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.