(Formerly Paper 5.6) Building a physical model of water transfer by rain splash in a 3-D canopy structure

The physical interaction between rainwater and plant surfaces occurs under three main hydrodynamic mechanisms following drop interception by plant organs : drop coalescence and dripping, stemflow, and splash droplet production. Rain-splash plays an important role in dispersal of splash-borne biotic particles at short distances ( <1-2m).

The purpose of this study is to model the mechanism of water transfer by splashing of raindrops impacting a canopy structure. In most models of water transfer in canopies, the vegetation is considered as a turbid medium. In this work, we suggest a discrete approach of rain and crop structure including an explicit description of drop size distribution and both geometry and localization of plants organs. With this approach, the balance of splashed water amounts can be obtained for each plant organ, canopy layer or whole canopy.

This mechanistic model of water transfer by rain splash in a canopy consists of several components : splash droplet production from impacting raindrops, droplet motion in the free canopy space, and droplet interception by plant elements. Forcing variables are rain characteristics in terms of drop size and velocity distribution and canopy structure. Aerial transport of droplets production is well know whereas the physics of splash droplet production remains largely unknown because of its hydrodynamic complexity. This part of the model is based on experimental results obtained using single incident drops. Numerical integration techniques rely on Monte Carlo methods with importance sampling and also on the explicit calculation of droplet trajectories using the Runge-Kutta method.

In the case study proposed in this paper, the chosen 3D structure is a cylinder identical to the one used in the splashmeter methodology (Shaw,1987) designed to evaluate the splash potentials of a given rain under natural conditions. This cylinder (diameter=0.08m, height=0.8m) is placed vertically in the middle of horizontal ring (external diameter=20 cm, internal diameter=25 cm) delimiting a free water area exposed to rainfall. Our objective is here to test the model ability to simulate vertical profiles of water splashed using a virtual splashmeter as 3D structure. Simulated profiles will be compared to those obtainable using the splashmeter.

To conclude, we will show how this modeling approach can be extended to the case of a whole 3D canopy structure.