Edge-to-Stem Variability in Wet-Canopy Evaporation From an Urban Tree Row

Evaporation from wet-canopy ((E_mathrm{C})) and stem ((E_mathrm{S})) surfaces during rainfall represents a significant portion of municipal-to-global scale hydrologic cycles. For urban ecosystems, (E_mathrm{C}) and (E_mathrm{S}) dynamics play valuable roles in stormwater management. Despite this, canopy-interception loss studies typically ignore crown-scale variability in (E_mathrm{C}) and assume (with few indirect data) that (E_mathrm{S}) 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 (E_mathrm{C}) and (E_mathrm{S}) 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 0.67 (hbox {mm h}^{-1}). Mean (E_mathrm{S}) (0.10 (hbox {mm h}^{-1})) was significantly lower ((p < 0.01)) than mean (E_mathrm{C}) (0.16 (hbox {mm h}^{-1})). But, (E_mathrm{S}) values often equalled (E_mathrm{C}) and, when scaled to trunk area using terrestrial lidar, accounted for 8–13% (inter-quartile range) of total wet-crown evaporation ((E_mathrm{S}+E_mathrm{C}) scaled to surface area). (E_mathrm{S}) 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.