Canadian Forest Service Publications

Edge-to-stem variability in wet canopy evaporation from an urban tree row. 2017. Van Stan, J.T. II; Norman, Z.; Meghoo, A.; Friesen, J.; Hildebrandt, A.; Côté, J.-F.; Underwood, S.J.; Maldonado, G. Boundary-Layer Meteorol. 165 : 295-310.

Year: 2017

Issued by: Laurentian Forestry Centre

Catalog ID: 38382

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1007/s10546-017-0277-7

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Evaporation from wet-canopy (EC) and stem (ES) surfaces during rainfall represents a significant portion of municipal-to-global scale hydrologic cycles. For urban ecosystems, EC and ES dynamics play valuable roles in stormwater management. Despite this, canopy-interception loss studies typically ignore crown-scale variability in EC and assume (with few indirect data) that ES is generally <2% of total wet-canopy evaporation. We test these common assumptions for the first time with a spatially-distributed network of in-canopy meteorological monitoring and 45 surface temperature sensors in an urban Pinus elliottii tree row to estimate EC and ES under the assumption that crown surfaces behave as “wet bulbs”. From December 2015 through July 2016, 33 saturated crown periods (195 h of 5-min observations) were isolated from storms for determination of 5-min evaporation rates ranging from negligible to 0.67mm h-1. Mean ES (0.10mm h-1) was significantly lower (p < 0.01) than mean EC (0.16mm h-1). But, ES values often equalled EC and, when scaled to trunk area using terrestrial lidar, accounted for 8–13% (inter-quartile range) of total wet-crown evaporation (ES+EC scaled to surface area). ES contributions to total wet-crown evaporation maximized at 33%, showing a general underestimate (by 2–17 times) of this quantity in the literature. Moreover, results suggest wet-crown evaporation from urban tree rows can be adequately estimated by simply assuming saturated tree surfaces behave as wet bulbs, avoiding problematic assumptions associated with other physically-based methods.

Plain Language Summary

This study demonstrate that the amount of water that evaporates off stems after a rainfall, as measured with suitable instruments, is on average five times higher than the indirect estimates made in the past. The researchers also found that the evaporation from the stems was equivalent to evaporation from the canopy.

Thanks to these results, the canopy’s role can be more accurately considered in rainfall management for urban ecosystems.

The study was carried out on a row of pitch pine, or slash pine, located on Georgia Southern University’s campus in the United States.