Artificially sub-surfaced drained watershed: A simpler or more complex hydrology?

In France, subsurface drainage concerns nearly 3 million hectares, which represent 10% of the agricultural surface. In waterlogged soil areas, it covers the main proportion of the catchment, up to 80% in the agricultural Brie area, East of Paris, France. This work focuses on experimental and modelling studies carried out at a small drained agricultural catchment, Goins (130 ha), in the Brie region. Rainfall and discharges from the catchment outlet were monitored since 10 years. Agricultural subsurface drainage (buried perforated pipes at 1 m depth and with a 10 m spacing) strongly affects the hydrological behaviour of this catchment: a continuous discharge is observed only during the winter wet period when precipitations are higher than PET and when the shallow water table rises the drain level. Catchment outflow corresponds mainly to drainage water, as surface runoff is limited thanks to the artificial drainage and as there is no deep groundwater contribution. In other words, the drainage system appears to be a hydrological shortcut, theoretically simplifying water transfer. Soil profile models are often used to describe the hydrology of small drained catchment provided that soils are homogeneous. Indeed, the lag time of water flowing through pipes can often be neglected. One of these models, DRAINMOD, is widely applied to predict water flows and water quality at the plot scale by calculating the water balance between two drains (soil profile scale). Thanks to the pipe network, changing from the soil profile scale to the watershed scale seems to be easier. From these considerations, we have used DRAINMOD to make rainfall/runoff simulations of Goins catchment. If the flow dynamics is well reproduced, peak events and consequently annual cumulative outflows are over-estimated. Actually, during the drainage season, we observed that the rainfall restitution (outflow amount vs. rainfall amount) was smaller (50%) compared to the simulated ones (around 80 %). A possible explanation of these results could be the existence of a deep infiltration from the perched water table towards the underlying Brie aquifer. Taking into account this deep infiltration in the model, we succeeded to well reproduce both cumulative outflows and peak events during the drainage season. But, a modelling problem remains for the transition periods (beginning and end of the drainage season): all the peak events are overestimated. One of the physical reasons suggested is that the perched water table may settle (or disappear) gradually in the watershed, so that only a part of the watershed surface contributes to the drainage outflow. This complexity in drainage hydrology is of great importance to assess the transfer of pesticides and fertilizers, which are commonly applied during these transition periods.