Abstract: Since its establishment as the capital of the Ningxia Hui Autonomous Region, the city of Yinchuan has been facing an increasingly serious urban-heat-island effect due to rapid urbanization. To provide insight into the city’s changes, thermal environment variation, and the response relationship of urban built-up area expansion and the thermal environment in Yinchuan during the past 28 years, this study used Landsat 5 TM and Landsat 8 OLI im? age data for 1989, 1999, 2010, and 2017 to extract urban built-up area using the index-based built-up index, and the expansion intensity, speed, compactness index, fractal dimension, and center of gravity from urban morphology evolution indices such as AGR, AGA, BCI for the same period. The study also used land surface temperature (LST) values retrieved through the thermal infrared bands based on a radiative transfer equation, and calculated the ur? ban-heat-island ratio index (URI) for these four years through normalized LST. Lastly, the space-time evolution of urban built-up area expansion and thermal environment variation were studied and their response relationship was discussed. The results showed the following: (1) During 1989-2017, urban built-up area expanded rapidly, increas? ing by nearly 506.13 km2. The expansion intensity and speed of different periods varied greatly; however, they pre? sented periodical characteristics with slow-rapid-steady expansion phases. The compactness index showed an up? ward trend while the fractal dimension showed a downward trend overall, which demonstrated that the urban spatial form tended to be compact and stable. The city expanded to the east and north and the city’s center of gravity shift? ed nearly 5.54 km to the northeast. (2) The heat island area gradually increased with urban expansion, with areas of higher temperature, high temperature, and extremely high temperature increasing by factors of approximately 9, 10, and 4 over the past 28 years, respectively. However, the heat intensity transferred from the region of high tempera? ture and extremely high temperature to the higher temperature region, thus relieving the heat island effect. The heat island’s spatial distribution showed that it spread from the old town area of Xingqing district to Helan County and Xixia district. Meanwhile, the Xingqing district heat island gradually evolved into an independent small secondary island, alleviating the intense overall heat island effect of Yinchuan, and URI showed an increasing trend first and then a decreasing trend but an overall increase. (3) The spatial distribution and expansion direction of the high tem? perature region of the LST is highly consistent with urban expansion. In addition, urban land, buildings, and bare land could increase LST, whereas grass land and water could decrease it, meaning that urban green spaces and wa? ter were able to effectively alleviate the heat island effect, with water demonstrating greater LST decreases than ur? ban green spaces.

Key words: urban build-up area expansion, thermal environment variation, Land Surface Temperature (LST), re? mote sensing, Yinchuan City