2.1 Study site
The study site is a five hectares cultivated field characterised by a mean slope of 0.7%. The field is located in the 25 km² Louroux lowland catchment (0°45′20.43″E,47°08′33.51″N) (Grangeon et al., 2017), in the Loire river basin around 250 km south-west of Paris, France (Figure1). The field is bordered by a ditch on the N-W and N-E sides, a farm road on the S-W side and adjacent to another crop field on the S-E side. During the study period, the field was cultivated with wheat; seeded on October 10th 2019 after a superficial tillage.
The field substratum is Helvetian shelly sands (Rasplus et al., 1982). A grain size analysis realized at the center of the field (Table 1) shows an abrupt change in clay content at about 40 cm depth: above 40 cm depth, soil has loamy clay texture and below 40 cm depth, soil is compact with a heavy clay texture. A soil augering campaign (52 points) confirmed the presence of a compact clay layer at 47.6 cm mean depth (30.0 to 85.0 cm). Redoximorphic features are observed from 25 cm depth, confirming the hydromorphic behaviour of the soil. This soil is classified as a Cambisol (FAO, 2014).
As in most fields of the catchment, two types of drainage are observed in the study site (i) a subsurface drainage with tile drains and (ii) a surface drainage with SDRs (Figure1).
For the subsurface drainage system, tile drains has been installed at 120 cm depth with a 10 m spacing during the 1980s. Two drain collectors of 16 cm diameter span the field (Figure 1). The main collector collects the runoff of 34 tile drains, i.e. 4.16 hectares and is connected to the ditch at the north of the field. The collector output is 16 cm above the bottom of the ditch and can be submerged during a runoff event. The second drain collector collects the runoff of 7 drains and drains other fields. The last cleaning of the drains occurred in 2017. Only the main drain collector is monitored, it drains 83% of the field.
After seeding operations, the farmer digs out a temporary artificial surface drainage rill network (SDR, figure 1). SDRs are U-shaped and shallow (15 cm). They either intersect the major slope or follow microthalwegs to enable the export of excess surface and hypodermic water to the surrounding ditch. These SDR network persist till the first tillage operation that follows the harvest. The SDR network is composed of primary SDR that are connected to a collector SDR, itself connected to the ditch network. The main collector SDR is connected to the N-E ditch and a second one is connected to the N-W ditch. The SDR collector drains 83% of the field.