PITCH: A model simulating the transfer and retention of pesticides in infiltrating ditches and channel networks for management design purposes.

Agricultural ditches are frequently included in the panel of landscape elements to be managed to minimize the negative impacts of agriculture on the environment, particularly water contamination. A new mechanistic model simulating pesticide transfer in ditch networks during flood events was developed for help in designing ditch management. The model considers pesticide sorption processes to soil, living vegetation and litter and is adapted to heterogeneous and infiltrating tree-like ditch networks, with a reach resolution. The model was evaluated with pulse tracer experiments conducted on two vegetated and litter-rich ditches and with two contrasting pesticides, namely, diuron and diflufenican. It appears necessary to consider exchange of only a small proportion of the water column with the ditch materials to achieve a good reproduction of the chemogram. The model simulates well the chemogram of diuron and diflufenican during calibration and validation (with Nash performance criteria values ranging from 0.74 to 0.99). The calibrated thicknesses of the soil and water layers contributing to the sorption equilibrium were very small. The former was intermediate between the theoretical transport distance by diffusion and the thicknesses usually considered in mixing models for pesticide remobilization by field runoff. The numerical exploration of PITCH showed that during flood events, retention in ditches is mainly due to adsorption of the compound by the soil and litter. Retention is thus driven by the corresponding sorption coefficients and by parameters controlling the mass of these sorbents such as ditch width and litter cover. The latter parameters can be modified by management practices. In some cases, infiltration can contribute significantly to pesticide removal from surface water and in return participate in soil and groundwater contamination. Finally, PITCH exhibits a consistent behaviour in predicting pesticide attenuation and is shown to be relevant for evaluating ditch management strategies.