All precipitation, falling above the trees, do not reach the ground. Part of it is retained in the canopy and eventually evaporates back into the atmosphere. This is known as intercepted rainfall. The process is influenced by various meteorological parameters of which we have mainly focused on drop size distribution.
Drop size distribution and rainfall in the open and throughfall under birch and pine trees have been measured since 2014 in Ljubljana, Slovenia. The results demonstrate that birch has intercepted 40% (± 32) and pine 68% (± 24) of rainfall on average per event. During the 146 individual events between 297 and 724,905 rain drops were recorded with the average drop diameter 0.73 mm (± 0.23) and average drop velocity 3.71 m/s (± 0.49).
We have closely analyzed the effect of DSD on interception by pine during two rainfall events of similar duration in June and July 2014. The June event was 75 minutes long heavy rainfall (10.2 mm) with short term intensity up to 0.36 mm/min. The July event was a moderate rainfall of 5.6 mm in 72 minutes with lower intensity up to 0.2 mm/min. The rainfall interception by pine was 53% and 65% during the first and the second event, respectively.
The rain drop diameter and velocity strongly affect the rainfall interception. In June event after 15 minutes drops with larger drop diameter (6.5 - 8.5 mm) were observed in the open causing an instant reduction of rainfall interception by pine for 30%. After that larger numbers of drops with diameter higher than average induced throughfall greater than the rainfall in the clearing. Similarly twice during the both events the groups of drops with higher velocity (7.6 - 10.4 m/s) were observed, causing the reduction in rainfall interception and for the June event inducing throughfall greater than the rainfall.