3.3.3 Water sources and pathways
The event of the February 2nd (event A, Table 3 and Figure 7) was selected to study water sources and pathway during a runoff event of the intense drainage period. During the 5 days prior the event A, 5 rainfall events occurred. A saturated zone is less than 6 cm below the soil surface before the event. The surface runoff occurs 22 min before subsurface runoff. Therefore, event A is characterized by the presence of a shallow saturated zone before the event, a surface runoff preceding the subsurface runoff and a high runoff coefficient.
The δ18O and δ2H composition of rainwater, soil water, surface and subsurface runoff are determined at low time-step during a storm event in order to identify water sources and pathways. No evaporation process can be identified from the isotopic signatures (Figure 6), thus, a variability of isotopic signature is due to a hydrological such as a change of water origin or mixing condition.
During the event A, the isotopic composition of rain, soil water, subsurface and surface runoff are significantly different (figure 6). The surface and subsurface runoff water isotopic composition are included between the composition of soil water and rain highlighting that surface and subsurface runoffs are a mixing between these two end-members. The ratio are different, the subsurface runoff is characterized by a higher proportion of soil water than the surface runoff, nevertheless, the proposition of soil water in the surface runoff is not negligible.
A two-end-member mixing analysis (EMMA) (Pinder & Jones, 1969) has been applied using isotopic signatures to estimate the proportion of soil water and rainwater contributing to the surface and subsurface runoffs. The isotopic composition of subsurface runoff before the flood event was considered as the end-member corresponding to pre-event soil water (-6.9 ±0.2‰). The assumption is supported by the fact that this value is close to the soil water composition (figure 6).
The isotopic composition of subsurface runoff varies along the storm event: during the rise of flow, the δ18O of subsurface runoff tends toward the rain composition underlining a rainwater contribution. At the time of the peak flow, the proportion of rainwater in the subsurface flow is estimated at 30%. During the falling limb, the δ18O decrease reveals an increase of soil water contribution : at the end of the sampling, the proportion of pre-event soil water in the subsurface runoff is estimated at 80%. For the surface runoff, the isotopic composition varies slightly except at the beginning of the storm event where the isotopic composition tend slightly toward the rainfall composition then stay constant. Thus, the surface and subsurface runoff are composed of 40% and 75% of pre-event soil water and 60% and 25% of rain water respectively.