i-Tree Streets/Design/Eco Rainfall Interception Model Comparisons

i-Tree Streets/Design and Eco estimate annual rainfall interception by urban forest based on different physics-based water balance models. Streets/Design is based on Xiao et al. (1998), while Eco is based on Wang et al. (2008) and improved by Hirabayashi (2013). In Xiao et al. (1998), annual rainfall interception percentage against the gross precipitation in Sacramento County, CA was estimated 1.0 % for the entire area and 11.1 % for the canopy-covered area. Applying Wang et al.’s model without Hirabayashi’s model to the same study area resulted in 0.6 % and 8.6 %, respectively. The results didn’t exactly match, but considering the differences in the two models’ parameterizations, it can be concluded that they were relatively in a good agreement. Although these results seem feasible to represent annual rainfall interception by urban forest, they may be too large if these results are interpreted as annual avoided runoff due to trees. Even if these trees were all removed, rainfall otherwise intercepted by trees may infiltrate into the ground or be stored on ground depressions and thus not all of the rainfall would runoff. To represent the annual avoided runoff, the effects by trees need to be isolated from Xiao et al.’s or Wang et al.’s results. Hirabayashi (2013) developed tree effect isolation method incorporating into Wang et al.’s model in i-Tree Eco. With this improvement, the annual avoided runoff for Sacramento County, CA was estimated 0.14 % for the entire area, about one seventh of Xiao et al. ’s annual rainfall interception (1.0 %).