Estimating fog deposition at a Puerto Rican elfin cloud forest site: comparison of the water budget and eddy covariance methods

Abstract

Abstract:The deposition of fog to a wind-exposed 3 m tall Puerto Rican cloud forest at 1010 m elevation was studied using the water budget and eddy covariance methods. Fog deposition was calculated from the water budget as throughfall plus stemflow plus interception loss minus rainfall corrected for wind-induced loss and effect of slope. The eddy covariance method was used to calculate the turbulent liquid cloud water flux from instantaneous turbulent deviations of the surface-normal wind component and cloud liquid water content as measured at 4 m above the forest canopy. Fog deposition rates according to the water budget under rain-free conditions (0Ð11 š 0Ð05 mm h 1 ) and rainy conditions (0Ð24 š 0Ð13 mm h 1 ) were about three to six times the eddy-covariance-based estimate (0Ð04 š 0Ð002 mm h 1 ). Under rain-free conditions, water-budget-based fog deposition rates were positively correlated with horizontal fluxes of liquid cloud water (as calculated from wind speed and liquid water content data). Under rainy conditions, the correlation became very poor, presumably because of errors in the corrected rainfall amounts and very high spatial variability in throughfall. It was demonstrated that the turbulent liquid cloud water fluxes as measured at 4 m above the forest could be only ¾40% of the fluxes at the canopy level itself due to condensation of moisture in air moving upslope. Other factors, which may have contributed to the discrepancy in results obtained with the two methods, were related to effects of footprint mismatch and methodological problems with rainfall measurements under the prevailing windy conditions. Best estimates of annual fog deposition amounted to ¾770 mm year 1 for the summit cloud forest just below the ridge top (according to the water budget method) and ¾785 mm year 1 for the cloud forest on the lower windward slope (using the eddy-covariance-based deposition rate corrected for estimated vertical flux divergence).

Interactions between payments for hydrologic services, landowner decisions, and ecohydrological consequences: synergies and disconnection in the cloud forest zone of central Veracruz, Mexico

“…Our cloud forest site falls at the low end of the global range for CWI, while the estimated ET (1325 mm yr -1 ) (Muñoz-Villers et al 2012) is at the higher end of the global range (Bruijnzeel et al 2011). These findings contrast with many other studies conducted in (foggier or windier) cloud forest sites that showed a much greater annual water recharge (i.e., CWI inputs minus ET losses) (Holwerda 2005, Holwerda et al 2006, McJannet et al 2007). This high global variability in CWI inputs, ET losses, and thus water yield underscores the fact that not all cloud forests are hydrologically similar and that sitespecific analyses are critical to accurately assess their water production services relative to other important ecosystem services (cf.…”

Evaluating the capabilities and uncertainties of droplet measurements for the fog droplet spectrometer (FM-100)

“…The FM-100 has been used in several ground based studies so far especially as part of an eddy covariance system to quantify fog water deposition fluxes in tropical mountain cloud forests (e.g. Eugster et al, 2006;Holwerda et al, 2006;Beiderwieden, 2007;Beiderwieden et al, 2008;Schmid et al, 2010), in temperate ecosystems (Burkard et al, 2002;Thalmann, 2002;Burkard, 2003), and deposition fluxes in rather arid areas (Westbeld et al, 2009). It has also been used as a single instrument for microphysical studies of fog (Gonser et al, 2011;Liu et al, 2011) and compared to other devices Schmid et al, 2010;Frumau et al, 2011).…”

“…One way of addressing the isoaxial sampling is to put the instrument onto a turntable and letting it continually turn into the main wind direction as done by Vong (1995), Kowalski et al (1997), Kowalski (1999), Wrzesinsky (2000), Burkard et al (2002), Thalmann (2002, Burkard (2003), Eugster et al (2006), andHolwerda et al (2006). Nevertheless, these procedures do not assure isokinetic sampling conditions.…”

The tropical mountain forest

“…In three montane rainforests of Australia, cloud interception constituted a significant extra input and accounted for 7 to 29% of the total water input (McJannet et al 2007). Another recent study on fog deposition, using an eddy covariance system in a Puerto Rican forest, yielded an estimate of ~ 785 mm yr -1 (Holwerda et al 2006). Although the estimation of the additional water input from fog remains technically difficult, these studies show that interception of cloud and fog water may play a very important role in TMF, especially in cloud forests.…”