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.