Assessment of the effect of meteorological drought on the performance of vegetation in erosion control projects using Landsat satellite images

 The best and most effective way to control wind erosion is to increase vegetation to cover the land surface. The roughness of the land surface is increased by vegetation. Because it increases the friction that causes a decrease in wind speed on the surface of the ground and the carrying capacity of sediments by the wind. By determining resistant species and more adapted to dry conditions, it will be possible to establish vegetation in these areas in different non-desertification projects to control and reduce wind erosion.

 In this research, in one of the critical centers of wind erosion in Gonabad County in northeastern Iran, investigated the performance of a biological project of non-desertification operations with Haloxylon aphyllum, Haloxylon persicum, Seidlitzia rosmarinus, Nitraria schoberi, Atriplex canescens and annual plants in different intensities of the drought for 2004 to 2021. At first, using the RDI index, drought intensities were determined in March, April, and May in the studied period. In the next step, the maximum, average, and minimum values of NDVI, TDVI, SAVI, and EVI indices were calculated using Landsat satellite images and data processing ENVI 5.3 software in each of the covered areas by desired specie in the studied months. In the final stage, the values of these vegetation indices were compared and analyzed for drought intensities in the areas and months.

 Based on the results, in all the indices, the area covered by Seidlitzia rosmarinus had a better condition than in other areas in the very dry drought intensity and with the highest value of 0.341 in the EVI index. But in the medium and mild drought intensities, the area covered with the Haloxylon sp had a better condition than in other areas and with the highest value of 0.456 in the TDVI index. However, all studied vegetation indicators did not show any significant difference between the planted areas. In March with the very dry condition, vegetation was more dependent on the intensity of dry conditions in February. The severity of the drought in February caused the values of all vegetation indicators in March in the studied areas to be negative, except in the annual species area. In March, the SAVI index, in April TDVI index, and in May TDVI and EVI indices had better ability to distinguish vegetation cover. The results of the Kruskal-Wallis test showed that in March, there was a significant difference between high, medium, and mild dry conditions only for the TDVI index at the level of 5% and the SAVI index at the level of 1%. In April, the NDVI and SAVI indices at the level of 1% and the EVI index at the level of 5% showed a significant difference between the three dry conditions. The results of the Mann-Whitney test showed that in May, only the SAVI index had a significant difference at the level of 1% between the moderate and mild dry conditions.

 The results confirmed the ability of vegetation indices obtained from Landsat satellite imagery to monitor the vegetation changes due to the drought. All the indices showed changes in the vegetation in the drought conditions, but no difference was seen between the vegetation areas. The resistance of the species to drought was one of the main reasons that caused there to be no significant difference between the vegetation areas, but the difference between the drought conditions was significant. Due to the adaptation and resistance of desert species to drought conditions, their sensitivity to drought in dry and desert areas is lower than in humid areas. In the condition that February is affected by drought, the cover conditions of annual plant species in the studied area in March were better than in other areas. But in March with very dry or moderate drought conditions, the cover conditions of Seidlitzia rosmarinus species were better coverage than in other areas. Based on the results, in the continuation and occurrence of moderate to high drought in April and May, the area of Seidlitzia rosmarinus showed a better cover than in other areas. In the condition of continued drought in March, annual plants do not have a chance to grow and the species that can use the moisture reserve in the deeper soil will have more opportunity to cover the surface of the ground, which this research showed that among the species in this area, Seidlitzia rosmarinus has more ability. Therefore, the principle of mixed planting and preventing single planting in the desert restoration and non-desertification projects should be emphasized and implemented.