Assessment of Water Surface and Quality Changes in Gorgan Bay Using Remote Sensing and Meteorological Data

Background and Objectives: Gorgan Bay, as one of the unique aquatic ecosystems in northern Iran, has faced serious environmental challenges in recent years. These challenges stem from various factors, including climate change, decreasing water levels of the Caspian Sea, increased human activities, and morphological changes in the region. Accurately identifying the factors leading to the degradation of this ecosystem is essential for sustainable water resource management and environmental protection. This study aims to analyze changes in water surface area and water quality in Gorgan Bay from 2000 to 2023 and identify the factors influencing these changes.
Materials and Methods: This research analyzes changes in water surface area and water quality in Gorgan Bay from 2000 to 2023 using satellite data from Landsat, MODIS, Sentinel, and Jason. Indicators such as water temperature, turbidity, and CDOM (Colored Dissolved Organic Matter) were examined to assess water quality in the region. Additionally, water surface area maps of the bay were created using MNDWI (Modified Normalized Difference Water Index) and NDWI (Normalized Difference Water Index). Pearson correlation coefficients (r) were utilized to analyze the relationship between meteorological and satellite parameters and the water surface area of the bay, identifying linear correlations among these variables. Shapley diagrams were employed to analyze feature importance and clarify complex impact patterns on the bay's surface area. A linear regression model was also applied to evaluate the linear relationship between input variables and the bay's surface area. Finally, to analyze the impact of factors such as the water level of the Caspian Sea, the area of the Caspian Sea, precipitation, temperature, and inflow discharge on the bay's surface area, a random forest model was utilized.
Results: The results indicate that from 2015 to 2023, the minimum water temperature in Gorgan Bay increased by an average of 2.3°C, primarily observed in the southern and western regions of the bay. Furthermore, there has been a continuous increase in water turbidity in recent years, particularly in 2020, 2022, and 2023, reaching unhealthy levels in the western and southern areas of the bay. The CODM index for Gorgan Bay in 2020, 2022, and 2023 remained in a suitable condition, with no significant pollution detected. In 2015, the water quality was higher than in recent periods but still did not reach unhealthy levels, with only small portions of the western areas approaching unhealthy conditions. The MNDWI and NDWI indices indicate that over 50% of the initial surface area of Gorgan Bay has been lost over these years. The analysis shows a significant reduction in the bay's surface area, particularly in 2020, 2022, and 2023, with large areas of the western, southern, and northern parts of the bay completely dried up. The study found that the decrease in the water level of the Caspian Sea is the most critical factor contributing to the drying of Gorgan Bay. The Pearson correlation coefficient between the bay's surface area and the Caspian Sea water level was calculated to be less than -0.90, with a coefficient of determination of 0.82, indicating a strong inverse relationship between these two variables. The relationships of other parameters, including the water area of the Caspian Sea, inflow discharge, precipitation, and temperature with the surface area of Gorgan Bay, were determined with coefficients of determination of 0.43, 0.40, 0.19, and 0.11, respectively, indicating that temperature has the least impact on the reduction of the bay's surface area. Additionally, the Shapley coefficient revealed that the water level of the Caspian Sea had the greatest variability across the horizontal axis, indicating its role in the surface area of Gorgan Bay. Other examined parameters, such as inflow discharge, the area of the Caspian Sea, temperature, and precipitation, also played significant roles in this process. Regression analysis to assess the role of the examined parameters on the reduction of Gorgan Bay's surface area based on coefficient values indicated that the water level of the Caspian Sea (coefficient of 0.54) had a more significant role in the drying of Gorgan Bay compared to other variables. In fact, the water level of the Caspian Sea had more than 50% greater influence on the drying process of Gorgan Bay than other features. The feature importance analysis using the random forest method showed that the water level of the Caspian Sea had a coefficient of 0.78, the area of the Caspian Sea had a coefficient of 0.14, while inflow discharge, temperature, and precipitation had coefficients of less than 0.1, indicating their lesser impact on the drying of Gorgan Bay. Moreover, the analysis of the water level of the Caspian Sea and the surface area of Gorgan Bay from 2000 to 2023 revealed a decrease in the bay's water surface area from 400 square kilometers in 2000 to 260 square kilometers in 2023, closely related to a decrease in the Caspian Sea water level, which has dropped by over 2 meters in the past 23 years. These changes are directly linked to the regional morphology of Gorgan Bay, particularly the reduction in depth and changes in the coastal shape, which have exacerbated the drying process and reduced water surface area.
Conclusion: The findings of this study indicate that Gorgan Bay has faced serious challenges from 2000 to 2023 due to declining water levels and water quality. Correlation analyses and regression models demonstrated that the water level of the Caspian Sea plays a primary role in these changes, with a direct and significant relationship between the decrease in water levels and the reduction of the bay's surface area. The results highlight the necessity for sustainable water resource management and the protection of Gorgan Bay as a sensitive ecosystem facing declining water levels and changes in water quality.