Evaluation of Drop Size Distribution Impact on Rainfall Interception by Trees

Abstract

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.

Influence of Raindrop Size Distribution on Throughfall Dynamics under Pine and Birch Trees at the Rainfall Event Level

“…Larger drop size distribution during event C was observed sooner, from minutes 4 to 7, as well as increased throughfall under the pine trees by 26%. A similar increase of 28% and 20% of throughfall under pine trees, after the onset of larger and faster raindrops during June and July 2014 events, respectively, was reported also by Zabret et al [24]. On the other hand, no influence from changes in the drop size spectrum on throughfall under the birch trees was observed in this study.…”

“…Throughfall under the pine trees after the occurrence of larger and faster drops immediately increased in both cases, but it exceeded the amount of rainfall in the open only in the case of event B (Figure 4). A similar phenomenon was also reported for the June 2014 event, which was 75 min long and delivered 10.2 mm of rainfall [24]. This may be the consequence of the time delay in the increase in the drop size spectrum and the amount of rainfall delivered until then (Figures 4 and 5).…”

“…The rainfall amount needed to induce throughfall during this event was only 1.2 mm and 1.8 mm for the birch and pine trees, respectively. A similar phenomenon of inducing throughfall by a cloud of larger drops was also shown in other studies [3,24]. For example, during the July 2014 event, throughfall under the pine trees started after 40 min from the beginning of the event, simultaneously with a drop diameter increase, when 1.8 mm of rainfall fell as well [24].…”

“…However, an increase in drop size distribution during event B was observed after 13 min, when 6 mm of rainfall had already fallen. Additionally, drop diameter and velocity increase during the June 2014 event occurred after 15 min, when 4.6 mm of rainfall had fallen [24]. By that time, the canopy storage capacity was full.…”

Influence of Rainfall on Canopy Interception in Mixed Broad-Leaved—Korean Pine Forest in Xiaoxing’an Mountains, Northeastern China

“…Under dry conditions, interception rates are primarily controlled by rainfall intensity [14]. Although rainfall characteristics were considered to be the most influential variables, the number of raindrops, raindrop diameter, and velocity were hardly included in the analyses of rainfall effects on canopy interception [15][16][17], except for in a study by Zabret et al [17], who reported that raindrops with a larger diameter (6.5-8.5 mm) and higher velocity (7.6-10.4 ms −1 ) reduced canopy interception of rainfall.…”