Reduction in the ratio of stemflow to rainfall during heavy rain in two Japanese cedar stands and the influence on rainfall partitioning

Abstract

Stemflow (SF) and gross rainfall (GR) were measured in two Japanese cedar stands with stand densities (SDs) of 5700 trees/ha and 9700 trees/ha. SF water was collected from four trees in each stand, and SF was derived by dividing the volume by the canopy area of the trees. In previous studies the highest SD for SF measurements in Japanese cedar and cypress was 2500 trees/ha with SF/GR of 23.3%. In this study SF/GR in the 5700-trees/ha and the 9700-trees/ha stands were 34.0% and 53.3%, respectively. Literature data together with the results of this study showed linear relationship between SF/GR and SD. In both cedar stands SF increased linearly with increasing GR on a rain event basis, however, a bending point in linearity that is defined as a threshold appeared for GR > 100 mm in the 9700-trees/ha stand, over which the slopes of the regression line became gentler. The 10-minute analyses also revealed the threshold and gentler slope of the regression line during intense rainfall. The threshold values were not constant but ranged from 4 to 8 mm/10 min. One of the causes of the threshold is splash droplets generated by raindrops hitting canopy and trunks, and another is SF water that breaks away from branches and stems. Both of them become throughfall (TF) and reduce rainwater allocated to SF during intense rainfall. The past measurements on TF and canopy interception (CI) conducted at the same site as this study showed the threshold, over which CI/GR increased. The existence of the threshold in CI/GR means some rainwater that was not allocated to SF but became TF evaporated as splashes before reaching the forest floor. Reduction in SF/GR influences rainfall partitioning in the stands.