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An article by Agroscope, Eawag

Complex Processes at Work in Tile Drain Losses of Plant Protection Products

Around one-quarter of the utilised agricultural area in Switzerland is drained. Plant-protection products (PPPs) may enter watercourses via this route. Agroscope analysed the influencing factors and evaluated measures against PPP loss.

Measurements taken in streams and rivers have shown many to be contaminated with plant-protection products (PPPs), particularly after heavy rains. Although PPPs enter watercourses primarily through runoff and erosion (Prasuhn et al. 2018), drainage systems also lead to PPP losses.

For this reason, the national Plant Protection Product Action Plan calls for the investigation of PPP losses via tile drains and the development of preventive measures. The present study takes this need into account by:

  1. developing a spatial map of the potentially drained areas in Switzerland;
  2. summarising our knowledge on PPP losses through tile drains in a conceptual model which will serve as a basis for developing measures to reduce PPP inputs into surface waters.

25% of utilised agricultural area is drained

Not all drained soils in Switzerland are recorded. For this reason, machine learning was used to create a map showing which agricultural soils tended to waterlogging, and could thus potentially be drained. The calculations showed that 25% of the utilised agricultural area has a moderate or high drainage potential, i.e. is highly likely to be drained.

Map of the potentially drained agricultural area in Switzerland. 240,000 ha (27% of the utilised agricultural area) have a low drainage potential (green), 120,000 ha (13%) have an average potential (yellow) and 110,000 ha (12%) have a high drainage potential (red).

Model: A wide range of interacting factors

Based on an extensive study of the literature, a conceptual model of potential tile-drain losses of PPPs was developed. The way in which water flows through the soil plays an important role in this context. Water and PPPs dissolved therein can enter tile drains quickly via preferential flow routes (e.g. macropores). The often-loamy agricultural soil in Switzerland tends to macropores, a quality which favours PPP losses via drainage.

EXPOSIT is unsuitable for worst-case risk assessment

We examined how well the German registration model EXPOSIT is able to predict peak concentrations of PPPs in watercourses after heavy rain. For this, Agroscope experimental data from Zurich-Affoltern was compared with the EXPOSIT recommendations. It transpires that EXPOSIT does not always supply worst-case scenario estimates for Swiss conditions.

Measures combatting PPP losses via drainage

We evaluated measures that could minimise PPP losses via drainage. Most of the measures that reduce PPP losses through runoff and erosion are also effective in the case of tile drains. Some, however, such as no-till, may be counterproductive, since macropores remain in the untilled soil, encouraging PPP losses. Caution should therefore be exercised with no-till farming on tile-drained soils. As a rule of thumb, no PPPs should be used on drained soils if heavy rainfall is expected, or if the drains are already active.

Conclusions

  • PPP losses through tile drains are of consequence. Although they are lower than PPP losses through runoff and erosion, they are higher than leaching losses.
  • The often loamy agricultural soil in Switzerland is prone to macropores, through which channel PPPs may enter the tile drains during heavy rainfall.
  • PPP losses through drainage depend on many interacting factors, the most important being the time elapsed between PPP application and heavy rainfall.
  • Measures that are also effective against PPP runoff, erosion and leaching are recommended. The few known mitigation measures specific to tile drains are costly and time-consuming.
  • An initial comparison of experimental data with forecasts produced by the German registration model EXPOSIT shows that the latter is probably not suitable for an objective worst-case risk assessment of PPP use on drained agricultural soils in Switzerland.

Scientific article

Complex Processes at Work in Tile Drain Losses of Plant Protection Products

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