2.3.3 Water sources and pathways
In order to study water pathways during the low drainage period, the runoff event that occurred on May 11th was selected (event B, Table 3 and Figure 7).
Before the event B,the soil water tension is – 45 cbar at 45 cm depth. During event B, for the first time of the year, the surface runoff follows the subsurface runoff (1.9 h lag time). Moreover, compared to the event B, the runoff coefficient is particulary low : 9%.
During the event B, the subsurface runoff δ18O was almost constant, ranging from -4.9‰ (±0.2‰) to ‑4.6‰ (±0.2‰). The surface runoff δ18O ranged from -4.9‰ (±0.2‰) to -3.8‰ (±0.2‰). Because rainwater is the only possible source for surface runoff, surface runoff δ18O variations indicate the rainwater δ18O varied during the event. At the beginning of Event B, surface and subsurface runoffs δ18O are similar. During the event, surface and subsurface runoffs δ18O progressively become significantly differents. It suggests that, at the TD was still drained rainwater of the beginning of the event.