Rainfall kinetic energy controlling erosion processes and sediment sorting on steep hillslopes: A case study of clay loam soil from the Loess Plateau, China L. Wang a,b , Z.H. Shi a,c,⇑ , J. Wang c , N.F. Fang a , G.L. Wu a , H.Y. Zhang a,b a State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, CAS and MWR, Yangling, Shaanxi Province 712100, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China c College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China article info Article history: Received 3 December 2013 Received in revised form 7 February 2014 Accepted 27 February 2014 Available online 12 March 2014 This manuscript was handled by Geoff Syme, Editor-in-Chief Keywords: Rainfall kinetic energy Sediment size Transport mechanisms summary Rainfall kinetic energy (KE) can break down aggregates in the soil surface. A better understanding of sed- iment sorting associated with various KEs is essential for the development and verification of soil erosion models. A clay loam soil was used in the experiments. Six KEs were obtained (76, 90, 105, 160, 270, and 518 J m 2 h 1 ) by covering wire screens located above the soil surface with different apertures to change the size of raindrops falling on the soil surface, while maintaining the same rainfall intensity (90 ± 3.5 mm h 1 ). For each rainfall simulation, runoff and sediment were collected at 3-min intervals to investigate the temporal variation of the sediment particle size distribution (PSD). Comparison of the sediment effective PSD (undispersed) and ultimate PSD (dispersed) was used to investigate the detachment and transport mechanisms involved in sediment mobilization. The effective–ultimate ratios of clay-sized particles were less than 1, whereas that of sand-sized particles were greater than 1, suggest- ing that these particles were transported as aggregates. Under higher KE, the effective–ultimate ratios were much closer to 1, indicating that sediments were more likely transported as primary particles at higher KE owing to an increased severity of aggregate disaggregation for the clay loam soil. The percent- age of clay-sized particles and the relative importance of suspension–saltation increased with increasing KE when KE was greater than 105 J m 2 h 1 , while decreased with increasing KE when KE was less than 105 J m 2 h 1 . A KE of 105 J m 2 h 1 appeared to be a threshold level beyond which the disintegration of aggregates was severe and the influence of KE on erosion processes and sediment sorting may change. Results of this study demonstrate the need for considering KE-influenced sediment transport when pre- dicting erosion. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Soil erosion by water involves the detachment of soil particles by raindrop impacts and the subsequent transportation of particles by overland flow (Pieri et al., 2009; Warrington et al., 2009). The importance of raindrop impact in sediment detachment has been shown in both laboratory and field settings (Martínez-Mena et al., 1999, 2002; Issa et al., 2006; Asadi et al., 2007a; Pieri et al., 2009) and has been incorporated into the most commonly used models for predicting soil loss, e.g., RUSLE (Renard et al., 1991) and WEPP (Nearing et al., 1989). Many previous studies have focused on the effects of rainfall properties, such as the drop size and velocity, drop shape, kinetic energy (KE), and intensity, on erosion processes (Riezebos and Epema, 1985; Jayawardena and Rezaur, 2000; Salles and Poesen, 2000; Martínez-Mena et al., 2002; Wei et al., 2007; Pieri et al., 2009). Rainfall intensity is a contributing factor to the runoff and sediment generation (Martínez-Mena et al., 1999; Wei et al., 2007). However, the role of rainfall intensity is ambiguous when the infiltration capacity of the soil is exceeded during short-duration, high-intensity storms as well as long duration, low-intensity storms, both of which may cause erosion. Energy parameters are now generally accepted as being better predictors of rainfall erosivity over a wide range of conditions (Stocking and Elwell, 1976). Data on the KE load of rain- storms are essential to developing and verifying models of soil detachment by raindrop impact on interrill erosion processes (Jayawardena and Rezaur, 1999). Relating KE to easily measured http://dx.doi.org/10.1016/j.jhydrol.2014.02.066 0022-1694/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China. Tel.: +86 27 87288249; fax: +86 27 80 02171035. E-mail address: shizhihua70@gmail.com (Z.H. Shi). Journal of Hydrology 512 (2014) 168–176 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol