Micro-Droplet Flux in Forest and its Contribution to Interception Loss of Rainfall – Theoretical Study and Field

Abstract

A new approach to explain forest interception was proposed by introducing micro-droplets of crushed raindrops during rainfall. The aerodynamic diffusion and transfer of both vapour and micro-droplets from canopy to upper air were described and calculated, and proposed formulas applied to eight rainfall events at the Okunoi Experimental Station, Tokushima, Japan. Contributions from droplet transfer were 0.9-58.2 times of contributions from vapour transfer, taking a majority portion in total interception loss. Accounting only the vapour transfer or evaporation loss as estimated by Penman equation was not able to account for actual interception loss. The micro-droplet flux component took major portion in the two heavily rained events, and completely made up the interception as happened in October 2004. The droplet flux could accommodate a high interception rate, even when the air was nearly vapour-saturated and vapour flux was zero. This approach provided a new explanation to extraordinarily high interception rates.

Experimental study on canopy interception using artificial Christmas trees to evaluate evaporation during rainfall and the effects of tree height and thinning

“…In each Run average E during rainfall ( = I R /(t 2n−1 − t 0 − Sbt) tends to increase with average R during rainfall ( = P G /((t 2n−1 − t 0 − Sbt) except for the rain event on Tray #3 on 30 September to 1 October (Table II). Hourly I increased with hourly P G as shown by Tsukamoto et al (1988), Murakami (2006) and Hashino et al (2010).…”

“…However, the mechanism of I has not been clarified, though the phenomenon seems to be a simple process of evaporation from the canopy surface, because there are some inexplicable observations related to I. Firstly, some studies show that vast amounts of evaporation by I, e.g. 13 mm h −1 for hourly rainfall of 58 mm h −1 , occur during a rain event when water vapor in the air is nearly at saturation and that hourly I is proportional to hourly rainfall (Tsukamoto et al, 1988;Murakami, 2006;Hashino et al, 2010). Secondly, a massive amount of evaporation during a rain event is not explainable in terms of heat budget, since the source of latent heat of vaporization cannot be found (Murakami, 2006).…”