Wind Speed Effects on Rain Erosivity

*Katharina Helming, Center for Agricultural Landscape and Land Use Research (ZALF), Eberswalder Str. 84, D-15374 Muencheberg, Germany. *Corresponding author: [email protected] ABSTRACT The kinetic energy of rainstorms plays a paramount role in surface sealing, runoff, and erosion processes. Typically, the kinetic energy rate is calculated based on terminal velocity of vertically falling raindrops. Few studies have investigated the effect of wind speed on raindrop velocity, rainfall energy and on inclination angles of raindrops. This paper reports an attempt to determine (i) the effect of wind speed on the kinetic energy of rainstorms, (ii) the relationships between rainstorm intensity and wind speed, and (iii) raindrop impact angle distributions with respect to wind speed, inclination angle, and soil surface geometries. Rainfall amount and wind speed were measured over 24 months. The kinetic energy and the inclination angle of the winddriven rain were determined with trigonometric functions combining the horizontal wind speed with the vertical drop velocity. The distribution of raindrop impact angles was determined for various soil surfaces represented by digital elevation maps. The results showed: 1) The rainstorm kinetic energy, determined with respect to the wind speed, reached a maximum of 3.5 times the value of the kinetic energy without the wind speed factor. On average, the portion of the raindrop energy derived from wind speed accounted to about one fourth of the total rain energy. 2) There was no association between rainstorm intensity and wind speed. 3) Soil surface roughness was more important for raindrop impact angle distribution than wind speed. The results suggest that wind speed has considerable effects on rainstorm energy, and thus impacts surface sealing and soil erosion processes.