Abstract: Objective　In recent years, China has experienced rapid urbanization. Inter-city connections are becoming increasingly close, and urban development is gradually forming a new pattern characterized by urban agglomerations. With the rapid economic growth of urban agglomerations, the intensity of human impact on the ecological environment is also continuously increasing. Consequently, ecological and environmental problems, such as the urban heat island effect, extreme weather events, and air pollution, have significantly deteriorated, severely undermining the sustainable development of urban agglomerations. This issue is particularly important for urban agglomerations in Northwest China. To provide scientific support for high-quality development, territorial planning, and resource conservation in urban agglomerations of Northwest China, the coupling and coordination status between urbanization and the ecological environment in these regions is studied in this work by employing the Visible Adjusted Normalized Urban Index (VANUI), the Remote Sensing Ecological Index (RSEI), trend analysis, and the Coupling Coordination Degree (CCD) model. Specifically, we investigated the spatiotemporal dynamics of urban built-up area expansion and ecological environment changes, as well as their coupling and coordination relationships, in four urban agglomerations of Northwest China, namely the Guanzhong Plain urban agglomeration, the Ningxia Yellow River urban agglomeration, the Lanxi urban agglomeration, and the northern slope urban agglomeration of the Tianshan Mountains, from 2000 to 2020. Methods　Firstly, based on nighttime light data and land use data, we employ the VANUI to assess the built-up area expansion of the Guanzhong Plain Urban Agglomeration, the Ningxia Yellow River urban agglomeration, the Lanxi urban agglomeration, and the northern slope urban agglomeration of the Tianshan Mountains, and analyzes and compares their urbanization characteristics. Then, using MODIS remote sensing products, we calculate the RSEI of these urban agglomerations through principal component analysis, including four indicators: Normalized Difference Vegetation Index (NDVI), Land-surface Temperature (LST), Wetness Component (WET), and Normalized Differential Build-up and Bare Soil Index (NDBSI). And the spatio-temporal variation characteristics of ecological environmental quality in the four urban agglomerations are examined using trend analysis. Finally, the CCD model is applied to explore the coupling and coordination relationship between the urban built-up area expansion and the ecological environment in the four urban agglomerations from 2000 to 2020. Results and Discussion　The results show that: (1) From 2000 to 2020, as the built-up land area of the Guanzhong Plain urban agglomeration, the Ningxia Yellow River urban agglomeration, the Lanxi urban agglomeration, and the northern slope urban agglomeration of the Tianshan Mountains steadily increased, the VANUI index of the four urban agglomerations exhibited an upward trend across all regions. Among them, the VANUI index of the Ningxia Yellow River urban agglomeration showed the highest growth rate, whereas that of the northern slope urban agglomeration of the Tianshan Mountains showed the lowest growth rate. (2) The mean RSEI values of the Guanzhong Plain urban agglomeration and the Lanxi urban agglomeration displayed a significant upward trend, increasing from 0.448 and 0.376 in 2000 to 0.565 and 0.496 in 2020, respectively. In contrast, the mean RSEI values in urban expansion zones of each urban agglomeration showed a downward trend, with the most pronounced decline observed in the Guanzhong Plain urban agglomeration, decreasing from 0.292 to 0.259. Generally, the ecological environment in the Guanzhong Plain and the Lanxi urban agglomerations was predominantly improving, the areas of improvement and degradation in the Ningxia Yellow River urban agglomeration were roughly balanced, whereas the ecological environment of the northern slope urban agglomeration of the Tianshan Mountains was primarily degrading. (3) The overall coupling coordination degree between urbanization and ecological environment across all urban agglomerations exhibited a fluctuating upward trend, with the average value increasing from 0.321 in 2000 to 0.622 in 2020. However, the spatial distribution of the coordination degree varied considerably. In 2020, the Ningxia Yellow River urban agglomeration achieved the highest CCD of 0.816, with Shizuishan City exhibiting the highest coordination grade within this agglomeration area. Conversely, the northern slope urban agglomeration of the Tianshan Mountains had the lowest CCD of 0.219 in 2020, with Turpan City recording the lowest coordination grade within this agglomeration area. Conclusions　This study has shown that the urbanization process in urban agglomerations of Northwest China is gradually accelerating, while the ecological environment quality in urban expansion areas is deteriorating. Therefore, in the process of urbanization, it is essential to fully recognize the value of ecological spaces and ecological products, prioritize ecological protection in key regions, strengthen vegetation-based ecological construction, and achieve coordinated development of urbanization and ecological environment.

Key words: urban expansion, spatio-temporal evolution, remote sensing ecological index, coupling coordination degree; urban agglomeration