Executive Summary The turf removal and urban heat study completed by DRI includes both a review and summary of the relevant literature on the heat characteristics of ground covers and a set of drone-based thermal heat measurements made prior and after removal of turf at Henderson city parks. There is a broad literature documenting the properties of various ground cover materials, and for many materials some estimate of how the material may mitigate or exacerbate heat impacts. Nevertheless, we found no clear consensus on a best replacement for turf. We have assembled from the literature a summary of surface material properties relevant to the impact of the material on near-surface heat. Studies of tree impacts find that treatments where new trees replace hard surfaces, cooling of surface temperatures by 0.5 °C to 1.0 °C can be achieved for approximately every 5% of total areal coverage of the tree canopy, potentially yielding as much as 4.7 °C cooling. Six parks in Henderson were comprehensively imaged by drone with a thermal infrared camera at multiple times of day from mid-morning to late afternoon prior to having been treated by removal of non-functional turf. Two parks (Russell Road Recreation Complex and Wells Park) were subsequently imaged by drone with a thermal infrared camera after treatment to remove non-functional turf. Analysis of the imagery shows that the inclusion of drip-irrigated vegetation and/or features composed of small or large stones is certainly mitigating some of the surface warming from converting turf to tan gravel surfaces.The heat signature around areas with mature trees shows that preservation of a mature tree canopy can help reduce the elevated heat signature of replacing turf with gravel by shading areas underneath and adjacent to trees.

The following report summarizes the results of research, information analysis, and data collection activities to advise the City of Henderson in complying with Assembly Bill 356 (AB356). AB356 prohibits the Southern Nevada Water Authority (SNWA) from using Colorado River water from being used to irrigate nonfunctional turf starting 2027. Nonfunctional turf includes grass that primarily serves as a decorative element, often in traffic circles, road medians, and outside buildings. Grass in the Las Vegas Valley consumes 75 gallons per square foot. A third of all grass in the Valley, about 4,000 acres, will be removed by 2026. However, the removal of grass risks exacerbating other challenges the valley faces. As a vegetative surface cover, grass reduces surface albedo an importance component of urban heat island (UHI) effects, transpiration can have a local cooling effect, vegetation cycles soil nutrients, maintains moisture better than bare soil, and offers aesthetic benefits valued by residents. Following AB356, the primary goal is water conservation. However, climate projections suggest warmer temperatures in the southwest and increasing frequency and intensity of extreme heat days. Decisions on ground cover to replace existing turf should seek to not increase urban heat island effects. Research investigating the influence of local design choices on extreme urban heat, as well as the efficacy of strategies to mitigate extreme urban heat, including through ground covers, is emerging.

Executive summary iii1. Introduction 12. Overview of surface type heat impacts 13. Heat mapping with drones 33.1 Pre-treatment sampling 43.2 Post-treatment sampling 54. Russell Road Recreation Complex: pre- and post-treatment comparison 64.1 Overview of the pre- and post-treatment comparison 64.2 Untreated control comparison: outfield areas of the two large baseball fields 84.3 Gravel vs. stone surface treatments 104.4 Gravel with drip-irrigated shrubs 115. Wells Park: pre- and post-treatment comparison 125.1 Overview of the pre- and post-treatment comparison 125.2 Untreated control comparison: grass in the grove of trees north of the baseball field 155.3 Comparison of treated surfaces pre- and post-treatment 156. Summary 196.1 Summary of key points from literature review 196.2 Summary of key points from drone heat mapping research 196.3 Summary of recommendations 206.4 Additional considerations 217. References 228. Appendix A: Summary of drone flights 239. Appendix B: Orthomosaic thermal maps of Russell Road Recreation Complex Soccer Fields 34

Contact Email Addresses Eric.Wilcox@dri.edu Marco.Giordano@dri.edu