EFFECTS OF SURFACE ROCK FRAGMENT CHARACTERISTICS ON INTERRILL RUNOFF AND EROSION OF A SILTY LOAM SOIL

The role played by rock fragments in water erosion has received much a ttention in recent years. Knowledge of the effects of rock fragment ch aracteristics on interrill erosion is incomplete. Hence, in order to i nvestigate these effects on a small scale, a simulation experiment was conducted in Braganca, Northeast Portugal. The experimental setup con sisted of 48 bottom perforated rectangular metal boxes (612 cm(2)), pl aced at a 10% slope, filled with 3.5 cm of a sieved silty loam soil ov er 2 cm of sand, covered by simulated rock fragments and maintained at near saturation. Twelve treatments, four replicates each, were expose d to 240 mm natural rainfall, comprising selected combinations of rock fragments size (small, medium-gravel range, and large-stone range), s hape (rectangular and circular), position (surface, half-embedded and embedded) and cover percentage (17, 30 and 66%), and bare soil. Infilt ration depth, runoff depth, washed and splashed sediment were repeated ly measured in appropriate collection devices. For bare soil, total wa sh 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 negat ively 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 t ime, too. Correlation with RC is positive for infiltration depth and n egative for runoff depth, both reflecting the seal development with ti me. The effects of rock fragments size, position and form were tested for 30% RC. Size has a positive effect on runoff depth, wash and splas h, and a negative effect on infiltration depth. The effect of rock fra gment 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. (C) 1998 Elsevier Science B.V. All rights reserv ed.