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Lookup NU author(s): Professor Ian Calder
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In a recent series of three articles Calder, 1996; Calder et al., 1996; and Hall et al., 1996 it was shown that an earlier stochastic model of rainfall interception could be developed to take account not only of the wetting up of vegetation by the primary impact of raindrops on vegetation canopies but also the wetting of lower layers of the canopy by impacts of secondary drops falling from the upper layers. It was shown that through the use of a rainfall simulator and an optical disdrometer ( Hall and Calder, 1993), species specific wetting parameters could be determined which allowed, through the use of the model, the calculation of secondary drop size and the wetting up response of the vegetation. It was also suggested that through the use of relationships between raindrop size and rainfall intensity the primary drop size striking the canopy could be calculated. An example of the use of the model was given for a site in Sri Lanka which showed that the observed net rainfall distribution was much better described by this model, which explicitly takes into account drop size, than by conventional models of rainfall interception, which generally were developed in temperate climates, under relatively low rainfall intensity (small drop size) conditions. It was proposed that the use of the stochastic model helps to explain some of the range in interception losses which were observed world-wide. Because of the generally small drop sizes in the primary rainfall (low rainfall intensities) and secondary drop sizes (small leaves) the wetting of temperate coniferous forests is very efficient and leads towards some of the highest losses, in both in percentage and absolute terms, that were recorded from any forests. By contrast, tropical rainforest subject to wetting by large drops in the primary rainfall, (often high rainfall intensities), and large secondary drop size wetting, (large leaves), tends towards inefficient wetting and low interception losses. Uijlenhoet and Stricker (1999) have questioned the use of a relationship used in one part of the model, that relating to rainfall intensity and drop size, and point out an inconsistency in the interpretation of the Sempere Torres ( Sempere Torres et al., 1994) general theory of raindrop size distributions. I am grateful to Uijlenhoet and Stricker for pointing out this inconsistency and for showing that with a more realistic relationship between rainfall intensity and drop size, drop sizes will be larger for a given rainfall intensity than had been calculated originally. This will have the effect of making the "stochastic" wetting effect of raindrops even more significant than had previously been anticipated, which would be most pronounced for sparse canopies growing in high rainfall intensity climates. For high leaf area canopies, whether growing in either temperate or tropical climates the effects of secondary wetting of drops falling from higher layers in canopies will predominate.
Author(s): Calder IR
Publication type: Article
Publication status: Published
Journal: Journal of Hydrology
Year: 1999
Volume: 127
Issue: 1-2
Pages: 164-165
Print publication date: 02/04/1999
ISSN (print): 0022-1694
Publisher: Elsevier BV
URL: http://sciencedirect.com
