مقایسه روش های زمین آمار در درون یابی سطح ایستابی و بررسی ویژگی های مورفومتری و گرانولومتری نبکا (محدوده مورد مطالعه هرات (شهرستان خاتم)

1. Introduction and Objectives

Nebkhas (phytogenic sand mounds) are aeolian landforms generated through the interaction of wind-driven sediment transport and perennial vegetation in arid and semi-arid landscapes. By trapping sand, modifying near-surface airflow, and enhancing soil moisture and nutrient accumulation, nebkhas exert a significant influence on land surface stability and ecosystem functioning in drylands. Owing to their sensitivity to changes in wind regime, sediment supply, vegetation characteristics, and groundwater availability, nebkhas are increasingly recognized as reliable indicators of land degradation and desertification processes.

Desertification remains one of the most critical environmental challenges worldwide, particularly in arid regions where groundwater depletion, climatic variability, and land-use change jointly accelerate soil degradation and wind erosion. Previous studies have emphasized the role of vegetation morphology, wind energy, and sediment texture in controlling nebkha morphology and spatial distribution. However, the combined effects of aeolian dynamics and groundwater fluctuations on nebkha development remain insufficiently quantified, especially in arid inland basins of Iran.

The Herat Plain, located in Khatam County, Yazd Province, is characterized by intense wind activity, declining groundwater levels, and extensive nebkha fields dominated by Tamarix species. This area provides an appropriate setting to examine the interactions among wind regime, sediment characteristics, vegetation structure, and subsurface hydrological conditions.

The objectives of this study are to: (i) characterize the wind regime and sand transport potential in the Herat Plain; (ii) quantify nebkha morphometric and vegetation parameters; (iii) analyze sedimentological properties of nebkha sands; and (iv) evaluate the relationship between nebkha spatial distribution and groundwater table variations using geostatistical techniques.



2. Methodology

This study integrates field measurements, laboratory analyses, wind regime assessment, and spatial modeling. Field investigations were conducted during three campaigns, during which 20 representative nebkhas were selected along west–east transects. Morphometric parameters, including nebkha height, basal diameter, slope, and estimated volume, were measured. Vegetation characteristics such as plant height and canopy diameter were also recorded. Nebkha volume was estimated assuming a conical geometry.

A total of 20 sediment samples were collected from the upper 10 cm of nebkha surfaces. Grain-size distributions were determined using dry sieving, and statistical parameters (mean grain size, sorting, skewness, and kurtosis) were calculated using GRADISTAT software. Selected sand grains were examined under a binocular microscope to assess surface texture and degree of rounding.

Wind data from the Herat synoptic station (2010–2019) were analyzed using WRPlot software. Drift Potential (DP), Resultant Drift Potential (RDP), and Resultant Drift Direction (RDD) were calculated following the Fryberger method, considering wind speeds exceeding the threshold for sand movement (12 knots). Wind directional variability was evaluated using the Unidirectional Index (UDI).

Groundwater level data from 36 piezometric wells covering the period 1999–2019 were obtained from regional water authorities. Spatial interpolation was performed using Kriging (spherical, exponential, and Gaussian models) and Inverse Distance Weighting (IDW) in ArcGIS 10.8. Model performance was evaluated using Root Mean Square Error (RMSE) after Box–Cox normalization.



3. Results

Wind regime analysis indicates that southwesterly winds dominate the study area on an annual basis. The highest sand transport potential occurs during winter (DP = 1539 vector units), whereas summer exhibits the lowest transport capacity. The annual RDP value of 730.8 reflects a relatively high aeolian energy environment. Seasonal variations reveal reduced wind directional consistency during summer, as indicated by a low UDI value (0.28).

Morphometric measurements show substantial variability among nebkhas. Larger and higher nebkhas tend to occur in areas aligned with dominant wind directions and closer to local sediment sources. Statistical analyses demonstrate significant positive relationships between vegetation canopy diameter and nebkha basal diameter and height, underscoring the importance of plant architecture in sediment trapping.

Grain-size analysis reveals that nebkha sediments are predominantly composed of fine to very fine sand, with a mean grain size of 2.574 φ. Sorting values (mean = 1.034) indicate moderate to good sorting, while skewness values (mean = –0.773) suggest near-symmetrical distributions. Kurtosis values (mean = 3.51) indicate elongated distributions. These characteristics suggest short-distance aeolian transport and a relatively homogeneous sediment source.

Geostatistical analysis shows that Kriging methods outperform IDW in interpolating groundwater levels, with RMSE values ranging between 0.36 and 0.57. Spatial patterns indicate that areas with deeper groundwater tables are associated with reduced nebkha density and smaller morphometric dimensions.



4. Discussion and Conclusions

The results indicate that nebkha development in the Herat Plain is controlled by the interaction of wind regime, sediment availability, vegetation structure, and groundwater depth. Strong southwesterly winds and high winter sand transport potential play a primary role in determining nebkha orientation and growth. Seasonal wind variability contributes to morphological heterogeneity.

Sedimentological evidence supports the interpretation that nebkha sands originate from nearby sources and are deposited through short-range aeolian transport. The relatively uniform grain-size properties reflect consistent transport conditions and limited sediment mixing.

Groundwater decline emerges as a critical factor limiting nebkha stability and growth. Deeper groundwater tables reduce vegetation vigor, thereby weakening sand-trapping capacity and increasing susceptibility to wind erosion. This finding reinforces the role of nebkhas as sensitive indicators of eco-geomorphological change in arid environments.

Overall, this study demonstrates that integrating nebkha morphometry, wind regime analysis, sedimentology, and groundwater modeling provides a robust framework for assessing land degradation processes. The findings have practical implications for wind erosion management, dryland restoration, and sustainable groundwater use in arid and semi-arid regions.