Assessment of Climate Change Transformations and Their Impact on the Hydrological Regime of Gorgan Bay Wetland

Introduction:

Wetlands are critical natural systems that provide essential ecosystem services, including water regulation, flood control, carbon sequestration, and biodiversity conservation. However, these ecosystems are increasingly threatened by climate change and human activities. Gorgan Bay Wetland, located along the southeastern Caspian Sea, is particularly vulnerable due to its unique geographic setting and reliance on upstream freshwater inflows. Over recent decades, rising temperatures, altered precipitation patterns, reduced snow water equivalent, and increased evapotranspiration have contributed to significant declines in both the surface area and storage volume of the wetland. This study aims to assess the historical hydrological trends of Gorgan Bay Wetland and forecast its future evolution under various greenhouse gas emission scenarios. By integrating remote sensing data, field measurements, and climate modeling, the research provides a comprehensive understanding of how climatic factors drive wetland dynamics and offers valuable insights for sustainable water resource management and conservation strategies.

Materials and Methods:

A multi-disciplinary approach was adopted to analyze the hydrological dynamics of the wetland. High-resolution Landsat Level-2 surface reflectance imagery, covering the period from 1984 to 2022, served as the primary source for delineating wetland boundaries. Spectral indices such as the Normalized Difference Water Index (NDWI) and the Modified Normalized Difference Water Index (MNDWI) were used to distinguish water bodies from other land cover types. These extracted boundaries were validated using ground control points (GCPs) collected along the wetland’s periphery. In October 2022, field surveys were conducted to measure water depth using differential GPS and digital depth sounders. These measurements were interpolated using the spline method to produce continuous bathymetric maps, and a Triangulated Irregular Network (TIN) model was developed to estimate the wetland’s storage volume over time.

Climatic parameters—including temperature, precipitation, potential evapotranspiration (PET), and drought indices—were obtained from the TerraClimate database. Statistical analyses, such as Pearson correlation and regression modeling, were employed to evaluate the relationship between these climatic variables and changes in the wetland’s area and volume. To project future hydrological changes, outputs from General Circulation Models (GCMs) presented in the IPCC’s Sixth Assessment Report (AR6) were downscaled using both statistical and dynamical methods. Four emission scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) were considered to capture a range of potential future climates, enabling robust scenario-based predictions for the wetland’s response to ongoing climatic shifts.

Results and Discussion:

Temporal analysis of Landsat imagery revealed that Gorgan Bay Wetland has experienced significant fluctuations in surface area and storage volume over the past few decades. A marked decline began around 2010, with the period from 2015 to 2022 showing a reduction of over 24% in surface area and more than 47% in water storage volume. These declines were strongly linked to climatic changes, particularly rising temperatures and reduced water inflows. Pearson correlation analysis indicated a statistically significant negative relationship between annual maximum temperature and both wetland area (r = -0.496, p < 0.01) and volume (r = -0.479, p < 0.01), underscoring the impact of higher temperatures on increased evaporation and reduced water retention. Conversely, a positive correlation was found between snow water equivalent and wetland area (r = 0.400, p < 0.05), emphasizing the role of snowmelt in sustaining inflows.

Regression analysis quantified the impact of temperature increases on the wetland, showing that for each 1°C rise in annual maximum temperature, there is an approximate loss of 3,280 hectares in wetland area. Future scenario modeling projects that, under a moderate emission scenario (SSP2-4.5), maximum temperatures in the region could increase by 1.3°C to 1.8°C over the next 20 years. This temperature rise is expected to result in a loss of around 4,494 hectares of wetland area by 2040—approximately 12.8% of its current extent. Under higher emission scenarios (SSP3-7.0 and SSP5-8.5), the decline in wetland area is anticipated to be even more severe, posing substantial risks of widespread degradation.

The integration of remote sensing, field data, and climate modeling in this study provides a detailed depiction of the interplay between climatic drivers and wetland hydrology. Despite inherent uncertainties in future climate projections and land use changes, the strong negative correlations observed reinforce the robustness of the current trends. The reduction in wetland area and volume not only threatens the ecological integrity of Gorgan Bay but also jeopardizes its ability to perform critical ecosystem functions such as flood mitigation, water purification, and habitat provision. These findings highlight the urgent need for adaptive management strategies that address both climatic and anthropogenic pressures.

Conclusion:

This study offers a comprehensive evaluation of the hydrological changes in Gorgan Bay Wetland driven by climate change. The integration of remote sensing, field surveys, and advanced climate modeling has revealed significant declines in both wetland area and storage volume, primarily due to rising temperatures and diminished water inflows. Future projections indicate that these trends will continue under most emission scenarios, potentially leading to severe ecological and environmental impacts. The results underscore the importance of revising water resource management policies, increasing environmental water allocations, and implementing modern irrigation practices in upstream regions to mitigate water deficits. Continued monitoring and model refinement are essential for ensuring the long-term sustainability of Gorgan Bay Wetland.