Raindrop Erosivity Depends on Rainfall Amount and Branch Height

Factors influencing the erosivity indices of raindrops in Japanese cypress plantations

Yoshinori Shinohara (a&b), Kirika Ichinose (b), Mayumi Morimoto (b), Tetsuya Kubota (b), Kazuki Nanko (c)

(a) University of Miyazaki, Miyazaki, Japan.
(b) Kyushu University, Fukuoka, Japan.

(c) Department of Disaster Prevention, Meteorology and Hydrology, FFPRI, Tsukuba, Ibaraki, Japan.

Catena, 171, 54-61, December 2018, DOI：10.1016/j.catena.2018.06.030( External link )

Japanese cypress is one of the main tree species for the plantations in Japan. Because the foliar litter of Japanese cypress tends to break into small pieces, it does not effectively protect the ground surface. In dense cypress plantations, less protection by understory vegetation causes soil erosion. The rainfall in forests (called throughfall) varies spatially and the size of large raindrops that drip from canopies varies among tree species. Thus, the factors affecting raindrop erosivity in forests are not well understood.

We prepared >500 splash cups to measure raindrop erosivity in many different points (Fig.1). Raindrops hit the sand in the splash cups and erode the sand away. The kinetic energy of the rainfall can be estimated by the amount of sand loss from the cup. This device enables us to closely investigate the conditions of a plot where the impact force of raindrops and the risk of erosion are high. We conducted measurements in seven different Japanese cypress plantations and found that raindrop erosivity was positively related to the amount of throughfall where the tree height of Japanese cypress was sufficient (15m or more). Where the trees were not sufficiently tall, the height of the lowest layer of branches as well as the amount of throughfall affected raindrop erosivity. This is because the falling velocity of throughfall drops remained low if the falling distance was short between the lowest layers of branches and the splash cups.

We have confirmed that spatial variation in rainfall as well as the height of the canopy and branches are important factors that affect soil erosion in forests. At the Forestry and Forest Products Research Institute, we conduct various research programs, often utilizing such new techniques, to estimate the potential risk of soil erosion associated with the growth of Japanese cypress plantations, and to study forest management techniques to sufficiently maintain the soil conservation capability of our forests.

 
Fig.1. Splash cup before (left) and after (right) rainfall. Photographs by Kirika Ichinose.