Comparison between four Methods to estimate Leaf Wetness Duration caused by Dew on Grassland

Leaf wetness duration is very important in modelling the dry deposition of ammonia. Ammonia is well dissolvable in water and therefore leaf water layers can be very important sinks for ammonia. The objective of this study is to evaluate four methods to estimate the leaf wetness duration caused by dew on grasslands. The first method is the dew parameterisation by Garratt and Segal (1988). The second method is the dew parameterisation by Pedro and Gillespie (1982). The third method is a constant threshold value for the relative humidity, which is currently used in the atmospheric transport model OPS at the National Institute for Public Health and the Environment (RIVM, The Netherlands). The fourth method is also a constant threshold value, but extended with a threshold value for the change of the relative humidity between two time steps. The results of the four calculation methods are verified with observations from a leaf wetness (grid) sensor. Observations are available for a period of more than 200 days in 2003. It is found that leaf wetness duration on grassland can best be predicted with the extended threshold method. The use of other methods may be favourable in certain situations.