Assessment of Urban Vacant Lands Greening Potential Based on an Urban Heat Island Network: A Case Study of the Core Urban Area of Hangzhou

Objective Amid global climate change and rapid urbanization, the heat island effect has emerged as a major challenge in urban environments. In densely built-up areas, high population concentration and reduction of green spaces contribute to the gradual merging of heat island areas, forming a networked structure that intensifies thermal environmental risks. Small-scale green spaces are vital resources for disrupting the connectivity of heat island sources and regulating the local climate, offering additional opportunities for optimizing the urban thermal environment pattern. However, in central urban areas where land resources are scarce and competition for land use is intense, creating large-scale new green spaces remains challenging. In this context, several key scientific questions remain unresolved: How does the spatial connectivity of heat island sources form and influence the urban thermal environment? Is there a structural correlation between the distribution of vacant land and the heat island network? How can we scientifically determine the greening priority of vacant land to effectively mitigate the heat island effect? To address these issues, this study analyzed the spatial distribution of vacant land within the urban thermal environment, assessed its greening potential, reasonably established the priority of conversion to green spaces, and proposed a phased implementation strategy, thereby providing a scientific basis for mitigating the urban heat island effect and optimizing the spatial pattern of green spaces. Methods Focusing on the core urban area of Hangzhou (China) and comprehensively considering both two-dimensional and three-dimensional urban environmental characteristics, research was conducted to systematically construct an evaluation framework for assessing urban vacant land greening potential based on the heat island network using the circuit theory method. Key procedures included the following: 1) Obtaining spatial location information for vacant land within the study area and classifying it based on land cover characteristics. 2) Identifying the distribution of heat island sources through land surface temperature retrieval, combined with morphological spatial pattern analysis and thermal environment connectivity analysis. 3) Comprehensively selecting 15 urban environmental indicators (8 natural environment and 7 built environment indicators) incorporating two-dimensional and three-dimensional factors to construct a heat island resistance surface. 4) Building a heat island network based on circuit theory to precisely identify key nodes within the urban thermal environment network, revealing the spatial relationship between vacant land distribution and the heat island network. 5) Determining the priority hierarchy for converting vacant land into green spaces based on the structural characteristics of the heat island network and heat island corridor width. 6) Evaluating the potential value and opportunities of vacant land in optimizing the urban thermal environment pattern and mitigating the heat island effect. Results The following results were derived: 1) A total of 16 primary heat island corridors with width of 540 m and length of 7.34 km, 42 secondary heat island corridors with width of 150 m and total length of 169.68 km, and 18 heat island pinch points with a total area of 0.65 km2 were identified within the study area. 2) Among the 228 vacant land parcels, 21 are located within the primary heat island corridors, with 2 directly intersecting heat island pinch points. Additionally, 37 are located within the secondary corridors. This distribution indicates an important spatial association between the vacant land and key structures within the heat island network. 3) The greening reconstruction of vacant land should be prioritized in three stages: first, “high-efficiency conversion parcels intersecting with primary heat island corridors and heat island pinch points,” followed by “high-efficiency conversion parcels intersecting with primary heat island corridors,” and finally, “high-efficiency conversion parcels intersecting with secondary heat island corridors.” This phased approach effectively blocks heat transfer and enhances the resilience of the urban thermal environment. 4) Based on the above results, this study further proposed the following gradient renewal pathway for vacant land. Transform the spatial association between vacant land and the heat island network into opportunities for ecological restoration, implement “targeted rescue” for 9 vacant land parcels within the core area that intersect with primary corridors, and carry out “batch restoration” for 33 vacant land parcels in peripheral areas intersecting with secondary corridors. This approach achieves precise allocation and efficient utilization of ecological resources and allows the remaining 186 vacant land parcels to be used for urban redevelopment purposes to balance land use costs. Conclusion The evaluation framework developed in this study provides methodological support and decision-making reference for improving the thermal environment and implementing precise greening interventions in high-density built-up areas facing land use constraints. By innovatively integrate both two-dimensional and three-dimensional environmental factors in the construction of the heat island resistance surface, and applying smooth curve fitting alongside threshold effect analysis to measure the width of heat island corridors, the spatial simulation accuracy of the urban heat diffusion process is substantially improved. Furthermore, incorporating the cost of vacant land reconstruction into the evaluation helps comprehensively balance the social, economic, and environmental benefits associated with vacant land reuse.