الگویابی و روند میرایی - توسعه نبکاهای ایران (مطالعه موردی: استان کرمان)

Introduction
Nebkha is a distinctive type of desert landform that develops when aeolian (wind-blown) sand accumulates around a vegetative anchor, such as shrubs, grasses, or small bushes. This phenomenon predominantly occurs in arid and semi-arid environments, where vegetation is sparse and the wind plays a dominant role in shaping the landscape. Nebkhas are not only geomorphological indicators of wind dynamics and vegetation interaction but also serve as ecological microhabitats for various desert organisms. These sand mounds are formed by the progressive deposition of wind-transported particles around the base of plants, where the roughness provided by vegetation reduces wind speed, causing particles to settle. Over time, this process creates crescent-shaped or irregularly domed sand dunes that can reach from a few centimeters to several meters in height, depending on environmental variables. The vegetation, in turn, becomes increasingly buried as the nebkha grows, often leading to a complex interaction between plant physiology and sediment deposition. In arid regions like Kerman Province in Iran, nebkhas act as natural windbreaks, reducing the extent of wind erosion and helping to stabilize otherwise highly mobile desert surfaces.The ecological significance of nebkhas goes beyond their role in sand trapping. They promote soil development, enhance microbial activity in the rhizosphere, and support biodiversity in desert ecosystems. By creating microenvironments with higher moisture retention and organic matter accumulation, nebkhas help establish favorable niches for other plant species and even desert fauna. These functions make nebkhas critical components in the study of desertification, land degradation, and ecological resilience.The primary objective of the present study is to explore the spatial and temporal patterns of nebkha development in Kerman Province, focusing particularly on the morphological evolution of these landforms over recent decades. Kerman is an ideal case study due to its varied climatic conditions, significant wind activity, and presence of vegetation-sand interactions in multiple zones. The research also highlights how climate variability—including droughts and episodic rainfall—can lead to the degradation or regeneration of these formations. By combining satellite imagery, meteorological data, and remote sensing techniques, this study provides a comprehensive understanding of how nebkhas are formed, how they evolve over time, and what their current status indicates about broader environmental trends. Such investigations are crucial for land management practices, especially in regions vulnerable to desertification and ecological stress.
Materials and Methods
To analyze the formation, spatial distribution, and dynamic changes of nebkhas across the study area, a multidisciplinary approach was adopted involving meteorological observations, satellite image analysis, and geospatial data processing. The methodology was designed to provide both qualitative and quantitative insights into the environmental processes governing nebkha dynamics. Wind Data Analysis:
The first step in the analysis involved collecting wind data—both speed and direction—from meteorological stations situated in Bam, Shahdad, and Kahnuj. Wind Rose and Sand Rose diagrams were constructed to visualize prevailing wind directions and intensities. These diagrams help identify dominant erosional forces and potential sediment transport pathways. The Sand Drift Potential (DPt) and Resultant Drift Potential (RDP) were calculated using standard equations in aeolian geomorphology, which quantify the potential for sand transport and its directional bias. This information is vital for understanding where nebkhas are most likely to form and how they might migrate over time. Remote Sensing and Image Analysis: To monitor temporal changes in nebkha distribution and morphology, a time series of Landsat 8 satellite images (30-meter resolution) was analyzed. Images from multiple years were preprocessed using atmospheric correction and geometric alignment to ensure consistency. The images were then imported into ImageJ, a powerful open-source software for image processing. In this software, negative imaging was applied to enhance contrast between vegetation and bare soil, making nebkhas more distinguishable from surrounding landforms. Image differencing techniques were used to detect spatial changes in nebkha features between different years. This technique involves subtracting pixel values of one image from another to highlight areas of change. In regions where pixel values significantly changed, either new nebkhas had formed, or existing ones had been degraded or displaced by active dunes. Automated Change Detection with Python: To improve the precision and reproducibility of the image analysis, custom Python scripts were developed. These scripts automated the pixel-level comparison across image sets, reducing human error and enabling more efficient processing of large datasets. The Python algorithms calculated pixel-by-pixel differences and flagged areas that showed significant morphological changes, enabling spatial mapping of nebkha dynamics over time. Climatic Data Assessment: In addition to wind data, the study analyzed long-term climatic data, including mean annual temperature and total annual precipitation over a 20-year period. These data were obtained from synoptic weather stations within and around Kerman Province. This climatic information helped assess how shifts in temperature and rainfall patterns correlate with the formation or deterioration of nebkhas. Vegetation Cover Mapping and NDVI Analysis: To evaluate the role of vegetation in nebkha stability, the Normalized Difference Vegetation Index (NDVI) was calculated from the Landsat images. NDVI is a widely used remote sensing index that reflects vegetation density and health. Higher NDVI values indicate dense, healthy vegetation, while lower values point to sparse or stressed vegetation. Land use maps were generated for different time points to identify zones of ecological degradation or improvement. These maps were then cross-referenced with nebkha locations to assess the relationship between vegetation dynamics and nebkha evolution. By integrating these diverse datasets and methods, the study provides a robust, multi-temporal analysis of nebkha development under the influence of climatic and geomorphological factors.
 
Results and Discussion

Wind Erosion Analysis: Wind dynamics play a pivotal role in both the formation and degradation of nebkhas. Based on the Wind Rose and Sand Rose analyses, distinct wind regimes were identified for each station:

Station  DPt       RDP     RDD     RDP/DPt
Bam      214.1    119.206 140°     0.0557
Shahdad            892.9    492.887 149°     0.552
Kahnuj  254.9    229.172 46°       0.899
The RDP/DPt ratio is critical in interpreting the unidirectionality of wind transport. In Bam, the low ratio suggests multidirectional winds, reducing net sediment transport in any specific direction. In contrast, Kahuna’s high ratio reflects a dominant wind direction, conducive to the linear accumulation of sand and nebkha formation. Shahdad represents an intermediate condition with substantial drift potential and a moderately focused directional bias. 2. Image Differencing and Spatial Changes in Nebkhas: Image processing in ImageJ revealed significant spatial and morphological changes in nebkha formations between different time intervals. Key observations include: Growth and densification of existing nebkhas in zones with recent rainfall and vegetation regeneration. Disappearance of smaller or younger nebkhas in areas increasingly affected by mobile sand encroachment. In some transitional zones, fragmentation of nebkhas was evident, possibly due to fluctuating vegetation health and inconsistent sand supply. These findings 
underscore the dual nature of environmental drivers: while certain areas demonstrate resilience and regeneration, others continue to degrade under the combined pressure of wind erosion and drought.3. Climatic Trends and Vegetation Dynamics: Analysis of the 20-year climatic data revealed a prolonged drought period lasting approximately 14 years, followed by three consecutive years of above-average rainfall. According to reports from the Kerman Meteorological Organization, this recent precipitation influx significantly improved soil moisture content and facilitated vegetative growth along the desert margins. This change is clearly reflected in the NDVI analysis, where notable increases in vegetation cover were observed, particularly in the northern and northeastern sectors of the province. These improved conditions led to the re-emergence of nebkhas in previously degraded zones. However, in southern areas with less precipitation, vegetation recovery was minimal, and many smaller nebkhas failed to survive. The study thereby confirms the hypothesis that climate—particularly rainfall—acts both as a degrading and restorative force in desert geomorphology.
 
Conclusion
The comprehensive investigation into the dynamics of nebkhas in Kerman Province highlights their significant geomorphological and ecological roles in arid landscapes. Nebkhas are not static landforms; they are dynamic systems shaped by the interplay of wind, vegetation, and climate. The study demonstrated that regions with high wind drift potential and sufficient vegetative cover are more likely to sustain large and persistent nebkhas. Conversely, areas subject to extreme drought and active sand encroachment show nebkha degradation and eventual loss. Key takeaways from the research include:
Morphological Patterns: Four primary nebkha patterns were observed—linear, fan-shaped, dense/sparse, and anthropogenic. These patterns reflect local wind regimes and vegetation types.
Wind and Sand Dynamics: The DPt and RDP values at the studied stations revealed varying degrees of susceptibility to wind erosion. Kahnuj had the most unidirectional winds, ideal for nebkha formation, while Bam had the least focused winds. Climate Variability: Despite a long-term drought, the recent increase in rainfall played a crucial role in reviving degraded areas and promoting nebkha regrowth. This underscores the importance of episodic climatic events in maintaining desert landforms.
Technological Integration: The use of satellite imagery, NDVI analysis, and automated pixel-level change detection provided a robust and replicable methodology for future studies on aeolian landforms. Ultimately, nebkhas serve as early indicators of ecological health in desert regions. Monitoring their evolution offers valuable insights into broader environmental processes, including desertification, climate adaptation, and landscape resilience. Continued research and satellite-based observation will be vital for managing and conserving these unique desert features under conditions of increasing climatic uncertainty