Spatio-temporal dynamics of vegetation and water bodies in the Lake Urmia Basin (1984–2022): an analysis using MODIS and Landsat data with new perspective

This study aims to quantitatively analyze the spatio-temporal changes in Lake Urmia's surface area, vegetation cover, and dam reservoirs from 1984 to 2022, using Landsat and MODIS data to comprehensively understand long-term environmental changes in the Lake Urmia Basin (ULB), northwest Iran . Landsat images from 15 selected years between 1984 and 2022, and MODIS data from August for each year 2000–2022, were used. The Normalized Difference Water Index (NDWI) and Normalized Difference Vegetation Index (NDVI) were used to assess long time water bodies and vegetation dynamics. Unlike previous studies that rely solely on conventional NDVI thresholding, this research not only uses conventional NDVI thresholding but also introduces several novel sampling-based approaches to improve classification accuracy. Conventional NDVI-based vegetation thresholds, along with newly developed methods based on representative sample pixels, were used to identify high-vigor vegetation, low-vigor vegetation, and non-vegetated areas. For lakes and reservoirs, the NDWI and sample pixels were used to separate water from non-water areas. The analysis revealed a substantial decline in Lake Urmia's surface area, decreasing from 491,597 ha in 1984 to 189,719 ha in 2022, with the most severe reduction observed in 2014 and 2015 ( ≈ 88 % decrease). In contrast, the area of water reservoirs behind dams increased from 4,788 ha to 8,118 ha over the same period. Vegetation cover expanded significantly, with MODIS-derived analysis indicating increases of 2.11 times in 2020 and 1.8 times in 2022 compared to 2000. Area of high-vigor vegetation which means vegetation with high index (NDVI 0.65–1) showed 13.51 times increase in 2020 and 6.81 times in 2022. Statistical analysis revealed a negative relationship between vegetation area and lake area, with correlation coefficients ranging from −0.71 to −0.32. These findings highlight the simultaneous expansion of vegetation and dam reservoirs alongside the continued shrinkage of Lake Urmia, suggesting significant hydrological and land cover changes in the basin over recent decades. Notably, this study covers a 38-year period from 1984 to 2022, making it one of the longest continuous analyses of the LUB. Therefore, policymakers and relevant authorities must make informed, data-driven decisions to prevent further deterioration of the region's hydrological and ecological conditions.