Assessment of land degradation using Landsat satellite data in the period 2011-2021 (Case Study: Isfahan city)

 Land degradation is one of the destructive phenomena that threaten the stability and security of ecosystems, especially in arid areas. Land degradation can lead to reduced soil fertility and productivity, population migration and displacement, food insecurity, and ecosystem destruction. Despite widespread efforts to combat land degradation, this problem has not only not diminished in recent decades but has gradually intensified. Therefore, monitoring land degradation and revealing its characteristics is essential for land management and recovery, and this monitoring in arid areas facilitates proper management and control of this phenomenon. Monitoring of land degradation in these areas is possible using remote sensing data so that this science will be widely used to monitor land degradation in areas. Considering the importance of land degradation and the need for land monitoring, this study was performed to understandthe degradation situation in Isfahan city properly. Also, this study tries to create appropriate and timely management for the spread of degradation using modeling of environmental indicators obtained from satellite data in the period 2011-2021.

In this study, Landsat satellite imagery, TM, and OLI sensors were used to study the trend of land-use change. In addition, the data from field visits were also used as ancillary information. Satellite images were processed and analyzed in ENVI software environment. The supervised maximum classification method was used to prepare a map of land-use changes. Then, all land uses in the study area were divided into agricultural lands, rangelands, barren and saline lands, and urban and man-made areas. Finally, the obtained layers were transferred to ArcGIS software to calculate the land use area and prepare a suitable output map. After investigating land-use changes, SI soil salinity indices and Albedo climatic index, NDVI, and the LSM vegetation index were designed using the maximum likelihood method. SI soil salinity index is one of the main indicators of land degradation assessment. This index extracted from satellite images can assess soil salinity in arid and semi-arid regions, calculated using Equation SI=√(ρ_Blue×ρ_Red ) (ρBlue and ρRed, are the red and blue bands on the TM and OLI sensors, respectively). The surface albedo index obtained from remote sensing data is a physical parameter that expresses the sun's surface reflection characteristics and short wavelengths. This physical parameter is affected by vegetation, soil moisture, and other surface conditions. Therefore, by studying the changes in Albedo, it is possible to look at the changes in the ground surface and the result of land degradation. Equation AIbedo = 0.356 ρ_Blue + 0.130ρ_Red +0.373ρ_NIR+0.085ρ_SWIR1+0.072ρ_SWIR2-0.018 (The ρ band corresponds to the Landsat TM and OLI sensor images) was used to calculate the surface albedo in TM and OLI sensors in this study. The NDVI index, which is obtained from Landsat satellite images, TM and OLI sensors, was used to study the vegetation in this study. This index is most sensitive to changes in vegetation and is less susceptible to the effects of climate and soil, except in cases where vegetation is low. Another important parameter for land degradation is soil moisture content, which was studied using changes in the LSM index. Finally, the primary component analysis (PCA) method between Albedo, SI, NDVI, and LSM indices was used to estimate land degradation (LD) in 2011, 2016, and 2021. First, the desired indicators were normalized, and then the amount of land degradation for each year was estimated. So that large amounts of land degradation indicate the maximum land degradation.

 The trend of land-use changes in Isfahan city in four uses of agricultural lands, rangelands, barren and saline lands, and urban and man-made areas in the period of 2011-2021 showed that between 2011-2016, agricultural lands and rangelands have decreased by 5.7 and 5.06, respectively. In contrast, barren and saline lands and urban and man-made areas increased by 10.45% and 1.51%, respectively. On the other hand, from 2016 to 2021, agricultural lands and rangelands have decreased by 0.75 and 1.25 percent, respectively, and barren and salty lands, urban and man-made areas have increased by 1.51 and 0.5 percent, respectively. Also, from 2011 to 2021, agricultural lands and rangelands decreased by 6.45 and 6.32 percent, respectively, and land use of barren and salty lands, urban and man-made areas increased by 11.96 and 0.8 percent, respectively. The study of the trend of land use changes showed that in this period of 10 years, the trend of destruction of agricultural lands and rangelands was decreasing, and barren and saline land and urban and man-made areas were increasing. The changes in desertification classes showed that the medium, high, and very high desertification classes have increased. The area of desert lands rose from 3428, 2817, and 1340 in 2011 to 4079, 4276, and 4302 Km2 in 1400, respectively. Low and very low classes have changed from 2826 and 5295 in 2011 to 574 and 2475 Km2 in 2021. These changes indicate an increase in desertification in Isfahan, which is due to land-use changes, especially the conversion of rangelands into agricultural lands and frequent droughts and drying of the Zayanderud River, which abandoned agricultural lands and turned them into barren and salty lands. On the other hand, with the dryness of the air, frequent droughts, and drying of the Zayanderud River, the soil moisture has decreased, which has caused salinization of the soil and increased unusable quality lands of this city. Also downstream of the Zayanderud River is Gavkhoni Wetland, one of the most important wetlands in Iran. Due to the reduction of incoming water, the surrounding beds have become barren and saline lands, which indicates the increasing desertification of this wetland.

 It can be concluded that by using the indicators estimated from remote sensing images, it is possible to monitor the destruction and desertification process with reasonable accuracy and put the necessary measures to deal with this destructive phenomenon on the agenda. In this study, the process of land degradation in Isfahan city was estimated over time, based on which the necessary programs and policies can be applied to deal with this phenomenon.