Final Report: Understanding the Relationship Between Urban Trees, Stormwater Runoff, and Cold-Water Streams in a Changing Climate
Dates
Publication Date
2019-03-29
Citation
Kevan Moffett, 2019-03-29, Final Report: Understanding the Relationship Between Urban Trees, Stormwater Runoff, and Cold-Water Streams in a Changing Climate: .
Summary
Climate change in the Northwest is causing warmer summer stream flows that can decimate cold-water species like salmon. This problem can be worsened in shoreline embayments or small streams that receive direct stormwater runoff from warm city environments. There has been little assessment, however, at the source: how urban warming – or cooling, such as by tree shade – affects curbside runoff temperatures from streets to storm drains. This research found that warm summer runoff might be cooled at its source via small adaptations in urban forestry, although with an important and unanticipated trade-off between managing street tree canopy type and amount. Evidence was drawn from monitoring a set of residential street gutters in Portland, [...]
Summary
Climate change in the Northwest is causing warmer summer stream flows that can decimate cold-water species like salmon. This problem can be worsened in shoreline embayments or small streams that receive direct stormwater runoff from warm city environments. There has been little assessment, however, at the source: how urban warming – or cooling, such as by tree shade – affects curbside runoff temperatures from streets to storm drains. This research found that warm summer runoff might be cooled at its source via small adaptations in urban forestry, although with an important and unanticipated trade-off between managing street tree canopy type and amount. Evidence was drawn from monitoring a set of residential street gutters in Portland, OR, during rain over two warm seasons. Street runoff at the storm drain was coolest if >35% of a half-street (centerline to curb) was covered by coniferous evergreen (CE) canopy. At <35% cover, broadleaf deciduous (BD) canopy was the cooler choice. Specifically, streets with dominantly CE street trees showed a 0.74 C average storm coolingbenefit for every 10% increase in street-overhanging CE canopy (ranging from average 19.9 C observed at 1.6% cover to 16.2 C at 50.9% cover). Runoff temperatures from streets with dominantly BD street trees, however, were unaffected by canopy cover fraction (17.0 C regardless of cover). These results document and quantify the magnitude of the under-recognized benefit of street tree canopy cover for mitigating urban thermal stormwater pollution, and furthermore one that could be cultivated by small changes in management (e.g., over-street pruning) or planting (type) of curbside tree resources. This"runoff temperature mitigation" ecosystem service of street trees is now being incorporated into a predictive module to add to the types of tree cost/benefit analyses city planners and managers use when considering urban greening impacts on local and downstream waters and ecosystems.
