"Cutting effect" of needles on the raindrop characteristics

Plant leaves (both in the canopy and on the land surface) can substantially alter raindrop characteristics, which link to rainfall evaporation, infiltration, raindrop energy, and land surface erosion. The effect varies depending on the leaf morphology. Because of the magnitude difference between needle diameter (similar to 1 mm) and raindrops (up to similar to 8 mm), there should be a "cutting effect" of the needle on falling raindrops. However, no study has beem performed to address how this effect alters raindrop characteristics. In this study, we investigated the effect using artificial water drops, high-speed photography, and staining method in the laboratory. Masson pine (Pinus massoniana Lamb.) needles were collected from the field including fresh green needles (FGN) withered yellow needles (WYN) partially decomposed needles (PDN). The needle properties (i.e., surface repellency, cross-section morphology, and flexural stiffness) were measured to determine the factor controlling the cutting effect. Results showed that after the cutting effect, one raindrop could be cut into up to 12 small raindrops. Their velocities owned a bimodal distribution. Compared to the diameter before the cutting effect occurred, the raindrop diameter decreased by an average of 59 %, and the raindrop velocity decreased by an average of 19 %. This caused a substantial decrease in the raindrop energy (similar to 85 %). The cutting effect included three sub-processes: collision, splitting, and interception. Our analysis suggested that the splitting process was the main reason for the cutting effect to reduce raindrop energy. The cutting effect was affected by the needle properties and raindrop falling heights. The cutting effect was enhanced when the needle developed from FGN and WYN to PDN. The cutting effect was positively related to the falling height of raindrops. Changes in the raindrop characteristics due to the cutting effect should enhance rainfall evaporation and infiltration and thus decrease soil erosion, which needs to be evaluated in further studies.