provide a new stimulus for advancing understanding of aeolian bedforms

Coupling leeside grainfall to avalanche characteristics in aeolian dune dynamics

Avalanche (grainflow) processes are fundamental drivers of dune morphodynamics and are typically initiated by grainfall accumulations. In sedimentary systems, however, the dynamism between grainfall and grainflow remains unspecified because simple measurements are hampered by the inherent instability of lee slopes. Here, for the first time, terrestrial laser scanning is used to quantify key aspects of the grainfall process on the lee (slip face) of a barchan sand dune. We determine grainfall zone extent and flux and show their variability under differing wind speeds. The increase in the downwind distance from the brink of peak grainfall under stronger winds provides a mechanism that explains the competence of large avalanches to descend the entire lee slope. These findings highlight important interactions between wind speed, grainfall, and subsequent grainflow that influence dune migration rates and are important for correct interpretation of dune stratigraphy

Coupling leeside grainfall to avalanche characteristics in aeolian dune dynamics

Avalanche (grainflow) processes are fundamental drivers of dune morphodynamics and are typically initiated by grainfall accumulations. In sedimentary systems, however, the dynamism between grainfall and grainflow remains unspecified because simple measurements are hampered by the inherent instability of lee slopes. Here, for the first time, terrestrial laser scanning is used to quantify key aspects of the grainfall process on the lee (slip face) of a barchan sand dune. We determine grainfall zone extent and flux and show their variability under differing wind speeds. The increase in the downwind distance from the brink of peak grainfall under stronger winds provides a mechanism that explains the competence of large avalanches to descend the entire lee slope. These findings highlight important interactions between wind speed, grainfall, and subsequent grainflow that influence dune migration rates and are important for correct interpretation of dune stratigraphy

Assessing landscape dust emission potential using combined ground‐based measurements and remote sensing data

Modeled estimates of eolian dust emission can vary by an order of magnitude due to the spatiotemporal heterogeneity of emissions. To better constrain location and magnitude of emissions, a surface erodibility factor is typically employed in models. Several landscape‐scale schemes representing surface dust emission potential for use in models have recently been proposed, but validation of such schemes has only been attempted indirectly with medium‐resolution remote sensing of mineral aerosol loadings and high‐resolution land surface mapping. In this study, we used dust emission source points identified in Namibia with Landsat imagery together with field‐based dust emission measurements using a Portable In‐situ Wind Erosion Laboratory wind tunnel to assess the performance of schemes aiming to represent erodibility in global dust cycle modeling. From analyses of the surface and samples taken at the time of wind tunnel testing, a Boosted Regression Tree analysis identified the significant factors controlling erodibility based on Portable In‐situ Wind Erosion Laboratory dust flux measurements and various surface characteristics, such as soil moisture, particle size, crusting degree, and mineralogy. Despite recent attention to improving the characterization of surface dust emission potential, our assessment indicates a high level of variability in the measured fluxes within similar geomorphologic classes. This variability poses challenges to dust modeling attempts based on geomorphology and/or spectral‐defined classes. Our approach using high‐resolution identification of dust sources to guide ground‐based testing of emissivity offers a valuable means to help constrain and validate dust emission schemes. Detailed determination of the relative strength of factors controlling emission can provide further improvement to regional and global dust cycle modeling

Assessing landscape dust emission potential using combined ground‐based measurements and remote sensing data

Modeled estimates of eolian dust emission can vary by an order of magnitude due to the spatiotemporal heterogeneity of emissions. To better constrain location and magnitude of emissions, a surface erodibility factor is typically employed in models. Several landscape‐scale schemes representing surface dust emission potential for use in models have recently been proposed, but validation of such schemes has only been attempted indirectly with medium‐resolution remote sensing of mineral aerosol loadings and high‐resolution land surface mapping. In this study, we used dust emission source points identified in Namibia with Landsat imagery together with field‐based dust emission measurements using a Portable In‐situ Wind Erosion Laboratory wind tunnel to assess the performance of schemes aiming to represent erodibility in global dust cycle modeling. From analyses of the surface and samples taken at the time of wind tunnel testing, a Boosted Regression Tree analysis identified the significant factors controlling erodibility based on Portable In‐situ Wind Erosion Laboratory dust flux measurements and various surface characteristics, such as soil moisture, particle size, crusting degree, and mineralogy. Despite recent attention to improving the characterization of surface dust emission potential, our assessment indicates a high level of variability in the measured fluxes within similar geomorphologic classes. This variability poses challenges to dust modeling attempts based on geomorphology and/or spectral‐defined classes. Our approach using high‐resolution identification of dust sources to guide ground‐based testing of emissivity offers a valuable means to help constrain and validate dust emission schemes. Detailed determination of the relative strength of factors controlling emission can provide further improvement to regional and global dust cycle modeling

Total vertical sediment flux and PM10 emissions from disturbed Chihuahuan Desert surfaces

Desert surfaces are typically stable and represent some of the longest-lived landforms on Earth. For surfaces devoid of vegetation, the evolution of a desert pavement of gravel and small stones protects the surface from erosion by wind and water and vegetation further protects the surface in arid and semi-arid rangelands. The susceptibility of the land surface to wind erosion is enhanced by mechanical damage to the desert pavement or vegetation losses resulting from fire or grazing. Despite the relatively rich literature on the effects of grazing and fire on plant community composition, land degradation, and the productivity of arid landscapes, little is known about the effects of moderate grazing or fire on the erodibility of soils in desert grasslands and shrublands. Here we investigate the effects of simulated moderate grazing, simulated livestock trampling, and of fire on the resulting wind erodibility and dust emissions of the affected soil surfaces. We surveyed 24 plots of the same size, 6 m × 0.6 m, at a research site in the northern Chihuahuan Desert including 6 plots in a shrub-grass ecotone, 12 plots in an adjacent grassland, and 6 plots in an area that had been burned by a natural wildfire 6 months earlier but had no vegetation recovery due to the time of year and drought. To evaluate the various effects of disturbances on the susceptibility of the surface to wind erosion and dust entrainment, replicates of three plots underwent different treatments including clipping, trampling, fire, and tillage. We subsequently tested each of the treated plots with a portable field wind tunnel run at 12.6 m s− 1. We found that moderate grazing and fire did not result in great soil loss in desert grasslands but that shrublands were more seriously affected by grazing and fire. Total removal of vegetation and disturbance of the soil surface did result in greater than order of magnitude increases of vertical sediment flux and greater than three-fold increases of dust emissions