Monitoring and forecasting of climatic factors affecting the mobility of sand dunes using Lancaster index (Case study: Sirjan desert)

Sand dunes are one of the most important landforms sensitive to wind erosion, the extent of which varies depending on the severity of the impact of various climatic and environmental factors. Sand dunes cover large areas of the land and are located mainly in arid areas. Long-term monitoring of these factors along with the mobility of sands dune in different areas can have a significant effect on reducing their adverse effects. Most of the stabilized sand dunes were most active in the late glacial period.  Vegetation is one of the factors modulating and weakening the action of wind on sand, the growth of which can lead to the stabilization of sand dunes.The phenomenon of moving sand dunes as one of the important processes of land degradation is a serious challenge in Iran. Therefore, studying and recognizing these factors in terms of their impact on the occurrence and intensification of mobility of sand dune is an undeniable necessity to prevent the aggravation of environmental crises in the future. Knowing the status of sand dunes can help us to protect them.

The study area, Sirjan desert, is located in Kerman province. For this purpose, for the present study, the average monthly values of climatic elements such as temperature, rainfall, hourly data related to the horizontal field of view, wind speed and direction and the code of various dust phenomena are required. In total, 100 meteorological codes (99-00) have been defined by the World Meteorological Organization for various phenomena, of which the code of 10-code represent local dust and one code (06) represents extraterrestrial events. To examine wind data including monitoring time and date, wind direction and speed, field of view and dust phenomena code including codes of 6, 7, 8, 9, 30, 31, 32, 33, 34 and 35 for statistical period 10 year (2011-2020) has been obtained from the Meteorological Organization of the country. In order to determine the direction of local dusty winds, hourly data related to speed and wind.  direction in local dusty hours were used. While to determine the direction of trans-local winds from the data related to code 06 was used, which indicates dust originating from outside the station. Rainfall and temperature data were used to calculate the amount of evapotranspiration by the Torrent White method. After calculating the percentage of erosive winds and drought index, the mobility status of sand dunes in the area was investigated using the Lancaster index. Sensitivity analysis test was used to predict the effect of possible change of climatic factors on the mobility of moving sand dunes.

On a monthly basis, the maximum speed of the strongest wind in the study area is in July. The prevailing wind direction in most months of the year is southeast. The highest and lowest frequencies of dusty days occurred in April and September, respectively. The results of wind speed frequency classification showed that about 89% of winds were blown at a speed of less than 6 m /s. The results of the analysis of the relationship between Lancaster index and the number of local dusty days showed that the impact of local dusty days from sand dune activity studies was 42% in the study area. The results of correlation analysis between climatic variables of sand mobility index showed that there is a significant negative relationship between rainfall and sand dunes activity in Sirjan desert synoptic station. As during the statistical period, the decrease in rainfall has increased the amount of sand dunes mobility index. Two-variable regression analysis between Lancaster index and drought index showed that there was a statistically significant relationship at 95% confidence level between these variables. In other words, 65% increase in wind sediment activity was due to changes in climatic conditions due to changes in rainfall and evapotranspiration potential of this region. About 90% of the statistical years were at high risk of desertification in terms of climate. Accordingly, with decreasing the amount of UNEP drought index and increasing environmental drought, the mobility of sand dunes in the whole study area and the risk of desertification will increase. The annual Lancaster index values at this station indicate that the status of active sand dunes is only at the top of the sand dunes. Based on the sensitivity analysis, it was found that if the frequency of erosive winds and evapotranspiration increases by 30%, the activity of sand dunes will increase by 69%. In other words, the active state of just at the top of the sand dines will change to the active sand dune state. Due to the combined reduction of the two mentioned parameters by the same amount, the activity of sand dunes will decrease by 51% and will change from active only at the top of the sand dunes to inactive.

In the study area, most forms of sand dunes are active type, and 90% of the statistical years were at high risk of desertification in terms of climate.  Around 65% of the increase in wind sediment activity was due to changes in climatic conditions due to changes in rainfall and potential evapotranspiration in this region.  It is predicted that the combined change of climatic elements, especially the speed of erosive winds and evapotranspiration in the future can affect the intensity of desertification in this region of the country.