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

Abstract

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.