## 4 Discussion
### Limitations and Future Work  
In this study, the spatial and temporal changes in the ecological environmental quality of Jincheng City from 1990 to 2022 were evaluated using the Resource City Ecological Index (RCEI). The results provided a scientific basis and guidance for the planning and development of coal mine areas, urban expansion, and ecological restoration in resource-based cities, particularly those reliant on coal. However, this study had some limitations:  
1. Limitations of Remote Sensing Imagery--The establishment of the RCEI was entirely based on remote sensing imagery, which is inevitably affected by cloud cover, resulting in missing images for some months. Although interpolation and multi-source data fusion techniques can supplement these images, they are not original data and may introduce small errors in subsequent operations. While these errors were within acceptable limits, they still imposed certain limitations on the research results. 2. Complexity of the Ecological Environment--The ecological environment of resource-based cities is complex and variable, with frequent human activities and intricate surface biophysical features. The RCEI consists of six ecological indicators, including the Indexed Coal Dust Index (ICDI) and the Enhanced Vegetation Index (EVI), which were added to represent the impact of coal mining on the atmosphere and vegetation establishment, respectively. However, the lack of socioeconomic factors (e.g., population density, economic activities, and policy changes) in these indicators limits their comprehensive description of resource-based cities. 3. Limitations of Model Validation--Although this study validated the reliability of the RCEI through the spatiotemporal correlation, coefficient correlation, and buffer zone analysis of two typical coal mines, complex large-scale coal mining and rapid urban expansion raised questions about whether the RCEI could adequately describe the temporal and spatial changes in the ecological environment of resource-based cities. Further research and validation are needed. 4. Deficiencies in the Analysis of Driving Factors--While we obtained data on the spatial and temporal changes in the ecological environmental quality of Jincheng City from 1990 to 2022, we did not deeply analyze the driving factors, such as climatic, anthropogenic, and topographical influences. Future work should focus on the following:  
1. In future studies, we plan to incorporate more socioeconomic factors (e.g., population density, economic activities, and policy changes) and coal mining data that are accurate in terms of working surfaces into the Resource City Ecological Index (RCEI) to more comprehensively assess spatial and temporal changes in ecological quality in resource cities. In addition, we plan to place ground-based field monitoring equipment, such as ecosticks, in different ecological change areas to obtain field-based ecological change data. These data will be processed and analyzed in parallel with remote sensing satellite data to effectively verify the accuracy and reliability of the ecological index of resource cities. 2. The RCEI could be applied to other coal resource cities for more detailed and reliable validation analyses. 3. The application potential of the RCEI on satellite sensors such as MODIS, ASTER, AVHRR, IRS, Sentinel, and Gaofen-5 should be explored. This could lead to enhancing the temporal, spatial, and spectral resolutions of remote sensing images through multi-source data fusion to establish a long-term, high-precision RCEI dataset for typical resource-based cities. 4. The analysis of the driving forces behind changes in the ecological environmental quality of resource-based cities should be strengthened and the impact of climate change and human activities on their ecological environmental quality should be quantified. 5. In subsequent studies, we will verify the reliability and validity of the PCA results using regression analyses, support vector machines (SVMs), decision trees, and neural networks.