Pine Needles as ‘Knives’ Cutting Falling Raindrops: How Do Sharper Knives Cut Larger Raindrops?

ABSTRACT Coniferous forests are one of the more ubiquitous forests on Earth. The needles in their canopy and on the land surface can cut falling raindrops, thereby altering rainfall characteristics. These characteristics strongly affect forest rainfall evaporation, under‐canopy soil moisture and soil erosion. The cutting effect should be critically controlled by the needle diameter, raindrop diameter and impacting velocity. In this study, a high‐speed camera system, coupled with a drop staining method, was used to systematically investigate the effect across five needle diameters, five raindrop diameters, and two falling heights. The pre‐ and post‐cutting raindrop characteristics and needle properties were analysed. Our results show that cutting reduced drop velocity and diameter by 22% and 61%, respectively. For a given needle diameter, the ratio of change in drop velocity decreased as drop diameter increased. These changes caused a 58% decrease in total drop energy. Needle diameter influenced the distribution of post‐cutting raindrop numbers, with larger needles causing a higher proportion of smaller drops. For different needle diameters, cutting reduced drop diameter, velocity, and mean energy by 75%, 23% and 91%, respectively. The effects increased linearly with needle diameter. However, the total raindrop water mass, partially reflecting water retention on the needles, decreased with increasing needle diameter. Analyses revealed that the influence of different needles was closely linked to their apparent flexural strength. Our results also reveal that the cutting effect remained significant even at a falling height, which generated a drop falling velocity similar to natural rainfall. These findings underscore the critical role of the needle‐cutting effect in modifying throughfall dynamics and highlight the need to incorporate it into models of sub‐canopy hydrological and ecological processes.