Warm stormwater runoff from streets has the potential to increase urban stream temperatures and degrade sensitive and threatened downstream cold-water aquatic habitats. Under future climate scenarios that predict lower streamflows and rising in-stream temperatures, warm urban runoff may further imperil these species. Managing urban forests, especially canopy overhanging paved streets, to help cool cities, manage runoff, and reduce runoff temperatures may be an effective way to climate-adapt urban environments using existing “green infrastructure.” While urban warming is well appreciated as an integral factor in today’s urban ecology and changing phenology, there is almost no data on the links between urban forests and runoff temperatures. We ask: to what degree is urban street runoff temperature controlled by street tree interception and runoff quantity reduction, street tree influence on air temperature, or direct pavement shading? To answer this question we measured temperature, water, and weather variables (air, pavement, runoff temperature, runoff volume, precipitation, solar radiation) and tree metrics (including canopy cover fraction) for 12 residential street blocks in Portland, Oregon during the most sensitive (warmest) late summer season of 2016, 3 streets for each combination of low vs. high street tree canopy cover and coniferous vs. deciduous tree type.
Runoff temperatures were significantly negatively related to the amount of street tree canopy cover. Air temperatures measured in full sun and in shade were different between the sets of streets dominated by coniferous vs. deciduous trees. Air temperatures were also strongly positively related to measured gutter pavement temperatures. The pavement-to-air temperature relationship also differed between streets dominated by coniferous vs. deciduous and high vs. low canopy. Impervious surfaces in sun on streets with less canopy were warmer than those on streets with a high cover. Within impervious surface types, house roofs were by far the warmest. The difference between sun and shade surface temperatures was very small for shrubbery/garden and some other pervious surfaces, but notably mulched ground surfaces in the sun were significantly warmer than in the shade. Mulch and bare ground in the sun were cooler on streets with high canopy fraction compared to low. This work begins to demonstrate how public investment in urban forest could help cool both runoff to sensitive aquatic ecosystems as well as urban air temperatures, important for human comfort and urban ecology, and also that private landscaping and roofing materials have a role to play.