A proposal for a new forest canopy interception mechanism: Splash droplet evaporation

Canopy interception was observed at a young stand of Chamaecyparis obtusa in a small Japanese experimental watershed for 2 years. Hourly canopy interception is linearly related to hourly rainfall on a rain event basis; this implies a dependence of the canopy interception on the rainfall intensity (DOCIORI). The DOCIORI became stronger from spring to summer and declined from fall to winter. Though canopy interception has been treated as evaporation from wet canopy surfaces, this concept cannot be accountable for (1) the DOCIORI and (2) the efficient canopy interception mechanism, as about 10–40% of the rainfall evaporates during rain events under high humidity conditions. A new concept is proposed to explain these contradictions: numerous small droplets are produced by splashes when a raindrop hits a canopy and they evaporate. It is well known that the specific number and the average size of raindrops increases with rainfall intensity, and, as a result, so do the number of small droplets produced by splashes and evaporation. This splash mechanism can explain both the DOCIORI and the efficient canopy interception mechanism based on simulations. A droplet of 25 μm in radius falling at its terminal velocity under a relative humidity of 95% evaporates and disappears after 1.7–2.8 m of fall distance, depending on the ambient temperature (15–25 °C), while one of 50 μm loses 20–32% of its original mass after 8 m of fall distance. However, a droplet of 100 μm in radius loses only 2–4% of its original mass with an 8 m fall distance. Seasonal changes in the DOCIORI are also partly explainable by the splash mechanism.