Evaluating the inﬂuence of canopy species and meteorological factors on throughfall drop size distribution Kazuki Nanko * ,1 , Norifumi Hotta, Masakazu Suzuki Laboratory of Forest Hydrology and Erosion Control Engineering, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan Received 25 September 2005; received in revised form 21 February 2006; accepted 23 February 2006 Summary Drop size distributions (DSDs) were continuously and simultaneously measured by laser raindrop-sizing instruments (LD gauges) in an open site and in three forest stands consist- ing of Japanese cypress (CY: Chamaecyparis obtusa), Japanese cedar (CD: Cryptomeria japon- ica), and sawtooth oak (SO: Quercus acutissima), during three rainfall events in Tokyo, Japan. Drop data during the whole observation period were used in an hourly based data set and divided into three meteorological condition groups: calm, heavy rain, and strong wind. Evalu- ating the inﬂuence of canopy species and meteorological factors on the D 50 and DSD difference revealed some throughfall DSD characteristics. First, throughfall had different DSDs among can- opy species under conditions of little canopy vibration with low rainfall intensity and wind speed; D 50 values were 2.00, 2.93, and 3.60 mm in CY, CD, and SO, respectively. Second, throughfall contained smaller drops under conditions of severe canopy vibration, with high rain- fall intensity and/or high wind speed, than under calm meteorological conditions. Vibration of the canopy could reduce water coalescence and increase spattered rainwater from canopies. Third, the inﬂuence of meteorological factors was different between canopy species; SO was readily inﬂuenced but CY was not. Moreover, results from this study implied that throughfall consisted of three drop components – free throughfall, drips, and splash droplets – and sug- gested a process for generating throughfall DSD that could explain the variations in throughfall DSDs between canopy species and that inﬂuenced by meteorological factors.  c 2006 Elsevier B.V. All rights reserved. KEYWORDS Throughfall drop size distribution; Drip; Splash droplet; Effect of meteorological factors; Effect of canopy species Introduction The interception of precipitation by vegetation canopies is a major component of the surface water balance in water- 0022-1694/$ - see front matter  c 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2006.02.036 * Corresponding author. Tel.: +81 3 5841 5214; fax: +81 3 5841 5464. E-mail address: nanko@fr.a.u-tokyo.ac.jp (K. Nanko). 1 JSPS Research Fellow. Journal of Hydrology (2006) 329, 422– 431 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jhydrol