Tests of models of cloudwater deposition to forest canopies using artificial and living collectors

Mechanistic cloud deposition models are very useful in the routine quantification of cloudwater deposition to forest canopies. In order to test, in a natural field situation, several assumptions in these models, a passive string cloudwater collector, a small artificial tree, and a living Norway spruce were exposed to cloudwater on a raised platform at the summit (elevation, 1686 m) of Whitetop Mountain, Virginia over a 5 month period. Cloudwater collection rates by these three collectors were used to examine relationships between these rates and measured values for two important meteorological variables in the models, liquid water content and wind speed, the product of which is the horizontal cloudwater flux. Collection rates for all three collectors were predicted moderately well by horizontal cloudwater flux (R² ranged from 0.54 to 0.73; p<0.0001) across all hours of observation, but were least strongly related when liquid water content was low, probably because of various measurement uncertainties under this condition. For all three collectors, simple linear regressions using the horizontal water flux to predict collection rates were not appreciably improved by inclusion of a cloudwater collection efficiency term or by conversion to binomial or curvilinear models. Cloudwater collection efficiency for all three collectors was related to the logarithm of horizontal water flux, as predicted by the models, only when this relationship was analyzed within individual cloud events. Between individual cloud events, collection efficiency varied across a wide range (0.12–0.50 for the spruce tree), with efficiencies much higher during events of short duration. Cloudwater collection efficiency was often lower than predicted by cloud deposition models, possibly because the models use wind speed measurements which do not take into account reductions in wind speed occurring within needle clusters on branches. Collection rates for all three collectors correlated highly with each other (R² ranged from 0.72 to 0.88; p<0.0001), as well as with a mature red spruce canopy. It was concluded that either the string collector or an artificial tree such as the one used in this study would serve as a good surrogate collector for living spruce tree crowns.