Spatiotemporal Dynamics and Assess of Ecological Quality in Sylhet Metropolitan Area amid Rapid Urbanization: A Remote Sensing Approach

Cities are living things that breathe, grow, and sometimes sicken. When a city expands, it often swallows the natural ground beneath it, replacing soil and plants with concrete and asphalt. This transformation changes how the land holds water, how it reflects heat, and how it supports life. Scientists who study these changes look for a way to measure the overall health of a city's environment, not just by counting trees or measuring temperature alone, but by seeing how all these factors work together. They want to know if a city is becoming a place where nature can still thrive or if it is turning into a landscape that struggles to support life. This question is especially urgent in places where populations are growing quickly and cities are spreading faster than planners can manage.

In the Sylhet Metropolitan Area of Bangladesh, a team of researchers set out to understand exactly how this rapid growth has altered the local environment over the last two decades. They focused on the years between 2003 and 2023, a period marked by intense urbanization. To get a clear picture, they did not rely on ground surveys alone, which can be slow and limited in scope. Instead, they used satellite images to look down on the city, capturing a wide range of environmental signals at once. They combined several different measurements: how much green vegetation was visible, how healthy that vegetation looked, how much moisture remained in the soil, how hot the surface felt, and how much of the land was covered by buildings and roads. By weaving these different threads together, they created a single score for every part of the city, a comprehensive index that tells a story about ecological quality.

The story that emerged from the data was one of a city that first improved, then began to struggle. In the early years of the study, from 2003 to 2011, the ecological quality of Sylhet actually got better. Areas that were previously degraded showed signs of recovery, and the overall health of the environment rose. This period suggested that perhaps the city was finding a balance, or that early planning efforts were working. However, the trend reversed sharply after 2011. From that point until 2023, the ecological quality began to decline steadily. The areas that were once healthy started to lose their vitality, and the pressure of human activity began to outpace the land's ability to recover.

The researchers found that the nature of this decline was specific and telling. The areas with the poorest ecological conditions, where vegetation was scarce and human activity was intense, did not stay in the same place. In the beginning, these struggling zones were mostly on the edges of the city, in the suburbs. Over time, as the city expanded outward, these poor-quality zones moved inward, shifting toward the urban core. The city's growth was not just a matter of adding new buildings; it was a process that pushed the healthiest parts of the environment to the margins while the center became increasingly dominated by concrete and heat.

To understand how the city was growing, the team looked at the shape of the new development. They found that the city was expanding primarily by spreading outward from its existing edges, like a stain soaking into a cloth, rather than by filling in empty spaces within the city or jumping far away to create new, isolated towns. This pattern of edge expansion meant that the city was constantly eating into the surrounding undeveloped land. The data showed that the area covered by urban settlements more than doubled, growing from about 3.30 square kilometers in 2003 to 6.91 square kilometers in 2023. This physical expansion was directly linked to the drop in ecological quality. As the buildings spread, the green cover shrank, the soil dried out, and the surface temperatures rose.

The relationship between the city's growth and its environment became even more complex over time. In the early years, the different factors of the environment, such as vegetation health and soil moisture, acted somewhat independently of one another. But by 2023, they had become deeply intertwined. The presence of buildings and roads began to strongly dictate the condition of the vegetation and the moisture in the ground. When the built-up areas increased, the vegetation health dropped significantly. The land surface moisture, which is crucial for cooling the city and supporting plant life, became the most dominant factor in determining the overall ecological score. This suggests that the city's water cycle has been fundamentally altered by the sheer volume of hard surfaces, making it harder for the environment to regulate itself.

Despite the overall trend of decline, the researchers noted that the city was not entirely lost. There were still pockets of excellent and good ecological quality, though they had become much smaller. The areas classified as having "excellent" quality, where nature was abundant and human impact was minimal, shrank from covering 2.60 percent of the city in 2003 to just 2.42 percent in 2023. The "good" areas, which were once the majority, covering nearly 67 percent of the city, dropped to just 11 percent. Meanwhile, the "moderate" category, representing a middle ground where the environment is under some pressure but not yet in crisis, grew to cover the majority of the city. This shift indicates that while the city has not collapsed into a state of total ecological failure, it has moved into a state of constant stress, where the natural systems are working harder just to maintain their current condition.

The study also highlighted that the changes were not uniform across the city. The transition from one ecological state to another showed that some areas were recovering while others were degrading. In the first period, some poor areas improved, but in the second period, the degradation accelerated. Areas that were once moderate began to slip into fair or poor conditions. The researchers observed that the city's ability to bounce back from damage was weakening. The places that had been healthy in the past were less likely to stay healthy, and the places that were struggling were finding it harder to improve. This suggests that the pressure of urbanization has reached a point where it is difficult for the environment to recover on its own.

The findings point to a clear conclusion: the rapid growth of Sylhet has come at a cost to its ecological health. The initial improvements seen in the first decade of the study were not enough to sustain the city in the long run. The subsequent decline shows that without specific interventions to protect green spaces and manage the spread of buildings, the natural environment will continue to lose ground. The researchers suggest that their method of using satellite data to create a comprehensive score is a valuable tool for city planners. It allows them to see exactly where the city is losing its ecological balance and where it might be possible to intervene.

Ultimately, the paper paints a picture of a city at a crossroads. The data shows that the current path of development is leading to a less resilient environment, one that is hotter, drier, and less capable of supporting biodiversity. The shift of poor ecological zones from the suburbs to the urban center serves as a warning that the core of the city is becoming increasingly vulnerable. The study does not offer a simple fix, but it provides a clear map of the problem. It shows that the trade-off between building more housing and preserving the environment is real and measurable. For the people of Sylhet, and for other cities facing similar rapid growth, the lesson is that the health of the city is not just about the buildings we see, but about the invisible systems of water, heat, and life that support them. If these systems are ignored, the city may grow larger, but it will become a poorer place to live.