International audienceThe paper presents a simplified model called PESTDRAIN. It simulates pesticide transport in a subsurface tile-drained field. It computes surface runoff and tile-drainage flow rates, along with the associated pesticide concentrations, with a variable event-driven time step. PESTDRAIN consists of three coupled modules: SIDRA, SIRUP and SILASOL. SIDRA and SIRUP are the water flow simulation modules in the saturated and unsaturated zones, respectively. SIDRA follows a simplified physically based approach while SIRUP follows a conceptual capacitive approach. SILASOL is the solute transport module for both the saturated and unsaturated zones and is based on transfer functions. It includes simple representations of adsorption and degradation of pesticides. PESTDRAIN was tested on field data sets collected for three drainage seasons at the La Jaillière experimental site in north-western France, for the wheat herbicides isoproturon (IPU) and diflufenican (DFF). After model calibration, relative errors for drainage and surface runoff flows over the season were 14% and 4%, respectively, and the Nash–Sutcliffe efficiency coefficient (Neff) value for drainage discharge was 0.58. A fair reproduction of a high temporal resolution IPU concentration data set in drainage discharge was also obtained (Neff=0.28). For the validation data sets, PESTDRAIN was able to simulate accurately drainage discharge with Nash–Sutcliffe efficiency coefficients of 0.57 and 0.69. The global Neff was 0.44 for all flow-weighted average weekly concentrations in drainage. Relative errors for the pesticide losses were 2.5% and 35% (IPU), and 60% (DFF). For surface runoff the results were not as accurate, but they remained correct in terms of time location and order of magnitude. Although further validation is necessary with more field data, PESTDRAIN appears as a promising tool for agricultural water management

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The Orgeval watershed (104 km(2)) is a long-term experimental observatory and research site, representative of rural areas with intensive cereal farming of the temperate world. Since the past few years, we have been carrying out several studies on nitrate source, transformation and transfer of both surface and groundwaters in relation with land use and agriculture practices in order to assess nitrate (NO3(-)) leaching, contamination of aquifers, denitrification processes and associated nitrous oxide (N2O) emissions. A synthesis of these studies is presented to establish a quantitative diagnosis of nitrate contamination and N2O emissions at the watershed scale. Taking this watershed as a practical example, we compare curative management measures, such as pond introduction, and preventive measures, namely conversion to organic farming practices, using model simulations. It is concluded that only preventive measures are able to reduce the NO3(-) contamination level without further increasing N2O emissions, a result providing new insights for future management bringing together water-agro-ecosystems.

International audienceMeasurements of soil bulk density profiles combined with thin-section analysis have been suggested to assess the structural seedbed degradation caused by rainfall. The effects of water table elevation and rainfall duration on surface sealing and seedbed slumping were studied on a repacked silt loam soil. Two initial water table elevations (0.3 and 0.7 m below the soil surface) and three simulated rainfall durations (15, 30, and 40 min at 30.5 mm h-1 followed by 180 min at 7 mm h-1) were used. Seedbed bulk density profiles were generated using x-radiography of resin-impregnated soil slices. Macroporosity measurements using image analysis and thin-section observations showed that infilling of eroded particles in interaggregate voids and compaction of the infilled particles were the main sealing processes. Below the seal, the seedbed exhibited coalescence and welding of aggregates into larger units, which affected mainly macroporosity. A model of sealing, exponential decrease in bulk density with depth, and slumping, linear increase in bulk density with depth, adequately reproduced the measured bulk density profiles (regression RMSE range 0.057-0.106 Mg m-3). The change in surface bulk density increased with rainfall duration, whereas this factor did not significantly affect slumping. The highest initial water table elevation led to the highest soil surface and internal seedbed bulk densities. It was suggested that high values of soil water content led to a decrease in aggregate cohesion. Moreover, the number of wetting and drying cycles and the water content during these cycles were shown to increase the magnitude of slumping

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