Characterization of deep infiltrations in subsurface drained agricultural system

Subsurface drainage practice is widely used in agriculture to eliminate temporary winter waterlogging of hydromorphic soils. Soil surface saturation is mainly due to the presence of an underlying layer (~1m deep) with a high clay content, considered as semi-impermeable. Generally, deep infiltration under this layer has been neglected in many hydrological studies. However, considering the variations in the ground water table levels, the recharge is mainly due to the deep infiltration. Understanding the dynamic of this infiltration is very important both for the quantitative management of groundwater resources and for the protection of its quality. Indeed, this infiltration can transfer spreading products (fertilizers and pesticides) used in agriculture, mainly the water-soluble molecules.

To understand the dynamic of the deep infiltration, hydrological and geophysical monitoring using ERT (Electrical Resistivity Tomography) method was set up on the drained experimental plot of Boissy le Châtel (Orgeval Observatory, in France). The water balance at the scale of the experimental plot highlighted the contribution of the deep infiltration to the groundwater table rise at the beginning of fall season.

Time-lapse geophysical survey coupled with water content monitoring on a 1.5m vertical profile showed the movement of a rewetting front from the soil surface towards deep layers during this very short transition period, which follows a precipitation event. After this period, during the intense drainage season, the deep infiltration below the drains continues (in the order of 0.12 mm/day compared to 2mm/day for subsurface drained flow) despite the rise of the water table to the surface layer. However, it is difficult to monitor its pathway using the passive ERT method, less sensitive to electrical resistivity variations in the range of soil water content close to saturation.