Hydro-Climatic Assessment of Climate Change Impacts on the Upper Watersheds of Gorgan Bay Wetland

Introduction: Wetlands play a pivotal role in supplying freshwater resources, controlling floods, and maintaining biodiversity. However, they have become increasingly vulnerable to the effects of climate change, such as the rise in greenhouse gas emissions, global warming, and altered precipitation patterns. Gorgan Bay Wetland, situated along the southeastern coast of the Caspian Sea, exemplifies a sensitive ecosystem under substantial pressure from sea-level fluctuations and reduced freshwater inflows from its upper watersheds. According to the Sixth Assessment Report (AR6) of the IPCC, rising temperatures and extreme weather events—including intensified drought and heavy rainfall—can significantly affect the hydrology and ecological functions of wetlands. In this study, we draw on TerraClimate data and outputs from General Circulation Models to examine hydro-climatic trends in the upper watersheds of Gorgan Bay Wetland over a 64-year span, and to project potential future scenarios of climate variability.
 
Material and Methods: In the first step, monthly data on temperature, precipitation, evapotranspiration, snow water equivalent, and drought indices were retrieved from the TerraClimate database for the period 1958–2021. These data were validated against observations from three meteorological stations located in the upper watersheds of Gorgan Bay. Subsequent trend analyses for temperature, precipitation, and drought indices were conducted using the non-parametric Mann–Kendall test, alongside ordinary least squares regression. Next, future climate projections were obtained from selected GCMs in IPCC AR6, corresponding to four Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5). Downscaling was performed through the Change Factor approach, which applies statistical corrections based on the discrepancies between historical observations and coarse-resolution GCM outputs. Finally, the results were interpreted with regard to sustainable water resource management strategies aimed at mitigating the impacts of climate change on Gorgan Bay Wetland.
 
Results and Discussion: Time-series analyses revealed a statistically significant upward trend in all temperature parameters (minimum, maximum, mean, and median) across the watersheds during the 64-year study period. Conversely, most areas did not exhibit a clear or substantial decline in precipitation. Only some eastern segments showed a slight yet statistically significant reduction in rainfall. Meanwhile, increases in temperature and evapotranspiration, coupled with decreases in snow water equivalent in certain mountainous areas, have led to a worsening of climatic water deficit and drought conditions—particularly since the 1990s. These findings partially explain the diminished surface inflows and reduced water levels in Gorgan Bay Wetland. Downscaled climate projections under different SSPs point to further warming in the coming decades. Under more optimistic scenarios (SSP1-2.6), annual temperatures are projected to rise by about 1.0 to 1.5 °C in the near future (2021–2040). However, in more extreme scenarios (SSP3-7.0 and SSP5-8.5), rapid warming alongside shifts in rainfall patterns could intensify drought conditions and reduce effective runoff. Although heavier rainfall events may occur, increased evapotranspiration and reduced snow storage could collectively diminish the net water input to the wetland. If realized, these changes would amplify drought severity, compromise the wetland’s ecological services, and impact local livelihoods.
 
Conclusion: Overall, the findings underscore the urgent need for adaptive management and planning to address the impacts of climate change in the upper watersheds of Gorgan Bay Wetland. Strategies such as adjusting cropping patterns, allocating adequate environmental flow, employing water-efficient irrigation technologies, and rehabilitating vegetative cover in upstream regions appear essential for mitigating climate-related risks. Ongoing monitoring of hydro-climatic variables, the integration of more advanced downscaling methods, and a focus on interdisciplinary approaches—including ecosystem-based management and economic-environmental modeling—could further enhance conservation efforts. The insights gained here reaffirm the importance of immediate and sustained policy actions to preserve the ecological integrity and socio-economic benefits of Gorgan Bay Wetland in the face of intensifying climate challenges.