WATER-MOVEMENT AND ISOPROTURON BEHAVIOR IN A DRAINED HEAVY CLAY SOIL .1. PREFERENTIAL FLOW PROCESSES

The processes and mechanisms that control pesticide transport from dra ined heavy clay catchments are being studied at Wytham Farm (Oxford Un iversity) in southern England. In the first field season field-drain w ater contained high concentrations of pesticide. Soil studies demonstr ated that the main mechanism for pesticide translocation was by prefer ential flow processes, both over the soil surface and through the soil profile via a macropore system that effectively by-passed the soil ma trix. This macropore system included worm holes, shrinkage cracks and cracks resulting from ploughing. Rainfall events in early winter rapid ly created a layer of saturation in the A horizon perched above a B ho rizon of very low hydraulic conductivity. Drain flow was initiated whe n the saturated layer in the A horizon extended into the upper 0.06m o f the soil profile; thereafter water moved down slope via horizontal m acropores possibly through a band of incorporated straw residues. Thes e horizontal pathways for water movement connected with the fracture s ystem of the mole drains, thus feeding the drains. Overland flow occur red infrequently during the season.