Influences of Street Tree Canopy Type and Amount on Stormwater Runoff Temperature

Thermally enriched air and stormwater runoff from urban areas have been identified as contributing heat sources to downstream surface waters. To date, the study of air quality, cooling, stormwater, and health benefits of urban forests have been widely covered, but surprisingly little research has focused on quantifying the relationship between urban air and stormwater runoff temperatures and furthermore, how these heat sources may affect downstream ecosystems. We measured temperature, water, and weather variables (air, pavement, and runoff temperatures, precipitation, solar radiation), and tree metrics (type, height, canopy cover fraction) at 12 study sites in Portland, Oregon during the warm summer season of 2016 and 2017. A two-factor factorial approach was used to select study sites that were of two tree types (broadleaf deciduous or coniferous evergreen) and varying levels of pavement-overhanging street tree canopy cover (high or low canopy fraction). Pavement temperatures in the absence of rain were measured ≤66◦C in the sun. Heat transfer from paved surfaces to runoff during low intensity summer storm events resulted in stormwater runoff temperatures that averaged 17.3◦C over 200 storm events analyzed. Coniferous evergreen dominated sites with high canopy cover fractions resulted in the coolest summer stormwater runoff temperatures on average (<16◦C at > 50% cover) and coniferous evergreen dominated sites with low canopy cover fractions the warmest (>19.5◦C for <2% cover). In contrast, runoff temperatures measured at broadleaf deciduous sites were surprisingly insensitive to the wide range in canopy cover fractions (1.6 % to 50.9 %) where canopy cover outside of the monitored catchment area may have had a confounding effect, potentially muting or masking effects of deciduous canopy cover fraction that might occur in other urban settings or sites. Average air temperature, accumulated precipitation, and the interaction between tree type and high or low canopy fraction during a storm explained 77% of the variance observed in runoff temperatures across all storm events. These results suggest that summer runoff temperature may be an under-appreciated benefit of street trees, when considering urban impacts on downstream surface waters, that is influenceable through management of tree type and cover.