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.