Time lag effects of rainfall inside a Platycladus Orientalis plantation forest in the Beijing Mountain Area, China

Rainfall interception by the forest canopy not only consumes a portion of rainfall but also changes the timing of rainfall inside a forest. Rainfall inside a forest always starts and ends later than it does outside a forest, an effect called the time lag effect. This time lag effect can influence the ecological and hydrological functioning of forest ecosystems, but the study of this relationship is only in its infancy. This study, performed at the Miaofeng Mountain forest farm in Beijing, used two laser raindrop spectrometers to synchronously monitor precipitation both inside and outside a Platycladus orientalis plantation. Based on these monitoring data, this paper analyzed the characteristics and major factors influencing the lag effect of rainfall inside a forest for 18 complete rainfall events. The results show all the start times of rainfall inside the forest were later than those outside the forest, with lag times ranging between 2 and 88 minutes (average 21 minutes). Most of the ending times of rainfall inside the forest were also later than those outside the forest except two light rainfalls of less than 0.1 mm; the lag time averaged 46 minutes and was as long as 158 minutes. Additionally, using a size-based analysis of large, medium and small typical rainfall events both inside and outside the forest, we found that the forest canopy delayed the peak rainfall intensity inside the forest, and rainfall intensity inside the forest was occasionally greater than outside the forest at same time during a rainfall event. Based on regression analysis, the delays of both the beginning and ending of rainfall occurring inside the forest decreased as rainfall intensity increased, which could be plotted as logarithm curves. This time lag effect of rainfall inside a forest may reduce flood peaks and effectively prolong conflux time, and it has positive ecological significance for land managers attempting to control flash floods, debris flows, landslides and other geologically related disasters.