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Abstract: Changes in surface conditions on cold-climate aeolian dunes are pronounced; during winter dunes are wet, snow covered, and/or frozen for extended periods of time. It is unknown how the critical wind speed for sediment transport (“threshold”) varies and how threshold may influence sediment transport predictions. Although the impact of surface conditions on threshold has been examined in synthetic experiments (wind tunnels), complicated feedbacks between threshold, sand transport, and surface conditions that occur in natural environments suggest that a ground-based empirical approach may provide enhanced insight. In this study we investigate threshold variability for 73days during fall–winter–spring surface conditions from 18 November 2008 to 30 May 2009 in the Bigstick Sand Hills of Saskatchewan, Canada. Simultaneous measurements of threshold and atmospheric variables (air temperature, relative humidity, solar radiation, wind speed and direction) were used to examine the extent to which surface erodibility was regulated by meteorology. Time‐lapse images of the surface from a co-located camera were used for quality control and interpreting changes in the surface affecting threshold. Results reveal that threshold varied throughout the deployment (25–75% quartiles: 6.92–8.28ms−1; mean: 7.79ms−1). Threshold variability was especially evident at two scales: (i) event timescale and (ii) seasonal timescale. Event-scale variability peaked during mid-winter; in one event the threshold varied by 6ms−1 in 2h with freezing and re-freezing of the surface and relatively constant atmospheric conditions. The causes of event-scale variability are complex though qualitatively related to changes of wind direction, antecedent meteorological conditions, and vertical variations of grain-scale bonding agents such as pore ice and moisture. Seasonal-scale changes manifested as an increase in threshold during fall, peaking in mid-winter, and decreasing in spring. Increased threshold in mid-winter was linked to lower insolation and air temperature, suggesting low erodibility due to the presence of pore ice. Correlation coefficients of threshold versus atmospheric variables yielded relatively weak correlations (air temperature: r =−0.322; relative humidity: r =0.388; solar radiation: r =−0.309) that also varied according to wind direction, suggesting that the link between atmospheric conditions and surface erodibility on cold‐climate dunes is complex. This contrasts with results from field-based studies in warmer climates and controlled wind tunnel experiments, which show a more direct link between atmospheric variables (temperature and humidity) and surface erodibility. Nevertheless, our results do show a seasonal pattern of threshold that could be important for modeling cold-climate aeolian sediment transport. [Copyright &y& Elsevier]

Abstract: Aeolian sediment transport threshold is commonly defined as the minimum wind speed (or shear stress) required for wind-driven sediment transport. Accurate and consistent quantification of this threshold is essential because it is an input variable in models used to predict wind erosion, dune activity, and dust emissions. The majority of threshold quantification has been performed with analogs (analytical models or wind tunnels); however, in the past few decades field-based approaches to threshold parameterization have become more common. Although several methods of calculating transport threshold from field data are available, their comparability is unknown. To address this issue we collected high resolution sediment transport and wind measurements (1Hz) on an active sand dune for 11days and compared four different methods of calculating threshold: (i) time fraction equivalence method (TFEM); (ii) Gaussian time fraction equivalence method (GTFEM); (iii) instantaneous method; and (iv) regression method. Time-paired measurements from the two most widely used methods (TFEM and GTFEM) were strongly correlated (r =0.977); however, correlations between other methods varied (from r =0.861 to r =0.261). To demonstrate the implications of using different threshold calculation methods we predicted mass transport, which ranged from 63.6 (instantaneous method) to 126.6kg per crosswind meter (regression method). This inconsistency suggests that the threshold calculation method could have an appreciable impact on transport predictions. Threshold values are similarly inconsistent when the measurement interval is modified. As such, we do not recommend comparing any measured threshold with another. We discuss several strategies that may mitigate the impact of this issue such as clarification of semantics and method standardization. We also discuss several criticisms of field-based threshold measurements and re-conceptualizations that could allow investigators to develop a better understanding of field-based measurements. Overall, results from this study could allow future investigators to improve threshold (and transport) predictions. [Copyright &y& Elsevier]

Progress in some areas of process-based aeolian sediment transport research is hampered by limited opportunities for data comparison, synthesis, and integration. This is partially due to a lack of reliable comparison methods. Many comparison methods are forms of calibration that are either restrictive (e.g., time-averages only) or non-existent (e.g., for field-based sediment transport thresholds or vertical mass flux profiles). It is believed that the adoption of standard methods for common measurements may improve inter-study comparison, add value and longevity to data, and advance integrative modeling efforts. Examples of approaches in allied disciplines where standards are used routinely are reviewed and we discuss how the mutual benefits of standardized data could outweigh perceived disadvantages. Overall, the goal of this commentary is to encourage discussion, self-assessment, and forethought with regard to measurement methods used in process-based aeolian geomorphology. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Piezoelectric sediment transport sensors (PSTSs) are commonly used to detect aeolian sediment transport. Detection of particles in the near-surface airstream can be used to derive measures of sediment transport threshold, which is an important parameter in sediment transport modeling. However, despite common usage, little comparative field data regarding the detection capabilities of PSTSs are available. This study compares the sediment transport detection of four PSTSs: Sensit H11-B, Sensit H11-LIN (10× configuration), Safire, and Sensit H11-LIN (1× configuration). These sensors were co-located on an active sand dune for 11days with data measured and recorded at 1Hz. During this period the relative proportion of time that sediment transport occurred was as follows: Sensit H11-B: 7.212%, Sensit H11-LIN 10×: 3.260%, Safire: 1.832% and, Sensit H11-LIN 1×: 0.089%. These relative differences demonstrate that the transport detection capabilities of the sensors are inconsistent. We find that the cylindrical design and variable sensitivities restrict straightforward prediction of field detection according to sensor specifications. From these data we demonstrate that the response of each sensor influences the estimate of sediment transport threshold. Regardless of the source of variability, the very presence of detection inconsistency is problematic. Overall, results from this investigation indicate that comparison of metrics derived from measures of sediment transport presence/absence (such as threshold) with different PSTSs is, at present, tenuous. [Copyright &y& Elsevier]

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