Fog Water Collection in a Subtropical Elfin Laurel Forest of the Garajonay National Park (Canary Islands): A Combined Approach Using Artificial Fog Catchers and a Physically Based Impaction Model

Fog precipitation has long been assumed as an additional water source in the relic laurel ecosystems of the Canary Islands, located at 500-1400 m MSL. However, to what extent fog water can contribute to the laurel forest water balance is not yet clear. Combining data from artificial fog catchers and a physically based impaction model, the authors evaluated the potential contribution of fog water captured by needle-leafed Erica arborea L. trees in a selected watershed of the Garajonay National Park (La Gomera Island) for a 2-yr period (February 2003-January 2005). Fog water collection was measured with artificial catchers at four micrometeorological stations placed at 1145, 1185, 1230, and 1270 m MSL. Average fog water collection was only significant at the highest measurement site (one order of magnitude greater than at lower altitudes), totaling 496 L m(-2) yr(-1) during the 2-yr period. The average fog water yield in the first and second annual periods ranged between 0.2-5.0 and 0.1-2.1 L m(-2) day(-1), respectively. Rainfall exhibited seasonality, distinguishing between rainy and dry seasons, while fog water collection was distributed more evenly throughout the year. Regarding fog water captured by the vegetation, the impaction model predicted a significant amount of fog water potentially collected by a single E. arborea tree, on the order of 1810-2090 L yr(-1). Taking tree population density into account, the yearly average water contribution to the soil surface by wind-driven fogs was 251-281 mm, whereas annual rainfall was 635 and 1088 mm, respectively. The hourly course of micrometeorological variables shows a 58% reduction in global radiation under foggy conditions and a concomitant 3 degrees-6 degrees C mean temperature decrease compared to fog-free periods. Thus, limiting evapotranspiration may also be a relevant effect of fog in this subtropical elfin cloud forest.