Canopy interception is the rainfall that is intercepted by the canopy of a tree and successively evaporate from the leaves. Precipitation that is not intercepted will fall as throughfall or stemflow on the forest floor.
Many methods exist to measure canopy interception. The most often used method is by measuring rainfall above the canopy and subtract throughfall and stem flow (e.g., Helvey and Patric [1965][1]). However, the problem with this method is that the canopy is not homogeneous, which causes that it is difficult to obtain representative throughfall data.
Another method that tried to avoid this problem is applied by e.g., Shuttleworth et al. [1984][2], Calder et al. [1986][3], and Calder [1990][4]. They covered the forest floor with plastic sheets and collected the throughfall. The disadvantage of this method is that it is not suitable for long periods, because in the end the trees will dry due to water shortage, and the method is also not applicable for snow events.
The method by Hancock and Crowther [1979][5] avoided these problems, by making use of the cantilever effect of branches. If leaves on a branch hold water, it becomes more heavy and will bend. By measuring the displacement, it is possible to determine the amount intercepted water. Huang et al.[6] refined this method later in 2005 by making use of strain gauges. However, the disadvantages of these methods are that only information about one single branch is obtained and it will be quite laborious to measure an entire tree or forest.