The shape, orientation, and spacing of sand ripples provide insights into local sand transport conditions. Consequently, mapping ripple patterns can enhance our understanding of wind regimes on Mars and Earth, especially in areas lacking local wind observations. However, manually mapping these ripple patterns is time-consuming and subjective, underscoring the need for automated methods to analyze large areas consistently. Our goal was to automatically quantify three types of ripple patterns—straight ripples, sinuous ripples, and complex textures—and to map their distribution over barchan dunes. We introduce two innovative and complementary methods for identifying these ripple patterns in high-resolution satellite imagery of Martian dunes. The efficacy of both approaches was assessed on 42 barchan dunes across 6 HiRISE sites.

The first method, a machine learning model known as U-Net, proved to be more reliable in classifying the ripple patterns, achieving 86% precision, 82% recall, and 82% F1-score for image tiles, as well as 82% precision, 77% recall, and 79% F1-score for complete dune mappings. The second method, a spatial autocorrelation analysis called the 2D semi-variogram, performed poorly in classifying ripple patterns over entire dunes, with precision reaching up to 56%, recall at 45%, and F1-score at 39%. However, it excelled in accurately measuring ripple spacing (R² = 0.78) and orientation (R² = 0.98). By combining the U-Net model's efficiency in ripple classification with the 2D semi-variogram’s precision in measuring spacing and orientation, we can conduct extensive analyses of ripples and local wind regimes across Mars. Furthermore, these methods hold potential for application in drone imagery of terrestrial dunes, paving the way for further research and exploration.

How to cite: Delobel, L., Moffat, D., and Baas, A.: Automatic Mapping and Characterising Of Ripple Patterns on Sand Dunes using U-Net and 2D Semi-Variogram., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6353, https://doi.org/10.5194/egusphere-egu25-6353, 2025.

Aeolian dunes are formed by the accumulation of wind-blown sand and its movement over time. While their shape may reflect wind directions during the period of formation, stratigraphic features in aeolian dunes record past climate and environments. In northern Fennoscandia, abundant dune fields serve as detailed records of Holocene arctic climate change. Specifically, buried soil and charcoal layers observed in mostly parabolic dunes preserve a rich history of past changes in climate, environment, fire history and land use. Understanding the evolution of the dunes in the face of these changes is crucial to project how Arctic environments respond to future climate change. Such studies require knowledge on the location, size, and shape of the aeolian sand dunes, which involves mapping them as polygon features. However, manual mapping of these landforms is very time-consuming. Semi-automatization can be used to develop a transferable, reproducible and scalable mapping approach, overcoming this issue.

Here we utilize a semi-automatic method involving machine learning models to map aeolian sand dunes in two study areas in northern Finland. While we rely on manual mapping based on a 2 m digital elevation model (DEM), its hillshade and Google satellite imagery for establishing a training dataset, segmentation and classification of sand dune objects is carried out using DEM derivatives such as slope, convergence index, general curvature, and topographic position index through support vector machines and random forest algorithms. We validate the training dataset and mapping results during field campaigns. Our semi-automatic object-based mapping method enables a mostly correct identification of dune objects, including attribute information on their size, shape, and morphological characteristics, compared to our field investigation. False positives occur at locations of great similarity between parabolic dunes and other topographic features. Future steps include reducing the false positives and transferring the approach to additional areas to ultimately develop a method for automated, regional-scale mapping of aeolian sand dunes in Arctic Fennoscandia.

How to cite: Campo, L., Hölbling, D., Stammler, M., Stevens, T., Abad, L., and Baykal, Y.: Semi-automated mapping of aeolian sand dunes: A case study from northern Finland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5666, https://doi.org/10.5194/egusphere-egu25-5666, 2025.

The postglacial landscape of Northern Fennoscandia is characterised by abundant depositional landforms, including eskers and aeolian sand dunes. Stratigraphic markers in sand dunes, such as buried soils or charcoal bands, record past environmental and climatic changes, preserving a history of landscape instability and sand movement caused by climate, fire, and land use changes. Although the shape and orientation of the dunes reflect the dominant wind direction during their formation, the mapping of aeolian sand dunes in Fennoscandia remains limited to a few studies and is generally not conducted at the polygon level. To systematically analyse the location, shape, and orientation of dunes over large areas, scalable and reproducible semi-automated mapping approaches based on remote sensing data are needed, with results validated in the field.

While analysing and interpreting digital elevation models, derived hillshades, satellite imagery, and aerial photographs, we observed features we referred to as ‘double-dune ridges’; dunes appearing in direct proximity to each other that mimic each other’s shape and orientation. Ground-truthing fieldwork showed that the ridges are unlikely to represent two separate, quasi-parallel dunes but rather the remnants of single dunes where the crests have been eroded and excavated. In their present form, these mostly parabolic dunes can be described by stable, vegetation-covered windward (stoss) and leeward (lee) sides, with open to low pioneer vegetation covering areas where the former crest once existed.

We discuss this example of dune degradation in Northern Fennoscandia and illustrate how an interdisciplinary approach can provide valuable insights into the dynamics involved. We strongly believe that integrating detailed mapping with chronostratigraphic, tree ring, and remote sensing data is essential for understanding the processes leading to the excavation of the former crest of dunes and, more broadly, to landscape evolution and degradation.

How to cite: Hölbling, D., Stammler, M., Stevens, T., Baykal, Y., and Campo, L.: What causes aeolian sand dunes in Northern Fennoscandia to lose their crests? From mapping and dating to process understanding, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11579, https://doi.org/10.5194/egusphere-egu25-11579, 2025.

The Otindag dune field, located on the Inner Mongolia Plateau and covers an area of approximately 52,000 km², ranks among the four largest dune fields in China. Located near the northern boundary of the East Asian monsoon, this region exhibits a cold temperate monsoon climate that transitions from semi-arid to arid conditions. The mean annual temperature ranges from 0 to 3 °C, while winter temperatures average approximately -18.3 °C, with an extreme minimum of -36.6 °C, reflecting the region’s significant seasonal thermal variation. In the southern Otindag dune field, nebkhas, a distinctive biogeomorphological aeolian landform created by the accumulation of sand around vegetation are distributed. The relatively thick sand layers within nebkha dunes are excellent sedimentary archives with great potential to capture past climate and environmental changes that occurred during the historical period and the Anthropocene. However, establishing a robust chronological framework for nebkha sediments remains a significant challenge due to limitations in existing dating methods.

In this study, we employed both luminescence and Cs-137 dating techniques to establish a chronology for nebkha sediments in the Otindag dune field. Luminescence dating results demonstrate that quartz luminescence signals from nebkhas are relatively strong and well-suited for age framework construction. The ages derived from the Central Age Model (CAM) and Minimum Age Model (MAM) are largely consistent, indicating effective bleaching of the quartz luminescence signals. Conversely, single-aliquot K-feldspar dating results exhibit a significant overestimation of ages. To address this discrepancy, we applied single-grain (SG) techniques in conjunction with the Minimum Age Model (MAM) to refine the dating of K-feldspar samples, yielding ages in agreement with quartz results. Cs-137 dating further validated the chronology, revealing that the primary development of nebkhas occurred within the past 100 years, which may be attributed to climate-driven or human-induced aridification in this region. Compared to other dune field regions in northern China, such as the Mu Us dune field, quartz from the Otindag dune field exhibits higher luminescence sensitivity, whereas K-feldspar signals display significant bleaching challenges. This study hypothesizes that regional variations in mineral luminescence properties may be attributed to differences in sediment provenance, which will be further investigated in future research. This study enhances the applicability of luminescence dating techniques for young nebkha sediments, providing a methodological framework for other nebkha fields across the globe.

How to cite: Yi, S., Wang, Y., Xu, Z., and Li, S.: Multi-method dating of typical nebkha sediments in the Otindag dune field, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17100, https://doi.org/10.5194/egusphere-egu25-17100, 2025.

Aeolian processes on planetary surfaces are governed by atmospheric dynamics and surface properties, yielding similar landforms across a wide range of environments. On Earth, recent climate change, particularly the decline in near-surface wind speeds (terrestrial stilling) and variations in other climatic factors, has significantly influenced aeolian landscapes. In the inland deserts of Central and East Asia, these changes have led to reduced dune migration, dune stabilization and transformation, fewer dust storms, and increased vegetation recovery in the drylands.

This study investigates the coupled effects of wind speed reduction and vegetation restoration on the mobility and stability of sand dunes in northern China, located at the junction of the Gobi, Badain Jaran, and Tengger deserts. Vegetation changes were assessed using multiple NDVI time series, which revealed a general increase in vegetation across the study area, despite notable spatial heterogeneity. This greening trend is primarily driven by wetter, warmer, and less windy climatic conditions during the past few decades, further amplified by large-scale ecological restoration programs. While vegetation expansion has not fully stabilized the dunes in most areas, it has contributed to reduced migration rates and altered dune morphology.

In extreme arid zones with negligible vegetation growth over the past decades, dune migration rates were analyzed using Landsat satellite imagery (1986–2021) and the COSI-Corr procedure, constructing a 35-year time series across multiple sites. The results demonstrate a consistent decline in dune migration rates, aligning with the trend of terrestrial stilling. This suggests that recent adjustments in atmospheric circulation under global climate change have significantly slowed dune migration. Model calculations further reveal reduced sand flux under declining wind speeds, consistent with the cubic relationship between wind speed and sand transport, validating theoretical sand transport laws at larger spatial scales.

These findings underscore the critical role of atmospheric circulation in shaping aeolian landforms and emphasize the combined effects of climate change and human activities in stabilizing sand dunes. By integrating these terrestrial insights into planetary aeolian research, this study offers valuable analogues for understanding dune dynamics under varying atmospheric conditions across planetary surfaces.

How to cite: Wang, L. and Xu, Z.: Dune Slowdown and Stabilization in Inland Deserts of East Asia: The Role of Wind Stilling and Vegetation Recovery over Four Decades, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6569, https://doi.org/10.5194/egusphere-egu25-6569, 2025.

Wind erosion is a major driver shaping landscape patterns in drylands. Vegetation is known to provide protection from wind erosion by sheltering the ground surface, extracting wind momentum, and trapping sediment. Significant progress has been made in parameterizing the protective effect of vegetation elements as a function of vegetation cover for wind erosion model development. However, the influence of single vegetation elements on shear stress patterns at the surface in two dimensions, particularly in the spanwise direction where increases in surface shear stress and subsequent soil erosion have commonly been observed, has not been included in drag partition schemes to date. Studies on wind speed around a shrub (or other obstacles) in the spanwise direction underscore the potential role of vegetation in intensifying erosion issues, e.g., dust emissions, desertification and challenges in land management. Here, we quantify wind erosivity patterns around a single shrub within a semi-arid rangeland environment for a range of wind velocity and plant phenology. Field data were collected 2/7/2023-7/19/2023 and 2/12/2024-6/20/2024 at the National Wind Erosion Research Network site at the USDA Jornada Experimental Range in south-central New Mexico, USA. A total of 18 Irwin sensors were installed around a honey mesquite shrub (Prosopis glandulosa Torr.), mainly perpendicular to the dominant wind direction, to quantify surface shear velocity (u_*s, m/s). A sonic anemometer, installed at 1 m above the surface 3 m upwind of the shrub measured turbulent shear velocity (u_*, m/s). We use the shear stress ratio (SSR= u_*s/u_*) to delineate and quantify shelter and acceleration zones around the shrub. Our objectives were to: 1) compare the spatial patterns of erosivity; 2) examine the shelter and acceleration effects around the shrub at different wind speeds; and 3) examine the plant phenological influence on shelter and acceleration effects around the shrub. Results show that the shrub's impact on erosivity patterns varied depending on its phenological phase. On average, the largest SSR, approximately 1.5 times greater than that at the upwind location, occurred during the dormant and green-up phases, at a spanwise distance of 2 meters (about twice the shrub height) from the shrub center. The smallest SSR, about 39% of that at the upwind location, occurred closest to the shrub during the green-up phase in the spanwise direction and during the leaf-on phase at the downwind location. In the dormant phase, the shape of the shelter zones became more streamlined as wind speed increased. In the green-up phase, the size of the shelter area increased with wind speed, reaching its maximum extent. In the leaf on phase, the acceleration zone expanded as wind speed increased. Our results suggest that incorporating the 2-D surface shear velocity impacts around vegetation, i.e., including both acceleration and shelter effects, is needed to increase the accuracy of wind erosion models in vegetated dryland ecosystems.

How to cite: Zhang, P., Edwards, B., Webb, N., Gillies, J. (., Trautz, A., Nikolich, G., Ziegler, N., Okin, G., Van Zee, J., and Wheeler, B.: The impact of a single shrub on the distribution of surface shear velocity in a semi-arid rangeland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13657, https://doi.org/10.5194/egusphere-egu25-13657, 2025.

The relationships between aeolian sediments in dune fields and adjacent sedimentary environments are critical for understanding arid landscapes. They provide valuable proxies for paleoclimatic reconstructions, as shown in various desert regions worldwide. Studies have highlighted how aeolian environments modulate sediment transport in fluvial systems, acting as buffers (e.g., East et al., 2015), an essential consideration for comprehensive source-to-sink sediment budgets. While research has primarily focused on fluvial-aeolian interactions, studies on alluvial fan-aeolian interactions are limited. Alluvial fans, when not bypassed, are excellent sedimentary archives for reconstructing paleoclimates in arid regions. It is known that increased aridity tends to expand aeolian coverage over fan surfaces, whereas increased runoff activity restricts aeolian environments to distal fan areas, which then serve as sediment sources for sand dune fields. However, there is a gap in understanding how fans and aeolian sediments interact when both operate simultaneously, independent of climatic variability. To address this, we studied alluvial fans in the Atacama Desert, where prolonged aridity provides a natural laboratory to explore interactions between aeolian and alluvial fan processes, with exceptional preservation of surface morphologies. Rare episodic storms generate runoff that transports sediments from catchments to alluvial fans, which may be partially or fully covered by aeolian sands. The selected fans exhibit debris flow lobes across all fan segments, not just at the apex.

Our study investigates how fan morphology (e.g., roughness and relief) (Cook and Pelletier, 2007) controls saltation transport processes and pathways, and examines the interactions between dune formation and debris flow lobes. By analyzing surface grain size and topography and leveraging Synthetic Aperture Radar (SAR) backscatter intensity data from C and L Bands, calibrated with field grain size distributions and laboratory analyses, we automated the mapping of fan sediment cover. Our findings reveal that aeolian covers, including barchan dunes, do not prevent debris flows from reaching mid and distal fan areas on fans with gradients of ~10°. This contrasts with observations from the southwestern US fans, where star dunes can obstruct debris flow pathways (Anderson and Anderson, 1990). The interactions we have identified are relevant for improving debris flow runout modelling, interpreting past fan sedimentary arrangements, and understanding fan evolution and sediment fluxes in arid environments. These insights have broader implications for the evolution of arid landscapes, sheeding light on the dynamic interplay between aeolian and alluvial fan processes.

How to cite: Cabré, A., Mather, A., Bufe, A., and Lang, A.: Interactions between aeolian dune fields and debris flows in alluvial fans., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13192, https://doi.org/10.5194/egusphere-egu25-13192, 2025.

Emergence and growth of sand dunes result from the dynamic interaction between topography, wind flow and sediment transport. While feedbacks between these variables are well studied at the scale of a single and relatively small dune, the average effect of a periodic large-scale dune pattern on atmospheric flows remains poorly constrained, due to a pressing lack of data in major sand seas. Here, we compare local measurements of surface winds to the predictions of the ERA5-Land climate reanalysis at four locations in Namibia, both within and outside the giant linear dune field of the Namib Sand Sea. In the desert plains to the north of the sand sea, observations and predictions agree well. This is also the case in the interdune areas of the sand sea during the day. At night, however, an additional wind component aligned with the giant dune orientation is measured, in contrast to the easterly wind predicted by the ERA5-Land reanalysis.

For the given dune orientation and measured wind regime, we link the observed wind deviation to the daily cycle of the turbulent atmospheric boundary layer. At night, a shallow boundary layer induces flow confinement above the giant dunes, resulting in large flow deviations, especially for the slower easterly winds. During the day, the feedback of the giant dunes on the atmospheric flow is much weaker due to the thicker boundary layer and higher wind speeds. Finally, we propose that the confinement mechanism and the associated wind deflections induced by giant dunes could explain the development of smaller-scale secondary dunes, which elongate obliquely in the interdune areas of the primary dune pattern.

Published article: Gadal, C., Delorme, P., Narteau, C., Wiggs, G. F., Baddock, M., Nield, J. M., & Claudin, P. (2022). Local wind regime induced by giant linear dunes: comparison of ERA5-land reanalysis with surface measurements. Boundary-Layer Meteorology, 185(3), 309-332.

How to cite: Gadal, C., Delorme, P., Narteau, C., Wiggs, G. F. S., Baddock, M., M. Nield, J., and Claudin, P.: Local wind regime induced by giant linear dunes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6096, https://doi.org/10.5194/egusphere-egu25-6096, 2025.

The transport of sand by wind has been the subject of extensive research. Despite numerous efforts, however, substantial discrepancies persist among theoretical models, wind tunnel experiments, and field observations concerning the magnitude of the critical threshold necessary to induce sand movement over heterogeneous surfaces.

In this study, we conducted field experiments to quantify surface shear velocities and wind velocities across seven heterogeneous surfaces characterized by a broad range of grain sizes. Surface shear velocities were measured using flush-mounted Irwin sensors (IS), while wind velocities were measured using a thermal anemometer (TA) positioned 30 mm above the surface and a 3-D ultrasonic anemometer (UA) 0.6 m above the surface. A high-resolution video camera monitored the surface to document sand motion. IS and TA sensors were emplaced within the view field of the camera. Reptating sand grains were captured using adhesive-coated plastic sticks. A total of 46, quality-controlled, threshold event data sets were acquired from the IS and TA measurements and 39 from the UA measurements.

For each event, surface shear velocities and wind velocities were block-averaged at 1-second intervals. Maximum and minimum values of shear (IS) and wind velocities (TA) were determined for each threshold event. Maxima are defined as the fastest shear and wind velocities without sand movement in the 5 seconds before an event, and minima are defined as the slowest velocities with movement in the 10 seconds before an event. Shear velocities in the constant stress layer (UA) were estimated with 2 minute-averaged Reynolds stress derivations. Median grain sizes for each event were estimated via photosieving. This approach enabled the estimation of threshold conditions corresponding to specific grain sizes, bracketed by their respective maximum and minimum values.

Our results from surface and near-surface data indicate no statistically-significant relationship between threshold shear or wind velocities and median grain sizes ranging from 131 to 475 µm. The mean of the maximum and minimum surface shear velocities for the 1-second block averages is 0.17 m/s (standard deviation, 0.02 m/s). The respective wind velocity threshold is 5.28 m/s (standard deviation, 1.64 m/s). Testing longer averaging intervals (1.5, 2, and 3 seconds) yielded consistent results, with a reduction in the differences between maximum and minimum values as the interval increased, but yielding no significant relationships. The shear velocity estimates from the ultrasonic anemometer data indicate a statistically significant, Bagnold-type relationship for the threshold, but with an “A” coefficient of 0.14 (standard deviation, 0.05) for the fluid threshold.

These findings support the concept of equal mobility, whereby a single threshold shear or wind velocity can initiate sand motion for a range of grain sizes on heterogeneous surfaces. The findings also indicate a decoupling of threshold conditions as measured at or near the surface and those as measured typically in the constant stress layer.

How to cite: Bae, J. and Sherman, D. J.: Field Estimates of Thresholds for Aeolian Sand Transport, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13976, https://doi.org/10.5194/egusphere-egu25-13976, 2025.

Previously, studies of barchans in Algeria have been limited to those close to In-Salah in the centre of the country and Mali has been entirely overlooked. This is primarily because the aeolian bedforms in these countries are predominantly linear dunes. However, 150-250km N-NE of Taoudenni, Mali one can find a number of barchans and megabarchans which straddle the Mali-Algeria border. To our knowledge, these dunes have not previously been studied. The widths of the dunes range from around 30m for the smallest barchans to 1.6km for the largest megabarchan, allowing us to test the robustness of scaling laws typically applied to barchans and the similarity between barchans and megabarchans. In this poster, we present preliminary analyses of the morphology and migration rates of the dunes measured using Google Earth imagery.

How to cite: Robson, D. and Baas, A.: Morphology and Migration of Megabarchans on the Mali-Algeria Border, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19261, https://doi.org/10.5194/egusphere-egu25-19261, 2025.

In this study, we analyze the surface aeolian record around the Coprates rise region, an extensive area that includes Thaumasia and Bosporos Planae as well as the western Noachis Terra lowlands. The main goals are: a) to test the systematic mapping of aeolian bedforms (PBRs, TARs, and/or paleo-megaripples ) at medium resolutions using CTX mosaics and the techniques introduced by Vaz et al. ( 2023); b) to evaluate the spatial distribution of aeolian bedforms and identify different populations in this large area; c) to use GCM predictions and state-of-the-art bedform formation models to test how well the mapped features fit the predicted current-day surface aeolian dynamics.  

The automated methodology accurately maps various sets of linear and periodic bedforms. We estimate that about 50% of the mapped bedforms are associated with or controlled by local topography, showing clear spatial associations with specific topographic settings: craters, incised valleys, grabens and wrinkle ridges. These were excluded from a comparative analysis with the GCM-derived predictions. 

We identified eight sets of bedforms with very different characteristics (e.g. trends, wavelength, morphologies, and preservation state). We found significant disparities between the mapped bedform trends and the GCM-based predictions, suggesting that most of the mapped sets are incompatible with current surface aeolian dynamics, as predicted from the GCM outputs. 

References

Vaz, D. A., Silvestro, S., Chojnacki, M., & Silva, D. C. A. (2023). Constraining the mechanisms of aeolian bedform formation on Mars through a global morphometric survey. Earth and Planetary Science Letters, 614, 118196. https://doi.org/10.1016/j.epsl.2023.118196

How to cite: Vaz, D. A., Cardoso, R., Torres, I., Bahia, R. S., Bohacek, E., and Silvestro, S.: Large-scale systematic mapping of paleo-bedforms in the Thaumasia Planum – western Noachis Terra region, Mars, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13879, https://doi.org/10.5194/egusphere-egu25-13879, 2025.