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Monitoring and modelling rainfall interception and canopy wetness in support of acidification research Bouten, W.; Schaap, M.; Aerts, J.C.J.H.; Vermetten, A. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. AbstractCanopy water storage amounts were measured with a newly developed measuring system based on the attenuation of a 10.26 GHz microwave signal. Every 5 min, vertical scans were made, over a period of 9 months. A physically based multi-layer interception model with empirical parameters was calibrated using a non-linear optimization technique. The calibrated model appeared to be capable of explaining up to 92% of the measured variance of water storage amounts for an independent validation period when using on-site measurements of meteorological variables. The performance decreased only slightly to 89% when other input sets were used for this period. These were necessary to extrapolate the results to longer time series required for evaluating canopy resistances in the study of deposition of airborne pollutants (Vermetten et al., Proc. 5th ZPSASEP Conf., Vol. 3, 1992). The resemblance between measured throughfall amounts and simulated results and the plausibility of the model parameters, although they were optimized without setting any limits, enhances confidence in the model results.

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The direct effects of sulphur dioxide (S0 2 ) and ozone (0 3 ) on the growth of two Douglas-fir stands are quantified from continuous measurements of the concentration of air pollution above the canopy combined with a quantitative description of the direct effects of gaseous uptake of pollutants on photosynthesis and respiration. Effects at the leafleve1 are scaled up to the stand level, using a deterministic model of forest growth that uses time steps of 1 day. This approach enables total uptake of gaseous air pollutants and their effects on canopy assimilation and growth to be quantified, taking ambient weather conditions and stand structure into account. By using a description of water balance in the model, the influence of limitations in soil moisture availability can also be incorporated, and the combined effects of air pollution and water shortage can be accounted for. Under Dutch conditions, with average daily concentrations of 10 Jlg m-3 for S0 2 and 50 Jlg m-3 for 0 3 , the short-term effects on tree growth appear to be minor, except near local sources, and during episodes of high concentrations under stable weather conditions. The possible magnitude oflong-term effects of exposure to 0 3 is assessed using a hypothetical relationship between dose (exposure multiplied by duration) and foliage loss. From this it is concluded that long-term effects may be significant in stands with a low leaf area index.