2.2.2 Runoff sampling and isotopic analysis
In order to study water pathways in the soil, two runoff events (Table 3) were selected, one during the period of intense drainage (event A, February 2nd) and the other occurring in late spring (event B, May 10th). These two events are representative of the runoff events of their respective period. These events was chosen because their representativeness and because the sampling realized. Stable isotope composition of soil water was only determined for the event A, i.e. when the soil water content was high enough to be sampled.
A rain collector is set next to the raingauge. During the runoff event sampling period, the rainfall is collected to represent only the study rainfall event by changing the rain collector just before and after the rainfall. Otherwise, the rainfall is collected by period of two weeks.
Surface and subsurface drainage runoff water is sampled using two automatic samplers. The sampling frequency depends on the runoff rate and it was manually adjusted for each event. For the subsurface runoff, the samples are taken at the beginning of the siphon (Figure 3) to avoid any discharge measurement perturbation. For the surface runoff, water is sampled at the middle of the “U” Venturi channel (Figure 3).
Soil water was sampled with ceramic cups. Four ceramic cups (15, 30, 45 and 80 cm depth) are parallel to and 3.5 m away from the tile drain axis. Two ceramic cups (30 cm depth) are perpendicular to and 1.5 m away from the drain axis. To sample soil water from the ceramic cups, a depression of 800 mbar was generated with a mannual vaccum pump.
Samples were filtered at the laboratory within a period of 24 hours after the sampling with 0.45 µm cellulose acetate membranes. Stable isotopes of water were analysed by laser spectrometry with a Picarro L2120-i. The results are expressed using the conventional notation (δ‰), as a deviation from the Vienna Standard Mean Ocean Water (V-SMOW). The analytical error for 18O and 2H are ±0.2‰ and ±0.7‰, respectively. The variability of δ18O and δ2H rain signature during a rainfall event have not been determined. From October 2018 to October 2020, 39 rainwater samples were collected allowing to determine a Local Meteoric Water Line (LMWL) as follows:
\(\delta^{2}H\ =\ 7.46\ \times\ \delta^{18}\text{O\ }\ +\ 5.61\)(Equation 1)