Bridging the scales of rainfall interception, from raindrop impacts on leaves to throughfall under a tree.

Precipitation falling onto vegetation is partly intercepted by the canopy and subsequently evaporates, while the remainder reaches the ground as throughfall or stemflow. Throughfall refers to precipitation that reaches the ground after crossing the canopy. It comprises free throughfall (raindrops not intercepted), drips, and splash droplets. Different rainfalls and foliage yield different number, size and velocity of each throughfall droplet type [1]. The resulting drop size distribution significantly affects infiltration and surface runoff processes [2]. Moreover, drips may induce the erosion and compaction of bare soil [3] while splash droplets may transport pathogenic spores [4]. Finally, the part of leaves that remains wet may experience significant leaching or water/nutrient uptake [5].

Predicting throughfall drop size distribution with physical models is complex because the physically relevant scale is that of a raindrop impacting a leaf, while the scale of interest is at least that of a tree. Previous studies (e.g., [6-8]) provided measurements at either scale but never at both. A few numerical models [4, 9-10] were proposed to estimate throughfall statistics and rain-induced transport by modelling interception at raindrop scale, but these models relied on strong and unverified assumptions on drop-scale dynamics.

In this original study, we first provide a detailed experimental characterization of interception at leaf scale. Hundreds of raindrop surrogates impacted single birch leaves. The leaf was weighed and imaged over time, and water storage variations were resolved at the scale of individual impacts. The storage capacity, the wetting-up time, the drip diameter and the splash fraction were measured as functions of the leaf area, the leaf inclination and the raindrop size. The results are extensively compared to previous studies at leaf scale.

Then rain interception is quantified at tree scale, with the same birch species and leaves in the same phenophase. Rain amount, intensity and drop size distribution in both open rainfall and throughfall were measured using two disdrometers positioned respectively above and below the canopy of a birch tree. Free throughfall, splash droplets and drips were separated for selected rainfall events with different intensities. The storage capacity and the wetting-up time were also estimated for each event. We relate these tree-scale measurements to the mechanisms observed at the leaf scale.

[1] D. F. Levia et al., Hydrol. Process. 33, 1698-1708 (2019)

[2] K. Nanko et al., Hydrol. Process. 24, 567-575 (2010)

[3] M. Beczek et al., Geoderma 347, 40-48 (2019)

[4] T. Vidal et al., Ann. Bot. 121, 1299-1308 (2018)

[5] T. E. Dawson and G. R. Goldsmith, New Phytol. 219, 1156-1169 (2018)

[6] C. Bassette and F. Bussière, Agric. For. Meteorol. 148, 991-1004 (2008)

[7] X. Li et al., Agric. For. Meteorol. 218, 65-73 (2016)

[8] C. D. Holder, Ecohydrol. 6(3), 483-490 (2012)

[9] Q. Xiao et al., J. Geophys. Res. 105 (D23), 29173-29188 (2000)

[10] R. P. de Moraes Frasson and W. F. Krajewski, J. Hydrol. 489, 246-255 (2013)