Satellite-Based Assessment of Vegetation and Hydrological Dynamics at the Garâa of Sejnane Ramsar Wetland (Tunisia): A Multi-Temporal NDVI and NDWI Analysis

Wetlands are among the most vital yet fragile ecosystems on Earth, acting as nature's sponges that filter water, store carbon, and provide a home for countless species. Because these landscapes sit at the boundary between land and water, they are incredibly sensitive to changes in rainfall, temperature, and human activity. To understand how these environments are changing over decades, scientists often turn to satellites orbiting high above the planet. These satellites capture images of the Earth's surface that can be processed to reveal two critical things: how much green vegetation is growing and how much open water is present. By comparing these snapshots taken years apart, researchers can track the slow, often invisible shifts in an ecosystem's health, revealing whether a wetland is thriving, drying out, or transforming in ways that might threaten the life it supports.

In the northwestern corner of Tunisia, a unique wetland known as the Garâa of Sejnane has long been recognized for its ecological importance, recently earning a place on the prestigious Ramsar list of protected wetlands. This vast area, covering more than 4,000 hectares, is a mosaic of temporary marshes, flooded fields, and wet meadows nestled in a depression surrounded by low hills. It is a haven for rare plants, including some found nowhere else in North Africa, which rely on a specific cycle of winter flooding to sprout in the spring. However, this delicate balance has been under pressure since the 1950s, when drainage works altered the landscape, and more recently from the growing impacts of climate change and shifting agricultural practices. To understand exactly how this wetland has changed over the last three decades, a team of researchers led by Imen Khemiri and Alaeddine Rouissi embarked on a long-term study, looking back at satellite images from 1992 all the way to 2024.

The researchers used a powerful tool called the Normalized Difference Vegetation Index, which essentially measures the density of green plant life, and the Normalized Difference Water Index, which maps the presence of surface water. They analyzed images from two different generations of satellites, Landsat 5 and Landsat 8, which provide a consistent view of the Earth with a resolution of 30 meters. By selecting images from specific dates across the 32-year period, they were able to compare the state of the wetland during different seasons and years, capturing both wet and dry conditions. The goal was not just to take a single picture, but to watch a movie of the landscape unfolding over time, seeing how the green cover and the water bodies interacted and evolved.

The results of this long-term observation tell a clear and concerning story. The analysis reveals a gradual but steady increase in the amount of vegetation covering the wetland, while the area of open water has noticeably shrunk. In the early 1990s, satellite images showed large, distinct patches of open water in the western and central parts of the depression, surrounded by vegetation. By 2019 and 2024, those same areas showed a significant reduction in open water, replaced by a denser spread of green plants. This shift suggests that the wetland is undergoing a fundamental transformation in its hydro-ecological balance. The researchers found that the vegetation in this area is deeply dependent on the winter floods; the plants need the water to germinate in the spring, but the data shows that the water is becoming less available over time.

This drying trend is visible in the water maps, where the deep blue colors representing flooded areas have faded to lighter shades and white, indicating dry ground or sparse vegetation. The researchers suggest that this change is driven by a combination of factors. The natural rhythm of the seasons plays a role, as the wetland naturally dries out in summer, but the long-term data points to a more permanent shift. Changes in rainfall patterns across northwestern Tunisia, likely linked to climate change, appear to be reducing the amount of water reaching the site. Additionally, human activities, such as changes in how the land is farmed and managed, are influencing the water flow. The study notes that while the area was once dominated by large bodies of water, it is now becoming more vegetated, possibly because the frequency of flooding has decreased, allowing plants to take hold in areas that were previously submerged.

The findings align with broader climate trends observed in the region, where shifts in rainfall and temperature are altering the natural cycles of wetlands. The researchers emphasize that the rare heritage plants of the Garâa of Sejnane are particularly vulnerable; without sufficient winter flooding, these species cannot germinate, and the ecosystem risks losing its unique biodiversity. While the increase in vegetation might seem positive at first glance, the study clarifies that it comes at the cost of the open water that defines the wetland's character. The authors caution that their work has limitations, such as the resolution of the satellite images which might miss small-scale details, and the lack of ground-level data for every single year. However, the overall picture is robust: the wetland is drying out, and the balance between water and land is shifting.

Ultimately, this study serves as a critical warning and a call to action. The Garâa of Sejnane is a living laboratory that demonstrates how climate change and human pressure can reshape a protected ecosystem over just a few decades. The researchers conclude that to save this unique habitat, management strategies must adapt to these new realities. Future efforts need to integrate better monitoring and adaptive plans to protect the rare flora and the hydrological integrity of the site. Without intervention, the delicate dance between water and life that has sustained this wetland for centuries may be lost, leaving behind a landscape that no longer functions as the vital ecosystem it once was.