Evidence-based urban green infrastructure planning in humid subtropical neighbourhoods to enhance outdoor thermal comfort

Strategic urban green infrastructure (UGI) planning is crucial to mitigate heat stress and foster climate-responsive urban areas that promote liveability, especially in hot and humid subtropical regions. However, paucity of empirical data on UGI-based heat mitigation has led to a dearth of effective urban green spaces in most Indian cities. This study, therefore, aims at developing actionable, evidence-based UGI planning strategies to enhance outdoor thermal comfort (OTC) by taking the case of two typical residential typologies in Dehradun, India. The selected neighbourhoods represent 1-2 storeyed plotted individual houses and 3-4 storeyed row block housing, respectively and include three urban settings: housing park, roadside plantation and private gardens or shared courtyards, for a more focussed analysis.

Context-specific ‘Quality and Quantity’ of UGI are critical for its cooling performance. This necessitates a need to understand the performance of subtropical tree species based on their traits, their placement in the aforementioned urban settings and the role of canopy cover in maximising OTC. Moreover, the comparative performance of trees and UGI types like green roofs and green walls needs to be understood in realistic neighbourhood settings particularly in Indian context. Therefore, we investigate the ‘Right: UGI type, Tree species, Planting arrangement and Canopy cover’ approach using microclimatic simulations on validated ENVI-met software.

The simulation results indicate that trees are significantly more effective in improving human outdoor thermal comfort as compared to green roofs and green walls. The existing trees on the study sites reduce average PET (Physiological Equivalent Temperature) between ~2-9°C under dry and well-irrigated soil conditions during the daytime heat hours (10 a.m. -5 p.m.). Besides, the cooling potential of different tree species varies with their morphological characteristics, and the dense canopy (high LAD) trees have maximum cooling impact during peak heat stress. The impact of LAD becomes even more pronounced in combination with tree height and canopy width due to more widespread shade and evapotranspiration. The simulation results also highlight the influence of planting arrangement on shade, wind speed, and direction on the site. The tree arrangements parallel to the wind and facilitating evenly distributed shade have greater impact on enhancing OTC. Another finding substantiates the beneficial role of increasing overall canopy cover on the site. However, the combined impact of greening strategies like ‘right tree in the right place’ is more beneficial, even in the case with lesser canopy cover than the existing one. This could be particularly beneficial in urban areas with land scarcity.

Therefore, the study provides several empirical evidences that confirm the significance of UGI in improving OTC, as well as a holistic approach for strategizing UGI planning for neighbourhood climate adaptation. The findings of this study can be useful for landscape planners, policymakers and similar actors in comparable urban and climatic contexts. Future research can also test the impact of vegetation diversity on heat stress mitigation to further promote biodiversity and resilience in urban areas. Role of all the UGI types can also be assessed for other ecosystem services, such as stormwater management, for comprehensive climate adaptation.