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Rainfall Interception by Six Urban Trees in San Juan, Puerto Rico

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Rainfall Interception by Six Urban Trees in San Juan, Puerto Rico Urban Ecosystems https://doi.org/10.1007/s11252-018-0768-4 Rainfall interception by six urban trees in San Juan, Puerto Rico Christopher J. Nytch1 & Elvia J. Meléndez-Ackerman1 & María-Eglée Pérez2 & Jorge R. Ortiz-Zayas1 # The Author(s) 2018 Abstract Rainfall interception is an important part of the urban hydrological cycle. Understanding is limited about the role of urban trees and other vegetation in the interception process. This study quantified interception losses by six trees in the Caribbean coastal city of San Juan, Puerto Rico, three representing a broadleaf evergreen, and three representing a broadleaf deciduous species. Rainfall was partitioned into throughfall for 13 storms to compare the results between tree types. Total rainfall ranged from 2.9 to 72.4 mm, and storm duration spanned 1 h to several days. Six of the storms analyzed were characterized by maximum hourly intensity rainfall rates categorized as heavy (> 7.6 mm/h). Strong northeasterly winds brought rain in sustained gusts up to 35 km/ h. Average interception losses totaled 19.7% for both tree types, 22.7% for the deciduous trees, and 16.7% for the evergreen trees. Throughfall exceeded 90% of total rainfall for each of the six trees on one or more occasions, and heavy intensity storms produced negative interception losses in one individual. The effect of tree type on interception was significant for storms of low and moderate intensity, but not heavy intensity. Differences in interception losses between storms of similar intensity and between the two tree types were influenced by leaf area and wind. Results suggest that individual urban tree canopies function as spatio-temporally dynamic storage reservoirs whose interception capacity can vary as micro-meteorological conditions change. These findings help advance understanding about interception processes in humid tropical urban settings. Keywords Interception . Rainfall intensity . Urban trees . Subtropical . Puerto Rico Introduction located in arid to tropical climates, with reported annual inter- ception losses ranging from 10 to 50% of total precipitation Rainfall interception by trees is an important part of the urban (Carlyle-Moses and Gash 2011). Relatively few analyses have hydrological cycle, and the spatial and temporal distribution investigated interception processes in urban areas and how of the hydrographic curve in cities (Xiao and McPherson they alter the timing and distribution of runoff. Nevertheless, 2002; Livesley et al. 2016). Interception processes are regu- urban forest canopies have been reported to intercept storm lated by vegetation at multiple scales and in diverse settings precipitation (Inkiläinen et al. 2013;Livesleyetal.2014;Xiao (Crockford and Richardson 2000; Llorens and Domingo and McPherson 2016), with consequent reductions in runoff 2007), but there is limited understanding of this heterogeneity volumes across the landscape (Wang et al. 2008; Berland and across urban land cover types. The majority of rainfall inter- Hopton 2014;SchoolingandCarlyle-Moses2015). Together ception studies have been conducted in rural forest ecosystems with other green spaces, urban and peri-urban trees can con- tribute to avoided flood control costs and lowered flood risk Electronic supplementary material The online version of this article (Zhang et al. 2015; Kim et al. 2016; Livesley et al. 2016). (https://doi.org/10.1007/s11252-018-0768-4) contains supplementary material, which is available to authorized users. Studies of urban trees in general have focused on temperate zones (Roy et al. 2012), and experimental interception studies * Christopher J. Nytch in urban areas are concentrated in northern temperate dry and [email protected] humid and subtropical dry climates: Xiao et al. (2000a), Xiao and McPherson (2011), and Xiao and McPherson (2016)stud- 1 Department of Environmental Sciences, University of Puerto ied interception losses of urban, open-grown hardwood trees Rico-Río Piedras, PO Box 70377, San Juan, PR 00936-8377, USA in the Mediterranean climate of central California; Guevara- 2 Department of Mathematics, University of Puerto Rico-Río Piedras, Escobar et al. (2007)examinedcanopyinterceptionbya PO Box 70377, San Juan, PR 00936-8377, USA broadleaf evergreen tree in the semi-arid climate of Urban Ecosyst Querétaro City in central Mexico’s high plateau; Asadian and The interception process Weiler (2009) investigated throughfall losses by urban conif- erous trees in the cool, humid climate of Vancouver, British Rainfall interacts with tree crowns, with some droplets passing Columbia; Livesley et al. (2014) monitored rainfall intercep- directly to the ground as free throughfall, or stored on leaf, tion and stemflow by eucalypt street trees in Melbourne, branch, and bark surfaces. Stored water can later fall as cano- Australia; and Holder and Gibbes (2017)examineddiffer- py drip and contribute to throughfall, be diverted via branches ences in rainfall interception among species and their effect and trunks to the ground as stemflow, or be intercepted and on urban hydrology in the semi-arid region of the Colorado evaporated back to the atmosphere (Rutter et al. 1975;Xiaoet Front Range. Rainfall interception studies in rural subtrop- al. 2000b; Carlyle-Moses and Schooling 2015). Intercepted ical and tropical forest settings abound (see Schellekens et rainfall can be calculated as the difference between total rain- al. 1999; Marin et al. 2000; Muoghalu and Johnson 2000; fall above the canopy and net precipitation below the canopy Kumagaietal.2005;Holwerdaetal.2006; Fan et al. 2014; following extraction by plant surfaces (Xiao et al. 2000a; Chen and Li 2016), highlighting the importance of inter- David et al. 2006a). There are several factors that determine ception processes in the hydrologic cycle. Yet there is a the quantity of precipitation that is captured through intercep- disproportionate lack of knowledge about interception pro- tion processes. These include local and micro-meteorological cesses in the urban tropics. One notable exception is a study conditions (e.g., total rainfall amount, rainfall intensity, event by Alves et al. (2018) on the interception capacity of nine duration, storm frequency, air temperature, wind speed and city trees in Uruaçu, Brazil. In Central America, Calvo- direction, and humidity), which control precipitation inputs Alvarado et al. (2009) conducted a study in the tropical as well as the rate of evaporation from the tree canopy; the lowlands of Costa Rica. However, few to no urban inter- morphological structure of individual tree canopies (e.g., stem ception studies have been published for the Caribbean branching patterns and angles, leaf and stem surface area, gap region. fraction), which influence the volume and timing of within- The hydrological cycle of the humid tropics is driven by canopy flow; and the spatial characteristics and structure of intense convective precipitation events that are anticipated to trees in a given stand (e.g., species, tree height and canopy increase in response to global climate change (Wohl et al. dimensions, stem density), which regulates the collective vol- 2012). Urban centers in the humid tropics are also experienc- ume and timing of flow within a stand (Crockford and ing burgeoning population growth as people migrate to cities Richardson 2000; Xiao et al. 2000b; Staelens et al. 2008; from rural areas (DeFries et al. 2010;Setoetal.2012). Xiao and McPherson 2016). Urbanization processes typically increase the proportion of The structural morphology of urban forests and microcli- impervious surfaces that contribute to stormwater runoff matic conditions can vary from that found in natural forest (Shuster et al. 2005; Jacobson 2011). This combination of stands, with consequent effects on interception losses (Xiao factors will contribute to an increase in the number of urban et al. 1998, 2000a; Inkiläinen et al. 2013). Many street and dwellers living in flood-prone areas in the next few decades park trees are open-grown or isolated, without competition (Güneralp et al. 2015). Assessing interception by subtropical from other vegetation, which may lead to greater amounts of and tropical urban trees is essential to broaden the body of intercepted rainfall by individual trees due to larger canopies knowledge about green infrastructure and urban ecosystem (Xiao and McPherson 2002; Schooling and Carlyle-Moses services (e.g., Wang et al. 2008;JimandChen2009; 2015). Broader, exposed crowns are also subjected to higher Livesley et al. 2016; Xiao and McPherson 2016). It is also wind speeds (Xiao et al. 2000a), as well as elevated ambient relevant to informing management practices related to temperatures typical of urban areas (Zipperer et al. 1997; stormwater drainage, environmental quality, and Shepherd 2006). Together these factors can modify intercep- sustainability/urban resilience planning for adapting to climate tion volumes through changes in the angle of incidence at change (Demuzere et al. 2014; Liu et al. 2014; Silvennoinen et which rainfall strikes the canopy (King and Harrison 1998; al. 2017). For example, numerous cities, including Xiao et al. 2000a; Carlyle-Moses and Schooling 2015), and Philadelphia (Philadelphia Water Department 2011), increased evaporation (Xiao et al. 2000a; Guevara-Escobar et Singapore (Lim and Lu 2016), and Melbourne (Ferguson et al. 2007; Asadian and Weiler 2009). Alternatively, municipal al. 2013) have allocated tens of millions of dollars toward maintenance practices such as
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