Forest Structure Influences on Rainfall Partitioning and Cloud Interception
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Agricultural and Forest Meteorology 150 (2010) 265–275 Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet Forest structure influences on rainfall partitioning and cloud interception: A comparison of native forest sites in Kona, Hawai’i Kate A. Brauman a,*, David L. Freyberg b, Gretchen C. Daily c a Emmett Interdisciplinary Program in Environment and Resources, Yang and Yamazaki Environment and Energy Building, MC 4210, 473 Via Ortega, Suite 226, Stanford University, Stanford, CA 94305, United States b Department of Civil and Environmental Engineering, Yang and Yamazaki Environment and Energy Building, MC 4020, 473 Via Ortega, Rm 257, Stanford University, Stanford, CA 94305, United States c Department of Biology and Woods Institute for the Environment, 371 Serra Mall, MC 5020, Stanford University, Stanford, CA 94305, United States ARTICLE INFO ABSTRACT Article history: Vegetation can play a major role in the hydrologic ecosystem service tradeoffs resulting from land use Received 21 May 2009 change: by affecting the volume of rain water that reaches the ground surface, vegetation affects water Received in revised form 9 October 2009 supply. But because canopy interception of rainfall is affected in complex and competing ways by forest Accepted 15 November 2009 structure and ambient weather conditions, both of which vary at small and large scales, predicting the impacts of vegetation change is challenging. Keywords: We explore rainfall and cloud interception in two native forests sites on leeward Hawai’i Island and Canopy interception find that although our study forests are superficially similar, with identical dominant species and no Fog drip history of logging, throughfall in one forest is nearly double that in the other. Using micrometeorological Hawaii Ecohydrology and vegetation data collected over 20 months, we examine the dominance of different hydrologic Rainfall partitioning processes at each site. Tropical montane cloud forest Direct fog water input accounted for at least 12% of total water input in the North forest site, an average of about 0.1 mm/day, and 27% of total water input in the South forest, an average of 0.3 mm/day. In the North forest, canopy interception of rainfall dominates, and annual throughfall amounts to only 64% of rainfall. A large canopy surface area and low rainfall rates cause the high rate of interception. Direct interception of clouds by the canopy dominates in the South forest, where throughfall is 113% of rainfall. Increased throughfall at this site is not attributable to increased fogginess. Instead, taller trees and a denser mid-canopy increase canopy surface area, causing increased cloud interception. The denser forest structure is likely a result of cattle exclusion and limited grazing in this forest. This study illustrates the effects of subtle differences in vegetation structure on hydrologic fluxes and, by extension, the hydrologic effects of land use change. It also underscores the importance of replicate sites in ecohydrologic investigations. ß 2009 Elsevier B.V. All rights reserved. 1. Introduction Payment schemes to enhance watershed ecosystem services have been created throughout the world (Engel et al., 2008); better Forests are prominent features of the watersheds upstream of understanding the effects of vegetation on hydrologic fluxes will approximately 57% the world’s population (Millennium Ecosystem improve the effectiveness of these payments. Assessment, 2005). In these forested ecosystems, the partitioning In most cases, net rainfall under a vegetated canopy is less effects of vegetation on rainfall determine the distribution of water than gross rainfall in unobstructed areas. During a rain event, available for surface runoff, groundwater recharge, and water use some drops fall through the canopy directly to the ground while by plants. When water supply catchments come under develop- others are intercepted by trees, shrubs, and grasses. Water that ment pressure, comparing the effects of existing and projected falls through the canopy or drips from leaves is measured at the vegetation on water supply provides one basis for assessing the ground as throughfall; water preferentially funneled down tradeoffs inherent in land conversion (Brauman et al., 2007). stems and trunks is measured at the ground as stemflow; water collected on leaves and branches is described as intercepted; and water that evaporates from vegetation directly into the * Corresponding author. Tel.: +1 650 380 0387; fax: +1 650 725 4139. atmosphere constitutes interception losses (Crockford and E-mail address: [email protected] (K.A. Brauman). Richardson, 2000). 0168-1923/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2009.11.011 266 K.A. Brauman et al. / Agricultural and Forest Meteorology 150 (2010) 265–275 Vegetation structure determines rainfall partitioning, but 2. Methods interception of rainfall in a vegetated canopy and drip from a saturated canopy are complex processes that are often difficult to 2.1. Site description predict, particularly in the tropics (Herwitz, 1985). Throughfall in different forest types and climates varies widely: throughfall We quantify throughfall-rainfall relationships in two native ranges from 70 to 90% of rainfall in broadleaf and coniferous forest sites and adjacent pastures on the western side of Hawai’i temperate forests and from 60 to 95% of rainfall in tropical forests Island in the districts of North and South Kona. The North site is (Berger et al., 2008; Bruijnzeel, 2004; Crockford and Richardson, located inland of Kahalu’u Bay; the South site is located 13 km to 2000; Germer et al., 2006; Levia and Frost, 2006; Llorens and the south, inland of Kealakekua Bay. The sites are 6.5 and 8 km Domingo, 2007; Lloyd and Marques, 1988; McJannet et al., 2007b; from the coast, and both are at approximately 1000 m asl (Fig. 1; Vernimmen et al., 2007; Zimmermann et al., 2007). Weather is one Table 1). Both sites have a westward aspect (2508) and an average cause of these differences: more water is retained in an slope of 78. unsaturated than a saturated canopy, so long periods between Microclimate in Hawai’i has been described by a number of storms, high vapor pressure deficit, short storms, and low intensity investigators, most focusing on windward areas of the islands rainfall can decrease throughfall (Cuartas et al., 2007; Levia and where weather is tradewind-dominated (Giambelluca et al., 2009; Frost, 2006; Staelens et al., 2008; Zeng et al., 2000). Tree height, leaf Juvik and Nullet, 1994). The leeward side of Hawai’i Island, isolated size and shape, and canopy density also promote different from the tradewinds, has a convective land-breeze sea-breeze interception and drip processes (De Schrijver et al., 2007; Germer weather system that brings afternoon clouds and rain to the et al., 2006; Keim et al., 2005; Staelens et al., 2008; Zimmermann mountain slopes throughout the year (Juvik et al., 1978; Leopold, et al., 2007). Interactions between weather and forest structure 1949). The climate of leeward Maui is well-described (Giambelluca may cause forests with similar species inventories to have and Nullet, 1991), though leeward Maui is less isolated from the dramatically different throughfall rates. For example, large trees tradewinds than is Kona. On leeward Hawai’i, most previous work with substantial canopy surface area decrease throughfall by has been outside the heavy cloud area, though some transects have intercepting a substantial fraction of rainfall, but the effects of overlapped with this study (Bean et al., 1994; Nullet et al., 1995). interception are mitigated if rainfall continues after canopy In leeward Hawai’i, rainfall increases with increasing elevation, saturation and the bulk of intercepted water drips to the ground reaching an annual average of 1500–2000 mm at approximately (Llorens and Domingo, 2007). 1000 m asl, where, as illustrated in Fig. 1, our study sites are Vegetation can also intercept fog, increasing water input below located, then decreasing at higher elevations (Giambelluca et al., the canopy. We use the terms fog and cloud water interchangeably 1986). Variability in rainfall is high inter-annually, intra-annually, in this paper to refer to incidence and interception of clouds that within, and between sites. For example, three NCDC gauges at occur near the ground surface. Reviewing a number of studies, approximately 400 m asl exhibit a period of high rainfall lasting Bruijnzeel and Hamilton (2000) report that cloud interception from June through October, and we define the summer by this increases throughfall to 80–100% of rainfall, and in particularly period. However, another government-maintained rain gauge, at foggy, exposed locations throughfall can reach 115–130% of higher elevation and closer to our sites, shows no statistical rainfall. Predicting the extent and significance of cloud intercep- difference between summer and winter rainfall over the past 20 tion is difficult because weather and forest structure have varying years. effects on competing hydrologic processes. A large canopy surface Afternoon fog and clouds occur year round, and seasonal area can intercept a substantial fraction of rainfall, reducing the oscillations in other meteorological variables are small. During the volume of water that falls directly to the ground, but a large canopy period of measurement, summer maximum (minimum) tempera- area also increases