CATENA vol. 14, p.149-155 Braunschweig 1987 SPLASH DETACHMENT: RUNOFF DEPTH AND SOIL COHESION D. Torri, M. Sfalanga and M. Del Sette, Firenze SUMMARY A set of laboratory experiments has been carried out in order to investigate the im- portance of soil cohesion and the inter- ference due to the depth of the surface runoff on the detachment rate of soil particles and aggregates. Experimental data show that the soil detachment rate decreases as the runoff depth increases. This indicates that the deta6hment power of the raindrops is partially dispersed by the water layer. An inverse relationship between soil detachment and soil cohesion has been confirmed. Nevertheless the grain-size distribution must also be used in order to define soil detachability. 1 INTRODUCTION The impact of raindrops on the soil sur- face is one of the agents of soil detach- ment and erosion (ELLISON 1947). When a drop hits a solid surface it generates a crown of lateral jets of wa- ter. The flow velocities of the jets attain initial values close to 8 times the falling velocity of the impinging drop (ENGEL 1955) if the solid surface is a smooth one; then a very rapid decrease in velocity has been observed. Data of GHADIRI & ISSN 0341-8162 (~)1987 by CATENA VERLAG, D-3302 Cremlingen-Destedt, W. Germany 0341-8162/87/5011851/US$ 2.00 + 0.25 PAYNE (1981) on drops hitting a soil paste confirm the basic data of ENGEL. Also a computer simulation based on the Navier-Stokes equations agrees with the already mentioned observations (HAR- LOW & SHANNON 1967). Shear stresses are generated at the solid-liquid interface by the lateral jets. Those shear stresses can detach soil par- ticles and aggregates ejecting them far from the impact area. AL DURRAH & BRADFORD (1982a,b) found out that an inverse re- lationship exists between the angle of ejection of the soil particle and the soil cohesion (rs) and a hyperbolic relation- ship between the amount of detached soil and Zs. Many authors (PALMER 1963, MUTCHLER 1967, MUTCHLER & YOUNG 1975, DE PLOEY 1981, POE- SEN & SAVAT 1981, TORRI & SFA- LANGA, 1986) pointed out that the depth of surface runoff influences the amount of the splashed soil, even if con- trasting trends have been observed. This paper aims to investigate the im- portance of soil cohesion and the inter- ference due to the surface runoff depth on the detachment rate of soil particles. CATENA An lnterd,sophnary Journal of SOIL SCIENCE HYDROLOGY GEOMORPHOLOGY