Limited extent with big intent: Effects of urban residential landscaping at the heat-water nexus

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Victoria K. Dearborn, Graduate Group in Ecology, University of California - Davis, Davis, CA and Mary L. Cadenasso, Department of Plant Sciences, University of California, Davis, Davis, CA

Background/Question/Methods
Semi-arid cities face a combination of climate vulnerabilities, including extreme heat and drought. Urban heat has serious implications for energy consumption, air quality, and heat related illness and mortality. Urban vegetation, especially well-irrigated green space and tree canopy, has the potential to mitigate urban heat. However, semi-arid cities must weigh the cooling effects of urban vegetation against water conservation needs. Many cities have adopted residential outdoor water use reduction policies, resulting in an increase in drought-tolerant yard landscaping. Unfortunately, drought-tolerant landscaping has the potential to increase surface air temperatures across urban environments, exacerbating urban heat. Still, questions remain about whether drought-tolerant landscapes affect microclimates at the scale at which residents differentially experience heat: within their own yards. We investigated the effects of landscaping choices on temperature in yards across Sacramento, CA, asking, 1) Are yards landscaped for drought-tolerance hotter than those with conventional turf grass? and 2) Does tree canopy have the potential to mitigate heat in drought tolerant yards? To answer these questions, we measured air temperature, relative humidity, and wind speed and direction at a 2m-height in 147 drought-tolerant and 107 conventional turf grass residential yards nested in 29 neighborhoods representing a range of tree canopy cover.
Results/Conclusions
We found that yards with drought-tolerant landscaping were significantly warmer than their conventional turf grass counterparts; however, the magnitude of this difference was relatively small (0.3 °C) and is unlikely to be a determining factor of heat-related illness. Canopy cover within the yard offered a cooling effect across both landscaping types, but shaded drought-tolerant yards were still hotter than shaded turf yards. Drought-tolerant yards included a wide range of landscaping materials, such as gravel, mulch, succulents, and shrubs. We found that drought-tolerant yards with predominantly living landscaping (e.g., succulents, perennial grasses) were 0.8 °C cooler than those with predominantly non-living landscaping (e.g., pavement, gravel). This demonstrates that the heterogeneous land cover composition of these yards may differentially affect air temperature, and thus additional fine-scale research is needed to determine which specific landscaping configurations exacerbate heat. Our study demonstrates the importance of quantifying how fine-scale changes in urban landscapes can affect the severity and distribution of heat archipelagoes within cities. Advancing our understanding of heat archipelagos within cities also allows individual residents and municipal policy-makers to make informed landscaping decisions at the intersection of drought and heat risk.