Jože Rakovec

Part of precipitation is intercepted by forest canopies, while the rest reaches the ground as throughfall or stemflow. This process is influenced by various meteorological variables, of which we have mainly focused on drop diameter and velocity. Rainfall in the open and throughfall under birch and pine trees have both been measured since 2014 in Ljubljana, Slovenia. The results demonstrate that the total throughfall during 3.5 years was 73% and 53% of rainfall under birch and pine trees, respectively. During the 236 analysed events, the median volume diameter was 1.8 mm (±1.7 mm), and kinetic energy between 0.01 mJ/cm² and 23.3 mJ/cm² was recorded. We closely analysed the effect of rainfall microstructure on throughfall under pine and birch trees during three specific rainfall events. The increase in drop diameter and fall velocity during a rainfall event instantaneously increased throughfall under pine trees between 25% and 47%, whereas no such changes were observed under birch trees. This may be the consequence of different tree properties of the two species. Additionally, in the case of a saturated canopy, throughfall under pine trees exceeded rainfall in the open after an onset of larger and faster drops.

Not all precipitation, falling above the trees, 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 diameter and velocity. 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 41% (±32%) and pine 70% (±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 mm) and average drop velocity 3.71 m/s (±0.49 m/s). We have closely analyzed the effect of drop size and velocity on interception by pine during two rainfall events in June and July 2014. In June event 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%. Furthermore, 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.