The role played by rock fragments in water erosion has received much attention in recent years. Knowledge of the effects of rock fragment characteristics on interrill erosion is incomplete. Hence, in order to investigate these effects on a small scale, a simulation experiment was conducted in Bragança, Northeast Portugal. The experimental setup consisted of 48 bottom perforated rectangular metal boxes (612 cm2), placed at a 10% slope, filled with 3.5 cm of a sieved silty loam soil over 2 cm of sand, covered by simulated rock fragments and maintained at near saturation. Twelve treatments, four replicates each, were exposed to 240 mm natural rainfall, comprising selected combinations of rock fragments size (small, medium—gravel range, and large—stone range), shape (rectangular and circular), position (surface, half-embedded and embedded) and cover percentage (17, 30 and 66%), and bare soil. Infiltration depth, runoff depth, washed and splashed sediment were repeatedly measured in appropriate collection devices. For bare soil, total wash and total splash were equivalent to 42.2 g m−2 and 70.6 g m−2, respectively. Infiltration and runoff represented 52% and 13% of total rainfall, respectively. Wash has a negative exponential relationship with rock fragment cover (RC). The regression coefficient varies negatively with cumulative precipitation, decreasing significantly after a surface seal is formed (at about 80 mm cumulative precipitation). The relationship between splash and RC, linear and negative, varies with time, too. Correlation with RC is positive for infiltration depth and negative for runoff depth, both reflecting the seal development with time. The effects of rock fragments size, position and form were tested for 30% RC. Size has a positive effect on runoff depth, wash and splash, and a negative effect on infiltration depth. The effect of rock fragment size on infiltration, runoff and erosion is more pronounced than that of position. The effect of shape was less significant than that of size and position.
