5. CONCLUSIONS
In this study, we established the water balance of a surface and
subsurface drained field and compared the runoff processes of surface
and subsurface drains according to the soil hydric status. The
quantification of the surface and subsurface fluxes allowed to find the
characteristics of the discharges of the drained fields. In our case,
the period of intense drainage extends from November to March and
corresponds to a period during which the saturated zone is close to the
soil surface. During this period, most of the subsurface discharge is
due to a slow matrix flow of water into the soil but macroporosity seems
to be active and contributes to up to 30% of the subsurface discharge.
Thus, it appears that rapid flows of water in soil to subsurface drains
are present throughout the year and regardless of the hydric state of
the soil. When the entire soil profile is saturated, a slow matrix flow
of water from the soil to the drains occurs and stays dominant as long
as the soil moisture content is greater than the field capacity. Surface
drainage seems mainly due to a saturation-excess runoff but when the
saturated zone approaches the soil surface, water from the first few
centimeters of the soil can be transferred to surface drains. Moreover,
according to the water balance, 44% of the water transiting through the
soil infiltrate into the S2 horizon. At the end of the intense drainage
period, the decrease in water content of the LA and S1 horizons to below
field capacity results in the discontinuation of base flow for
subsurface drainage. From mid-march to August, subsurface tile drains,
like surface drains, only operate during intense rainfall events.
However, the subsurface discharge comes from a preferential flow through
the macropores while surface drainage is only due to a refusal of
infiltration.
The calculation of the water balance proposed in this study makes it
possible to predict the hydrological functioning of the drained field
for the period of intense drainage. More particularly, it constitutes a
model that makes it possible to anticipate the initiation of subsurface
drainage. Provided that the variability of the soils and the
characteristics of the drainage networks of the other fields in the
watershed are taken into account, the simplicity of setting up this
model should make it possible to extrapolate it to other fields. The
application of this model on all the drained fields could then lead to
the understanding of the hydrological functioning of the watershed and
participate in the study of dissolved and particulate transfers from the
fields to the watershed.
Moreover, according to the soil hydric state, water sources variations
of subsurface runoff can be one of the factors explaining the variation
in dissolved and solid transfers over the year in drained systems.
Concerning surface drainage by digging SDRs, it can be observed that, in
addition to the runoff process usually encountered in drained fields,
there is a lateral flow of water from the soil to the SDRs when the
saturated zone reaches the depth of the SDR. Such a flow is therefore
likely to be associated with transfers of dissolved substances stored in
the first few centimeters of soil.