Investigating geomorphological indicators and their relationship with the vegetation of sand dunes. Case study (Aran and Bidgol region)

Introduction

On a global scale, the importance and danger of wind erosion is less than that of water erosion, but its size and magnitude is sometimes greater than that of water erosion. Wind deposits are important for the development of desert landscapes, especially in arid and semi-arid lands. In fact, wind erosion is a major process in arid and semi-arid regions and the landscapes created by it include sand sheets and different forms of sand dunes. Sand dunes are one of the types of natural hazards that cause a lot of damage every year, especially in dry and desert areas of the world. Depending on the wind regime and sand supply, the hills include several shapes such as barkhans, transverse hills, linear star hills and satellites, each of which has its own characteristics such as height, width and distance. Dunes are a potential source of dust caused by wind erosion, which strongly affects loess formation and biogeochemical and ecological processes by supplying nutrients to marine and terrestrial ecosystems. Sand dunes are always important in terms of human threats, so it is necessary to study these dunes because of the effects they have on water and soil resources, plant and animal life, and communication facilities and roads.

Materials and methods

The purpose of this study is to investigate the morphology of sand dunes under the influence of plant communities using telemetry indicators (Normalized Differential Vegetation Index (NDVI)) and also to investigate the effectiveness of each species in the field in the Kashan Citadel region during the period 1995-2020 was done. In order to check the height and slope, Google Earth images were used, and to calculate the vegetation, Landsat TM and OLI8 images and field visits were used. Correlation between morphological factors and vegetation was investigated using geographically weighted regression (GWR).

Result

The identification of sand dunes indicated the presence of six different types of dunes in the study area. Quantification of height and slope based on images extracted from Google Earth showed an average height of 33 meters and a slope of 3.6%. The changes in the height and volume of the sand dunes of Arg Kashan showed an increasing trend, so the height change rate and its correlation were calculated as 0.0455 and 0.85, respectively. Also, the volume change rate and its correlation were calculated as 0.216 and 0.9, respectively, and the surface volume of sand dunes in Kashan region increased by 1.02 cubic kilometers during a 25-year period.

Also, the temporal changes of NDVI increased in this study period, and the relationship between the height and slope of the sand dunes and NDVI showed that the highest amount of vegetation cover was in the fixed hills and the lowest amount was in the Star and Buklieh hills. Barkhani is located. The results of the correlation between the morphological characteristics of sand dunes and NDVI through GWR showed that there is a positive correlation between these factors in all sections (r= 0.57 and r=0.46). The sand-loving species of the region were Calligonum comosum, Cyperus conglomerates, Smiriniva iranica and Stipagrostis plumosa species. The results of the analysis of the efficiency of these species in the morphology of hill ranges showed that the above species do not have the same performance. So that the combination of the species and the Stipagrostis plumosa species had the greatest effect on the partial slope of the range by increasing the partial slope of the range by 40% and 30%, respectively, and the Smiriniva iranica species had the least effect on the partial slope of the range.

Discussion and conclusion

The height of the sand dunes at the beginning of the citadel is low and gradually increases until it reaches its maximum value in the center of the citadel and then decreases, which is due to the prevailing winds in the area. According to studies, the winds in Kashan region are capable of carrying a lot of sediment throughout the year due to their abundance, which play an important role in the formation of sand dunes. The prevailing winds in the study area are mostly east to north-east, which affects the sand dunes. Converging and local winds have caused the formation of sand pyramids and thus increased the height and volume of sand dunes, especially in the central parts of Kashan citadel. In terms of spatial distribution, the highest value of NDVI was observed in the southwestern and western part of the study area, which was consistent with the stabilized sand dunes, which can be seen mostly by man-planted forests. The reason for increasing the stability of the range in the case of a combination of species and basket species can be said that the height and volume of the canopy of the plant has an important role in the height of the sand dunes because the taller and bulkier the plant is, the more sand it traps. and as a result creates a higher sand dune with a greater slope. In the case of tree and shrub species such as skambil, it can be said that tree and shrub species act more as a protector than a sediment trap due to the lack of significant aerial organs. The results of the analysis of the efficiency of these species in the morphology of hill ranges showed that the above species do not have the same performance. Morphological changes in sand dunes provide important information with which we can understand the possible arrangement of ancient wind dunes. Therefore, this work is an important step in developing improved models to describe stratigraphic complexity and heterogeneity in desert environments. Also, the variety of vegetation has led to the variety of yield in the morphology of the hills, and this shows that the ecosystem uses the role of all species against the wind process.