Effect of throughfall type on drop size distribution and soil erosion potential in a teak plantation in northern Thailand

As a commercially valuable species in tropical regions, it is important to minimize soil erosion in teak (Tectona grandis Linn. F.) plantations. Forest canopies alter not only the amount of rainfall beneath the canopy but drop size distribution (DSD) and erosive potential of throughfall. DSD is useful index to estimate soil erosion and canopy interception processes. Throughfall is composed by three types: free throughfall, splash throughfall, and canopy drip. Working in a teak plantation, we simultaneously measured drop size and velocity of both open rainfall and throughfall with five laser disdrometers in Mae Mo district, Lampang Province, northern Thailand. Throughfall drops were measured at four points: near stem, beneath one canopy, beneath two canopies, and canopy gap. Preliminary analyses for a rain event (13.6 mm of precipitation in three hours) revealed that volume percentage of throughfall type, throughfall DSD, and throughfall kinetic energy were different among the measurement points. The point at canopy gap received more free throughfall (43% of throughfall volume), whereas the other three points beneath canopy mainly received canopy drip (73-86% of throughfall volume). Throughfall had higher kinetic energy than open rainfall, 1.22-1.71 times higher in total kinetic energy and 4.56-7.13 times higher in total effective kinetic energy. Points under the canopy received higher kinetic energy than the canopy gap and near the stem because they received more canopy drip. Although the point near stem received a similar amount of canopy drip with the points under the canopy, the falling velocity of canopy drip was lower because the near stem had shorter distance to the lowest branch. This work provides managers of teak plantations with information to help mitigate soil erosion.

This research was supported by JSPS KAKENHI (Grant Number JP17780119 and JP17KK0159) and the CREST Program of JST.