4.1 Impact of subsurface drainage on the water balance
From Sept. 2019 to Aug. 2020, surface and subsurface flows represented
11.3% and 8.8% of the total annual rainfall, respectively. The
simultaneous measurements of subsurface and surface drainage discharges
during an entire year in a field drained by temporary artificial rill
and tile drains is an originality of the present study. Previous studies
quantifying both surface and subsurface drainage discharges have been
conducted on fields only drained by tile drains (Schwab, G. O. et al.,
1980; Skaggs et al., 1994; E. Turtola & Paajanen, 1995; Eila Turtola et
al., 2007). In these studies, surface runoff pathways are free. In the
present study, surface runoff is oriented manmade SDRs. So the surface
runoff measurement is realized for a precise area that is the same all
along the study period. Nevertheless, present results suggest that the
surface runoff guidance by artificial rills do not modify the relative
contribution of the two drainage networks: some studies report that the
subsurface runoff represents 34% to 68% of the total runoff.
In order to have a better understanding of the hydric functioning of the
field, a water balance could be established for the year 2019-2020. The
calculation of the water balance (Figure 8) was performed on a daily
time step with the components presented in Equation 2: precipitation
(P), real evapotranspiration (RET), surface and subsurface runoff (D),
and soil infiltration. Soil infiltration is composed of two terms: the
variation of the water stock of the LA and S1 surface horizons (ΔS) and
the infiltration to the underlying horizon (I), from S1 to S2. As there
is no measurement of the variation of the water stock of the LA and S1
horizons throughout the year, the variations were estimated. The hydric
capacities of the horizons were estimated from a pedotransfer function
proposed by Saxton (1986) and based on grain size. The horizons (LA+S1)
have an estimated cumulative total available water of 75.68 mm. The
estimation of water transfers to LA and S1 is based on the following
assumptions:
- (H1) The water stock can not exceed the water stock at field capacity
- (H2) The water stock can not be lower than the water stock at the
permanent wilting point
In addition, the distribution of water inputs or losses is based on the
following hypothetical processes:
- (H3) When the effective rainfall is positive, water input is first
allocated to the LA and S1 horizons provided that it does not contradict
assumption (H1), then to the S2 horizon.
- (H4) When the RET is higher than the rainfall, the water losses are
allocated in priority to the LA and S1 horizons, provided that they do
not contradict hypothesis (H2), and then to the S2 horizon.
- (H5) At the beginning of the hydrological year, the soil water content
is considered to be close to or equal to the water content at the
permanent wilting point, both for the (LA+S1) horizons and for the
underlying horizon.
The discharges and stock changes calculated under these assumptions are
presented in Figure 8. After the rains of September and October, the
water content of the LA+S1 horizons reaches the water content of the
field capacity. This is confirmed by manual tensiometer measurements
taken in early November. Variations in the measured depth of the
saturated zone even indicate that the LA and S1 horizons can be rapidly
saturated during a rainfall event, which is consistent with a pre-event
water content close to field capacity. Once the water content of the
LA+S1 horizons reaches the field capacity water content, flows from both
surface runoff and tile drains were monitored during some rainfall
events. Flow from tile drains becomes continuous from late November to
mid-December (Figure 5.). At the same time, water percolation from S1 to
S2 begins.
This is consistent with manual measurements of soil tension: at 95 cm
depth, soil water tension is divided by 4 between early November and
mid-December. By mid-December, 62% of the volume of water infiltrated
to S2 during the intense drainage period has percolated. Until this
date, percolation from S1 to S2 can represent up to 96% of the daily
precipitation. Little percolation was calculated in January, suggesting
that the S2 horizon was saturated. At the end of the intense drainage
period, the volume of water percolated from (LA+S1) to S2 is 1.4 times
the volume of water drained: 193.8 mm and 139.5 mm, respectively. By
mid-March, no tile drain flow was measured, because of decreased soil
water content in the LA+S1 horizons. This result was confirmed by
experimental measurements. From mid-March, the water stock in the LA and
S1 horizons did not reach the water stock at field capacity and no
transfer from S1 to S2 is calculated. S2 water stock progressively
decreases from mid-March to the end of August. Over the entire
hydrological year, S2 water stock increased by 50.7 mm and subsurface
drainage captures 54% of the water transiting through the soil (Table
5).
The water balance highlights the importance of percolation from S1 to S2
in the distribution of flows within the field. In particular, it seems
that the clayey S2 horizon plays a buffering role in the initiation of
subsurface runoff: subsurface runoff intensifies when the storage
capacity of the S2 horizon is reduced. However, almost the half of the
flow is not intercepted by the subsurface drains.
All of the assumptions made for the calculation of the water balance
appear to be consistent with the measurements made during the intense
drainage period. However, these assumptions, and more particularly
assumptions (H3) and (H4), are less suitable for the low drainage
period. Indeed, the development of the crop and the intensity of
evapotranspiration mean that water loss must certainly affect the LA/S1
and S2 horizons in a more distributed manner. In addition, the presence
of cracks significantly modifies the distribution of water in the soil
(Koivusalo et al., 1999) and may lead to direct flow to S2. The seasonal
contrast of the soil water content in the superficial horizons suggest
changes in soil structure that may have consequences on the soil water
pathways. In the following part, soil water pathways under dry and wet
conditions are described.