UC Davis

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. Questions remain about whether drought-tolerant landscaping affects microclimates at the scale at which residents differentially experience heat: within their own yards. I investigated the effects of landscaping choices on temperature and thermal comfort in yards across Sacramento, CA, asking, 1) Are yards landscaped for drought-tolerance hotter than those landscaped with conventional turfgrass? and 2) Does the tree canopy interact with landscaping to affect yard-scale heat? To answer these questions, I sampled drought-tolerant and conventional turfgrass yards that had a range of tree canopy cover, at both yard- and patch-scales. I measured air temperature, relative humidity, incoming solar radiation, and wind speed and direction within the yards, and estimated thermal comfort using the NOAA Heat Index (HI), Universal Thermal Comfort Index (UTCI), and Physiological Equivalent Temperature (PET).

When considered independently from the effects of tree canopy, air temperature anomalies were +0.30°C greater in drought-tolerant yards. Canopy cover within the yard offered a cooling effect across both landscaping types. The effect of high amounts of tree canopy on improving thermal comfort was more dramatic in drought-tolerant yards. In yards with low canopy cover, drought-tolerant landscaped yards exhibited an average UTCI anomaly of +3.32°C, while conventional yards were cooler at just +1.47°C. In yards with high canopy cover, however, drought-tolerant yards had an average UTCI anomaly of -3.69°C and conventional yards had an average anomaly of -2.66°C. Drought-tolerant yards were extremely heterogeneous and included a wide range of landscaping materials. Regardless of canopy cover, drought-tolerant yards with predominantly living landscaping (e.g., succulents, perennial grasses) had cooler air temperatures than those with predominantly non-living landscaping (e.g., hardscape, gravel).

This study demonstrates the importance of quantifying how changes in urban landscaping at the scale of an individual yard can affect the severity of heat experienced by residents. Advancing our understanding of localized heat and vegetation dynamics allows residents and municipal policymakers to make informed decisions at the intersection of water conservation and heat risk.