Seasonal variation of 24D export through surface runoff from pasture

Abstract

One day after the herbicide 24D was applied to 1050 m² of a pastoral hillslope in Waikato runoff was generated with a sprinklertype rainfall simulator and 24D transport in surface runoff was measured The runoff coefficients differed significantly between an autumn (47) and a spring (19) event in spite of similar preevent soil water conditions Saturation excess with a variable contributing area had earlier been identified as the main runoff generating process for the autumn event In contrast infiltration excess possibly induced by treading effects and hydrophobicity are proposed as runoff causing processes for the spring event The eventaveraged 24D concentration in runoff was higher in autumn (049 mg/litre) than in spring (024 mg/litre) Correspondingly the exported 24D loss in autumn totalled 75 g/ha which equals 7 of the applied mass compared to only 14 g/ha (or 1) during the spring event