It goes both ways: measurements of simultaneous evapotranspiration and fog droplet deposition at a montane cloud forest

Beiderwieden E, Wolff V, Hsia YJ, Klemm O

Abstract

Fluxes of latent heat, sensible heat, and water vapor, including turbulent deposition of fog droplets. were measured for two months in autumn 2005 within a subtropical montane cypress forest in Taiwan. The of the study was to determine whether significant evapotranspiration can occur during foggy conditions. Water vapor fluxes, Q(w), as determined with the Bowen Ratio method, were compared to those simultaneously measured with the eddy covariance method. The median Bowel Ratio was 1.06. and the median Q(w) flux was 5.2 x 10(-5) kg m(-2) s(-1). The vertical gradients of temperature and specific humidity over the forest. Delta T and Delta q, peaked around noon during days without fog, and were reduced during foggy conditions. For 66% of the data points. Delta T and Delta q were negative, corresponding to positive (upward) fluxes of sensible heat Q(n) and latent heat Q(L). A Monte Carlo simulation proved that statistically significant evapotranspiration rates, i.e upward water vapour fluxes occurred during fog. At the same time, deposition fluxes of fog droplets occured. Our results show that even during fog events, significant evapotranspiration may occur. Copyright (c) 2008 John Wiley & Sons, Ltd.