Source area effects on herbicide losses to surface waters—A case study in the Swiss Plateau

Reducing herbicide applications on hydrologically contributing areas has been suggested as an effective approach to mitigate diffuse herbicide pollution. We report on controlled herbicide applications in two small neighboring catchments in the Swiss Plateau investigating the persistence of contributing areas in time and the influence of herbicide properties on the spatial loss patterns and on the extent of herbicides loads. A mixture of atrazine, dimethenamide, metolachlor, and sulcotrione was applied to four corn fields on the same day. Simazine was used as an additional tracer to the wettest part of one field. Irrespective of rainfall timing and for all compounds, spatial differences in the herbicide losses relative to the amounts applied were large (factors 15–18). The water table fluctuations in the fields suggest that soil hydrology differentiated between “high” and “low loss” fields. The spatial patterns were consistent with results obtained under very different weather conditions in an earlier study indicating that contributing areas prevail in time. Simazine losses from a wet hot spot were exceptionally large exceeding 24% of the applied mass. Avoiding atrazine applications on this small area corresponding to 1% of the field size would have resulted in a 30% decrease of the overall losses. Hence, land use strategies should not only account for soil (hydrological) differences between fields but should pay sufficient attention to small-scale heterogeneity within fields.