Holwerda, F., R. Burkard, W. Eugster, F. N. Scatena, A. G. C. A. Meesters,
and L. A. Bruijnzeel (2006), Estimating fog deposition at a Puerto
Rican elfin cloud forest site: Comparison of the water budget and eddy
covariance methods, Hydrol. Processes, 20, 2669– 2692.
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 h1) and rainy conditions
(0Ð24 š 0Ð13 mm h1) were about three to six times the eddy-covariance-based estimate (0Ð04 š 0Ð002 mm h1). 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 year1 for the summit cloud forest just below the
ridge top (according to the water budget method) and ¾785 mm year1 for the cloud forest on the lower windward
slope (using the eddy-covariance-based deposition rate corrected for estimated vertical flux divergence). Copyright
2006 John Wiley & Sons, Ltd.