Agricultural and Forest Meteorology 149 (2009) 1179–1187 Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet Fog precipitation and rainfall interception in the natural forests of Madeira Island (Portugal) Susana Prada a,1, Miguel Menezes de Sequeira a,1, Celso Figueira b,*, Manuel Oliveira da Silva c,2 a Dept. de Biologia/Centro de Estudos da Macarone´sia da Universidade da Madeira, Campus Universita´rio da Penteada, 9000-390 Funchal, Madeira, Portugal b Centro de Estudos da Macarone´sia da Universidade da Madeira, Campus Universita´rio da Penteada, 9000-390 Funchal, Madeira, Portugal c Dept. de Geologia da Faculdade de Cieˆncias da Universidade de Lisboa – GeoFCUL, Edifı´cio C6 – 28 Piso Campo Grande, 1749-016 Lisboa, Portugal ARTICLE INFO ABSTRACT Article history: Situated in the Atlantic Ocean, Madeira is a within-plate volcanic island, approximately 600 km Received 27 September 2007 northwest of the Western African coast. Cloud cover formed mainly of orographic origin persists on Received in revised form 19 January 2009 Madeira for more than 200 days per year between 800 m and 1600 m altitude. Since vegetation occupies Accepted 9 February 2009 2/3 of the island’s surface, fog precipitation, which occurs when fog droplets are filtered by the forest canopy and coalesce on the vegetation surfaces to form larger droplets that drip to the forest floor, is an Keywords: important hydrological input. Rainfall interception and fog precipitation data were collected between Fog precipitation 1996 and 2005 in the natural forests of Madeira. Six throughfall gauges were placed under the canopy of Interception three different types of forest: high altitude tree heath forest (1580 m), secondary tree heath forest High altitude heath tree forest Secondary heath tree forest (1385 m) and humid laurisilva forest (1050 m). Fog precipitation is higher under high altitude heath forest (average canopy interception was 225% of gross precipitation) and dependent both on altitude Humid laurisilva forest À Madeira Island and vegetation type, due to different tree architecture and leaf shape. Although results are conservative estimates of fog precipitation, they point towards the importance of fog-water as a source of groundwater recharge in the water balance of the main forest ecosystems of Madeira. ß 2009 Elsevier B.V. All rights reserved. 1. Introduction According to Twomey (1957) vegetation can directly influence the rainfall of a given area, i.e. water reaching the ground, because Fog precipitation is due to the simultaneous presence of a thick in an elevated region (frequently covered with low clouds), cloud and persistent cloud cover, wind, and dense vegetation cover. It water intercepted by trees and other vegetation may constitute an occurs when fog droplets are filtered by the forest canopy and appreciable fraction of the total runoff. coalesce on the vegetation surfaces to form larger droplets that During a rainfall event, vegetation intercepts precipitation and drip to the forest floor (Kittredge, 1948; Prada and Silva, 2001). stores water in the canopy, a large proportion of it being Fog precipitation is also known as occult precipitation, horizontal evaporated thereafter (Kittredge, 1948). As a result of the process precipitation, fog drip, cloud drip, cloud milking, occult condensa- of interception, a rain gauge placed in the open normally receives tion and precipitation by direct interception of cloud water more water during a rainfall event than throughfall gauges placed (Kittredge, 1948; Twomey, 1957). Precipitation by direct intercep- under a forest canopy. tion of cloud water best describes the process of fog precipitation, as The purpose of this study was to quantify fog precipitation and it suggests that in the absence of interception, there is no significant interception in the main forest types in Madeira, corresponding to deposition. Fog droplets stay suspended in the atmosphere because the cloud cover altitudinal range, and to evaluate the importance of their drop velocity is smaller than the velocity of the ascending fog water to the water resources of the island. currents inside the cloud. Only a very small percentage of droplets, dragged by the wind, can precipitate directly on the soil in quantities 2. Methods and materials that do not exceed 0.2 mm/day (Cunha, 1964). 2.1. Site description * Corresponding author. Tel.: +351 291 705 331; fax: +351 291 705 399. Situated between 328380 and 328520N and 168390 and 178160W, E-mail addresses: [email protected] (S. Prada), [email protected] Madeira is a within-plate volcanic island, approximately 600 km (M. Menezes de Sequeira), celso_fi[email protected] (C. Figueira), [email protected] (M.O. da Silva). northwest of the Western African coast. It is a hot-spot originating 1 Tel.: +351 291 705 395; fax: +351 291 705 399. from a mantle plume dating back to Miocene times, about 5.6 My 2 Tel.: +351 217 500 151; fax: +351 21 750 00 64. (Ribeiro et al., 2005). 0168-1923/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2009.02.010 1180 S. Prada et al. / Agricultural and Forest Meteorology 149 (2009) 1179–1187 Fig. 1. Location of study area in the Paul da Serra massif, Madeira Island. With a 737 km2 surface area, a length of 58 km, a 23 km wide tree heath forest dominated by the heath Erica platycodon subsp. and a maximum altitude of 1861 m (Pico Ruivo), the island forms maderincola and the madeiran blueberry (Vaccinium padifolium). an E–W oriented barrier, with deep valleys. Above 1400 m a high altitude tree heath forest, dominated by Erica Madeira’s climate is heavily influenced by the Azorean arborea trees, corresponds to the climax vegetation. This high Subtropical Anticyclone’s intensity and location, but also by relief, altitude tree heath forest was severely damaged by overgrazing altitude and orientation. The trade winds are predominantly and fire and only a few small woods still stand. blowing from the northeast for most of the year. Precipitation under a canopy (throughfall) and rainfall in a Since the island barrier (E–W) has an almost perpendicular nearby clearing were compared at three sites representing three orientation with the prevailing wind direction (NE), temperature different types of vegetation (Table 1) selected on the northern and rainfall vary remarkably on each of the slopes. The northern slope of ‘‘Paul da Serra’’ massif (Fig. 1). slope is more humid than the southern at the same height, and the This area was selected because it has a well-preserved amount of rainfall increases with altitude on both slopes (Prada, vegetation cover (included in the Madeira Natural Park, and part 2000). of Natura 2000 NET), is exposed to the predominant winds from The highest precipitation is 2966.5 mm at Bica da Cana (1580 m northeast and has a high annual rate of fog days (235 days). Mean altitude), and precipitation decreases above this height. The lowest slope is 50%, from at about 600 m to the summit at 1600 m at Bica precipitation occurs on the southern slope’s lowlands in locations da Cana. Following the bioclimate classification proposed by Rivas- like Funchal and Ponta do Sol, where the annual precipitation is Martı´nez (2004) and according to the bioclimate map proposed by 513 mm and 583 mm, respectively (Prada, 2000; Fig. 1). Mesquita et al. (2004) the sites correspond to a temperate Cloud cover over the island is higher than over the sea. This macrobioclimate, to a mesotemperate termotype (superior to effect is due to the formation of orographic clouds and fog when inferior) and to ultra to inferior hyperhumid ombroclimate. the humid maritime winds ascend along the island slope. Air cools The first site is a high altitude tree heath forest at 1580 m (‘‘Bica and condensates the water vapour into small particles that stay da Cana’’, Fig. 2) exposed to winds from all directions, with suspended, originating in fog and clouds, whether the condensa- prevailing north-eastern direction (36% of the time) in a 5.6 ha tion occurs near or far from the ground, respectively. homogenous stand. This site bioclimate corresponds to a superior In Madeira, fog is almost exclusively of orographic origin, mesotemperate termotype and to a ultrahyperhumid ombrocli- forming in the North and dissipating to the South of the island. In mate (Mesquita et al., 2004). Data were collected with two gauges Bica da Cana, as much as 235 days of fog per year were registered, for 955 days between October 1996 and September 1999. whilst in Pico do Areeiro the value is 229 days of fog per year The second site is a secondary tree heath forest at 1385 m (INMG, 1979). Cloud cover persists between 600–800 m and (‘‘Fonte do Rentroia’’, Fig. 3) exposed to the northern winds in a 1600 m and its liquid water content varies between 0.1 g/m3 and large area (about 50 ha) of continuous vegetation. It is bioclima- 0.25 g/m3 (Frisch et al., 1995). tically similar to the first site, but the ombroclimate is dryer Recently, Madeira’s vegetation was described by Capelo et al. (inferior hyperhumid, Mesquita et al., 2004). Data were collected (2004). Above 300 m on the North slope a humid laurissilva forest, with two gauges for 394 days between May 21, 2004 and July 13, dominated by stink laurel (Ocotea foetens), islands laurel (Laurus 2005. Gauges malfunctioned for 25 days between November 23 novocanariensis) and Clethra arborea is the climax vegetation. and December 17, 2004. However, mainly due to human actions (fire and grazing) this The third site is a humid laurissilva forest at 1055 m (Fig. 4) multi-stratified forest up to 30 m high was replaced by a secondary exposed to northern winds in large area (about 50 ha) of Table 1 Geographical and ecological characteristics of sites.