斑块面积对林地降温效率影响的能量平衡机理研究

Urban greenspace provides important ecosystem services in urban heat mitigation. Both the percent coverage and spatial configuration of urban greenspace significantly affect urban thermal environment. With limited space for greening in cities, how to optimize the spatial configuration of urban greenspace to maximize its cooling efficiency, that is, the cooling capacity of the greenspace of a unit area, is increasingly becoming important for improving the urban thermal environment and the living comfort of the residents. Here, we choose Shenzhen, one of the fastest developing cities in China, as a case study, to investigate how the patch size of woody vegetation (i.e., trees) affects its cooling efficiency. We will use unmanned aerial vehicle (UAV) platform to acquire the area and height of woody patches and their surface temperatures, and then we can calculate the energy that is reduced by the two key cooling processes of woods patches: evapotranspiration and shading. We will retrieve the latent heat caused by evapotranspiration and simulate the solar radiation reduced by shading. We will then calculate the cooling efficiency of all the patches based on patch sizes. Base on the relationship between woody patch area and its cooling efficiency, we will explore the existence of optimal threshold of patch area, at which maximum of the cooling efficiency occurs. In addition, we will further investigate whether such threshold varies by different local climate zones (LCZ) with varied temperature and humidity, and what are the driving forces of the change in the threshold. The results from this study can help us better understand the cooling efficiency of urban greenspace, and provide useful information for urban planning and management.

城市热环境受到绿地覆盖及其空间配置的显著影响。在绿地空间有限的前提下，如何优化其景观格局，最大化降温效率（即单位面积绿地降温能力），是城市热环境研究的热点问题。本研究以深圳为研究区域，以降温效果最强的绿地类型－林地为研究对象，探讨斑块面积与降温效率的定量关系及其能量平衡机理。项目拟通过无人机航拍获取林地斑块和三维信息，结合实地观测与模型估算等技术手段，基于林地降温的两个关键过程—蒸散与遮阴，计算不同面积林地斑块的蒸散潜热通量和遮阴减少的太阳能量输入，以能量的方式刻画林地的降温效率。本研究将量化林地斑块面积与其降温效率的关系，进一步分析该定量关系对不同的局地景观格局和微气候条件的响应，探寻是否存在最大降温效率的斑块面积阈值。本研究有助于深化对绿地降温机理的认识，可为改善城市热环境、提高城市绿地降温效率的规划设计提供科学依据，具有重要的科学价值和实践意义。

增加城市绿地可以有效改善城市热环境，然而城市空间有限，土地资源紧张，绿地空间总量受到限制。因此，如何优化城市绿地的空间配置，以最大限度地提高其降温效率对于改善城市地表热环境尤为重要。本项目以深圳作为研究区，从林地蒸腾和遮阴两个角度，探究了林地斑块面积与降温效率之间的关系及其机制，为面向城市热环境改善的城市绿地规划优化提供科学依据。主要研究成果如下：.1）揭示了具有最优降温效率的林地斑块面积阈值。遮阴效率和蒸腾效率随着林地斑块面积的增加呈“此消彼长”的趋势。林地遮阴效率随着斑块面积增加而降低，超过7hm2后其下降趋势逐渐平缓；蒸腾效率随斑块面积的增加逐渐增加，超过5hm2后上升趋势逐渐平缓。综合遮阴和蒸腾效率随斑块面积的变化趋势，具有最优降温效率林地斑块面积应不超过5hm2，最佳斑块面积的具体值因遮阴和蒸腾对总体降温作用的贡献程度而异。结果可为缓解城市热岛的绿地规划设计提供定量的科学支撑。.2）阐明了林地斑块面积对降温效率影响的机理机制。从遮阴的角度，林地斑块面积的变化将改变边界密度从而影响林地遮阴效率。当绿地总面积一定时，随着平均斑块面积的增加，边界密度下降，导致遮阴效率下降。从蒸腾的角度，揭示了林地斑块面积—斑块内部温湿度—植物蒸腾速率的联动影响机制。林地斑块面积的增加通过林地总量以及对外界环境响应程度的变化，改变了绿地的局地微气候环境，进而影响植物蒸腾作用，改变了林地的蒸腾效率。结果为绿地降温相关研究提供了新视角。.3）发现了局地景观格局与微气候对降温效率有显著影响。在开阔或紧凑高建筑群局地微气候类型，林地降温效率更高；在局地温度较低的密集森林局地微气候类型，林地降温效率更低。研究结果系统揭示了局地建筑布局、局地气温对林地降温效率的影响，深入认知林地降温作用在城市内部的空间异质性。结果可为系统性的城市规划设计提供有力支撑。.