SOIL-EROSION VIA PREFERENTIAL FLOW TO DRAINAGE SYSTEMS IN CLAY SOILS

Runoff measurements from artificially levelled land showed that rates of particle erosion to the drainage system was even higher than that f rom surface runoff. This effect was increased by tillage whilst no til lage reduced the losses significantly. The field site soil type was a silty clay loam. Artificial levelling at the site had exposed an unsta ble subsoil, susceptible to cracking. Soil structures with macropores and cracks were examined to find out more about water and particle tra nsport via preferential flow pathways. A field profile description of soil structure and CT scanning of monoliths from the plough layer, sub soil and backfill showed different density patterns and structural pro perties. The backfill consisted of clods and open voids with direct co ntact to the drainpipes. The soil down to 50 cm was cracked both verti cally and horizontally and some cracks were leading into the backfill. Cracks of up to 10 mm width were found, indicating that transport of particles through them was possible. The measured field hydraulic cond uctivities varied over 3 orders of magnitude, indicative of a preferen tial flow network. Infiltration with dye tracer visually demonstrated rapid flow of water through cracks leading directly to the drainage sy stem; with a hydraulic conductivity of 9.9 x 10(-4) m s(-1) in the soi l; and 5.8 x 10(-3) m s(-1) in the backfill. These results indicate th at particles can be eroded from the plough layer and transported both laterally and vertically, through macropores and cracks into the backf ill, and then directly to drain pipes.