Investigating the Dynamic Changes in the Area and Morphology of the Water Body of the Anzali Wetland Complex Using Sentinel-1 Radar Images, Alongside Ground-Based and Satellite Meteorological Data

Wetlands represent some of the most crucial natural ecosystems on our planet, facing alterations due to climate change and human activities. Coastal wetlands, including the Anzali Wetland complex, are undergoing considerable transformations influenced by various factors such as climate change, tourism expansion, urban development, and sea-level fluctuations. This research investigates changes in water coverage of the Anzali Wetland from 2017 to 2024. The complex consists of three primary sections: Siakshim, Sorkhangol, and Western Wetland, with radar imagery from the Sentinel-1 satellite utilized to analyze annual variations. Over the eight-year study period, the Anzali Wetland complex experienced significant changes in water body coverage, which were analyzed in relation to meteorological factors including temperature, precipitation, and evaporation, using data from Gilan's synoptic stations and satellite sensors. Radar imagery revealed a total reduction of 2,605 hectares in water coverage, with the Western, Sorkhankol, and Siakshim wetlands contributing 50%, 48%, and 2% to this loss, respectively. Compared to 2017, water coverage in these wetlands declined by 36%, 92%, and 6%. The study also examined NDVI indices from Landsat 8, Landsat 9, and Sentinel 2, comparing them with annual average land surface temperature (LST) data. Findings indicated increased vegetation growth in wetland areas, replacing portions of the water cover. This transformation resulted in a reduction of the temperature difference between maximum and minimum temperatures in recent years, in contrast to the beginning of the study period.
 
Extended Abstract
1-Introduction
Wetlands are vital ecosystems that provide significant benefits to natural energy cycles and ecological balance. They regulate biological, ecological, and hydrological processes at both regional and global scales, exhibiting higher biodiversity than many other ecosystems. Wetlands purify water and air, generate and store nutrients, manage floods, and provide essential habitats. They also support economic development through tourism. Importantly, wetlands contribute to reducing greenhouse gases, underscoring their crucial role in maintaining environmental health and sustainability. Anzali Wetland is recognized as a significant wetland and was among the first international wetlands listed under the Ramsar Convention, both in Iran and globally. However, despite this commitment, the wetland faces threats of degradation from various factors, including the discharge of urban, agricultural, and industrial wastewater, fluctuations in Caspian Sea water levels, the introduction of the invasive water hyacinth, and the construction of the Anzali Port bypass road. Given the region’s rainy climate, a large portion of optical sensor data is contaminated by clouds. This study focuses on monitoring changes in water coverage in Anzali Wetland by analyzing multiple radar images over the past eight years. While optical sensors such as Landsat and Sentinel-2 are affected by cloud cover, the radar-based Sentinel-1 sensor provides cloud-independent data with improved spatial and temporal resolution. Using Sentinel-1 images from 2017 to 2024, this research aims to assess changes in the Anzali Wetland Complex, a designated Ramsar site.
 
2-Materials and Methods
To achieve the study’s objective, over 83 Sentinel-1 images from each year were used to create annual average images classified as aquatic or non-aquatic. These 10-meter spatial resolution images helped analyze dynamic changes in water cover in the Anzali Wetland region. Additionally, the research examined time-series meteorological data from ground stations and satellite sensors to investigate the relationship between meteorological parameters and changes in wetland water cover over the study period. This comprehensive approach aimed to provide insights into the environmental dynamics affecting the wetland ecosystem.
 
3- Results and Discussion
The study used Sentinel-1 images to classify areas into water and land using the SVR method, achieving over 97% accuracy. The results showed that the Anzali Wetland complex lost 2,605 hectares of water cover over the past eight years, with significant losses attributed to the Western (50%), Sorkhankal (48%), and Siakshim (2%) wetlands. Specifically, water cover declined by 36% in Western, 92% in Sorkhankal, and 6% in Siakshim since early 2017. The decline in water coverage is closely linked to decreasing water levels in the Caspian Sea, as these coastal wetlands are interconnected with the sea through various streams, making their groundwater dependent on sea levels. Additionally, the analysis indicated correlations between temperature fluctuations recorded at meteorological stations in Rasht and Anzali and changes in water cover in Sorkhankal and Siakshim wetlands, while a similar trend was noted for the Western wetland with the Masuleh station. These findings underscore the impact of environmental factors, particularly temperature and sea level changes, on the water dynamics of the Anzali Wetland complex.
 
4- Conclusion
This research conducted a detailed analysis of water cover changes in the Western, Sorkhankal, and Siakshim wetlands using Sentinel-1 radar images with 10-meter resolution from 2017 to 2024. The study evaluated both temporal and spatial variations in wetland water cover and explored the relationship between Caspian Sea water level fluctuations and meteorological factors such as precipitation, temperature, and evapotranspiration. A significant finding was the identification of varying conditions across the Anzali Wetland, with the Sorkhankal Wetland showing notably severe changes in water coverage. Furthermore, a comparative analysis of land surface temperature and NDVI data from Sentinel-2 and Landsat-8 showed that while water coverage in the wetland complex declined in 2023 and 2024, increased vegetation cover following water body desiccation helped regulate local temperatures. The study emphasized that both Caspian Sea water levels and sedimentation in streams (based on previous research and field observations) significantly impact water coverage changes in the wetlands. These factors present critical challenges for the region, highlighting the need for professionals to develop strategies to effectively address these issues and promote sustainable management of the wetland ecosystems.