4.2 Soil water pathways and generation of runoffs
During the intense drainage period, a saturated zone is observed 75% of
the time, 30 cm below the soil surface or higher. The quasi-permanent
saturation of the soil leads to a low infiltration capacity and a
continuous water pressure above tile drains. Before event A, most the
entire soil profile was already saturated but the depth of the saturated
zone decreased during the event to reach less than 1.6 cm. The closer to
the soil surface the saturated zone is, the higher the subsurface runoff
flow is: it suggests subsurface discharge variations are caused by
variations of the water pressure above the drain. Moreover, as shown by
water tracing, subsurface runoff results from the mixing of both
rainwater and soil water. Such a transfer had also been observed by
(Coulomb & Dever, 1994) at the beginning of the intense drainage season
but not in the middle of the drainage season. The authors explain this
difference by the saturation of macropores during the drainage season.
Our results suggest that macroporosity was still active during the
studied event of the intense drainage period. Thus, a preferential flow
through the macropores is possible even when the soil is saturated.
Concerning the surface runoff, surface drainage may be caused either by
a refusal of infiltration or by a transfer of water from the first soil
centimeters to the SDRs. Isotope water tracing shows that surface runoff
is a mix of both soil water and rainwater. The presence of soil water in
the surface runoff may be due to the increase of the saturated zone
level and lateral subsurface flows. Following isoproturon transfer,
Haria et al. (1994) gave the same explanation and specified that
a lateral flow circulated over an impeding layer. The presence of such a
lateral flow is therefore also possible in our case. The lateral flow
would then flow directly into the ditch at the edge of the field and
could constitute a significant but unquantified part of the water
balance.
During the low drainage period, soil moisture is constantly low.
However, the studied event shows that a sufficiently intense event can
generate surface and subsurface runoff. For such events, subsurface
runoff occurs before surface runoff, proving that the process of
rainwater transfer to subsurface drains is faster than the generation of
surface runoff. Two reasons can be suggested: (i) the presence of the
wheat cover that slows the surface runoff and (ii) a preferential
vertical flow through macropores. Isotopic measurements support the
hypothesis of preferential flow through the macroporosity but our
results do not support any conclusion about the soil cover effect on the
surface runoff. During the studied event, surface runoff is only
composed of rainwater. The similarity of the isotopic signatures of
surface and subsurface runoffs suggest the subsurface runoff is also
only composed of rainwater. Moreover, cracks present at the soil surface
before the event constitute a macroporosity that can support a direct
water transfer from soil surface to tile drains. This result is
consistent with the conclusions of the dry and isotopic tracers based
studies conducted by (Øygarden et al., 1997) and (Klaus et al., 2013) in
fields with similar soils. Moreover, these authors underlined the role
of the macropores distribution and connectivity for the water transfer
to tile drains.