2.2.1 Hydrological and hydrodynamic measurements
Rainfall is monitored with an automatic raingauge located at the northeast corner of the field (Figure 1). Rainfall is logged every five minutes with a 0.2 mm accuracy. A total of 722.0 mm was recorded during the study period from September 2019 to August 2020, which is representative of the average cumulated rainfall in the area (ca. 685 mm 1981-2010, MeteoFrance) (Figure 2). However, the spring and summer are drier (-19%) whereas the autumn and winter are more humid (+23%) than the average. Potential evapotranspiration (PET) was estimated at hourly time step from the Penman–Monteith equation accounting for the hourly average of air temperature, wind speed, solar radiation, humidity and atmospheric air pressure. All measurements for the PET estimation were logged at a weather station of the Louroux catchment located 2 km northeast of the study field.
At the output of the SDR collector, the runoff water level height is measured every minute with a 2 mm accuracy in a Venturi channel (Figure 3a) placed at the output of the furrow collector. The runoff discharge is deduced from the water height using the constructor rating curve. An original device (Figure 3b) was designed to monitor the tile drain subsurface runoff flow. A horizontal PVC-siphon was placed in the continuity of the main drain collector output. At the extremity of the siphon, a doppler radar measured flow every minute. This set up allows the flow monitoring without flow modification and it allows measurements even when if the drain collector output is under water, a common situation in this region.
The soil hydric state was characterised by two types of measurements: the soil water tension on a plot on the study site and the upper depth of the saturated zone at 10 measuring points. The soil water tension was recorded every 10 minutes with autonomous WaterMark® tensiometers. Four groups of tensiometers were installed at a distance of 0.5, 1.5, 2.5 and 5 m of a subsurface tile drain axis. For each group, soil water tension was recorded at 15, 30 and 45 cm depth on each lines. Five manual tensiometers were installed to check the autonomous records. The upper limit depth of the saturated zone was recorded every 10 minutes from December 22th to June 7th with Mini-Diver™ probes compensated for atmospheric pressure using a Baro-Diver™ (Schlumberger Water Service) (Figure 1). The measurement accuracy is ±1 cm H2O. The Mini-Diver™ probes are installed in piezometers at a depth of 40 cm. The locations have been chosen with a stratified random sampling and probes are placed at the interdrain above the clayey horizon.