Factors influencing the erosivity indices of raindrops in Japanese cypress plantations

Severe soil erosion has been reported in monoculture forests without understory vegetation and litter on the floor. To clarify the factors that affect raindrop erosivity in monoculture forests, we measured raindrop erosivity in Japanese cypress (Chamaecyparis obtuse Endl.) plantations using splash cups. Splash cups can possibly measure raindrop erosivity from loss of sand (LoS). We first examined the relation between raindrop erosivity and LoS under natural rainfall in an open space. LoS was strongly correlated with raindrop erosivity factors, such as the kinetic energy and momentum multiplied by the drop diameter. We then observed LoS using 12–16 splash cups in seven plots with different canopy structures in Japanese cypress plantations. LoS at each point was compared with the amount of throughfall (T_f) and canopy structure factors, such as canopy openness (C_o) and distance to the closest branch (B_r). The relation between T_f and LoS was similar among the plots, except in one plot where the canopy and crown-base heights were the smallest. Our results imply that T_f is a dominant factor determining raindrop erosivity when canopy and crown-base heights are more than the threshold values (15.2 m for the canopy height and 10.5 m for the crown-base height in our forests). In plot 1, LoS was correlated with not only T_f but also B_r. When analyzing all the data together using generalized linear models, LoS was affected by B_r besides T_f. In monoculture forests with a high canopy, T_f measurements enable us to estimate raindrop erosivity. In monoculture forests with a low canopy, B_r (or the canopy height) should also be considered when estimating the erosivity of raindrops.