Herbicide loss in surface runoff from a pastoral hillslope in the Pukemanga catchment (New Zealand): Role of pre-event soil water content

Runoff production and herbicide loss in surface runoff were studied on a pastoral hillslope of 1050 m2 located in the Pukemanga catchment (New Zealand) during two rainfall events using a large sprinkler-type rainfall simulator. Simulated rainfall was applied for approximately 1 h at an average intensity of 35 mm h-1, 1 day after the herbicide application. In late autumn, the herbicide Pasture Kleen was applied at the recommended pasture application rate of 2 L ha-1 (active ingredient 1050 g ha-1 2,4-D ethylhexylester), while the pasture was treated with 4 L ha-1 MCPB 400 before the spring event (active ingredient 1600 g ha-1 MCPB sodium salt). The event-averaged MCPB concentration, normalised by the concentration of the solution applied, was twice the normalised 2,4-D concentration, reflecting the higher mobility of the MCPB sodium salt. However, the exported 2,4-D loss in autumn totalled 75 g ha-1, which equals 7% of the applied mass, and compared to only 4% for MCPB (61 g ha-1) during the spring event. Loads carried by sediments were calculated to be negligible. Herbicide loss was clearly determined by runoff volume, which was six times higher in autumn than in spring. The difference in runoff response in the two events could be explained by different pre-event soil water conditions. Saturation-excess with a variable contributing area in the convergent area of the hillslope section was identified as the main runoff generating process. Due to additional input of water by downslope lateral flow, saturation first developed close to the weir gradually extending further upslope. Further research on herbicide loads in subsurface lateral flow and on the spatial distribution of runoff response on the hillslope could identify high risk areas for herbicide loss through runoff. These critical areas could be managed better and for example excluded from herbicide applications to ensure environmentally sustainable agricultural management practices.