Lysimeter study to investigate the effect of rainfall patterns on pesticide leaching

The influence of five rainfall treatments on water and solute leaching thro ugh two contrasting soil types has been investigated. Undisturbed lysimeter s (diameter 0.25 m, length 0.5 m) from a sandy loam (Wick series) and a mod erately structured clay loam (Hodnet series) received autumn applications o f the pesticide isoproturon and a bromide tracer. Target rainfall plus irri gation from the end of November 1997 to May 1998 ranged from 235 to 414 mm. Monthly rainfall varied according to a preselected pattern or was constant . Leachate was collected at intervals and concentrations of the solutes det ermined. Total flow and losses of bromide increased with increasing inputs of water and were larger for the Wick than for the Hodnet soil. Leaching of isoproturon from the Wick soil increased with increasing rainfall. Losses from the Hodnet soil were larger than from the sandy soil for the driest an d the wettest treatment indicating an increased importance of preferential flow. For intermediate treatments, isoproturon losses were smaller from the clay loam soil. The results suggest that an extrapolation of data from sin gle studies to a broader range of climatic conditions is more difficult for soils with preferential flow than for soils where matrix flow is dominant.