Quantifying fog water deposition by in situ exposure experiments in a mountainous coniferous forest in Taiwan

A new approach for quantifying fog deposition rate was conducted in a mountainous forested ecosystem (Chamaecyparis obtusa var. formosana) in northeastern Taiwan where the frequency and duration of fog occurrence is extremely high and the annual number of foggy days often exceeds 350. The significance of fog in the hydrological budget of such ecosystems remains as an attractive ecological question. We studied the fog deposition rate by use of an in situ exposure experiment, in which the increase in weight of the leaves was recorded during several different wind and visibility conditions including foggy days of different fog density as well as some clear conditions. The fog capture rates of the leaves (fog water captured by unit leaf area per unit time) were measured at three heights of the canopy and multiplied with the respective leaf biomass for the deposition rates at the stand level. The exposure experiments showed a higher fog capture rate at the top layer of the canopy. Overall, the upper canopy contributed approximately 50% of the fog water captured by the forest stand, although this layer consisted only 34% of the total leaf biomass. Based on the Lovett micrometeorological model, an empirical model was developed which predicts fog deposition rate as an exponentially decay function of visibility. The annual fog deposition rate from March 2003 to February 2004 was calculated to be 328 mm year−1, which accounted for 10% of the total atmospheric hydrological input. The more frequent foggy conditions in winter resulted in a higher deposition rate in the cold season. We conclude that the use of a single parameter (visibility) accompanied by some point measurements in the field is effective and simple for predicting fog deposition rate. The limitations of the exposure experiment are discussed and possible improvements are proposed.