Monitoring and modelling canopy water storage amounts in support of atmospheric deposition studies

Canopy 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 IPSASEP 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.