Abstract
Agricultural drainage networks increase hydrological connectivity from
the field to the receiving environments. The response to the issue of
surface water quality therefore implies an understanding of the
hydrological processes related to drainage, particularly at the field
scale. Drainage by tile drains and drainage ditch are the two most
studied types at the plot scale. They can be complemented by temporary
surface drains to improve the removal of surface runoff. The
hydrological processes and functioning of tile-drained fields have been
extensively studied at the event scale. However, few studies have been
conducted over a full hydrological year and the description of water
pathways in the soil generally relies on either exogenous tracer
monitoring or irrigation experiments. In addition, only a few studies
have been conducted on fields combining tile drainage and temporary
surface drainage. In this study, high temporal resolution quantification
of runoff from surface and subsurface drainage was conducted for a full
year to establish one of the first water balances for a surface and
subsurface drained field. Soil water pathways were studied under dry and
saturated soil conditions tracing water by measuring stable isotope
concentrations (18O and 2H) on
rainwater, soil water, and surface and subsurface runoff. Runoff
quantifications showed that surface drainage and subsurface drainage
respectively evacuate 41% and 32% of the annual cumulated effective
rainfall. The water balance highlights the importance of infiltrations
to the deep horizons: 46% of the water transferred to the soil is not
captured by the subsurface drains. Water tracing showed that rainwater
was directly transferred to subsurface drains on dry soil, likely
through macropores. On saturated soil, soil water present before the
rain remains the main source of water to the subsurface drains, but
event-rainwater also reaches the subsurface drains and can constitute up
to 25% of the subsurface runoff volume.
Key words: tile drain, surface drainage, water balance, water
tracing, soil water pathways, percolation.
1. INTRODUCTION
Subsurface drainage constitutes an artificial water pathway that
directly links subsoil to surface waters. Since the 1950s,
production-oriented agricultural policies have led to draining
hydromorphic soils to increase the area of cultivated land and its yield
(Musgrave, 1994). According to the last data of the International
Commission on Irrigation and Drainage (2018), 11% of the arable lands
of the world are drained. Two types of drainage are commonly used:
drainage by digging ditches at the edge of the field and subsurface
drainage by laying buried pipes.
At the watershed scale, one of the first drainage impact studied is the
impact on flooding (Skaggs et al., 1994). In particular, drainage by
tile drains reduces the intensity of floods that have a low return
period, typically, less than 2 years (Nedelec, 2005). This is due to an
increased transit time of water percolating through the soil to the tile
drains in comparison to a direct transfer of water on the soil surface.
Studies have also focused on the ecological and biological impact of
drainage systems on receiving environments (Blann et al., 2009; Gilliam
& Skaggs, 1986). Reducing subsurface drainage impact at the watershed
scale implies to understand drainage impact at the field scale.
At the field scale, in temperate climates, the functioning of tile
drains is generally highly seasonal (Arlot, 1999; Gramlich et al., 2018;
Hirt et al., 2011; K.W. King et al., 2014). The decrease in
evapotranspiration during the fall leads to the formation of a saturated
zone in the soil. The saturated zone, or perched water table, is usually
present from fall to spring. Zimmer, (1988) defines this period as a
period of intense drainage during which runoff coefficients are at their
maximum. During the rest of the year, the drains only flow during
intense storm events. In addition, subsurface drainage has been shown to
reduce surface runoff most of the time. For example, Arlot
(1999),comparing drained and no-drained fields with Cambisol on altered
shale, showed subsurface drainage represents 90% of the total runoff
and decreases by a factor 10 to 20 the surface runoff. Grazhdani et al.,
(1996), in fields with clay loam, showed subsurface drainage increases
the water yield by 34% but reduces surface runoff by 40%. In this case
however, subsurface runoff contribution to the total runoff varied from
47% to 69%. The impact of drainage on surface runoff depends on the
type of soil but also on the design of the drainage system. In lowland
agricultural regions such as the Région Centre-Val-de-Loire in France,
subsurface drainage is complemented by a third type of drainage: surface
drainage by digging surface drainage rills (SDR). Contrary to tile
drainage and ditches, SDRs are temporary: they are installed after
seeding and destroyed by the first tillage following the harvest. This
type of drainage is used to improve the evacuation of excess surface
water and to direct surface runoff. It provides an additional water
transfer pathway. Drainage by SDR is therefore likely to modify the
hydrological functioning of the field. However, to our knowledge, no
work have been carried out on fields or watersheds with this type of
drainage. Therefore, no water balance has been established for a surface
and subsurface drained field.
Studies at the scale of the soil profile in drained contexts have shown
that macropores are likely to be an important pathway for dissolved
(King et al., 2015) and solid transfers (Michaud et al., 2019; Øygarden
et al., 1997). Understanding the pathways of water flow in drained soils
is essential to reduce the negative impacts of drainage. Water flow in
soils can be categorized in two types of flow: one through the soil
matrix (Skopp, 1981) and the other through macropores (Jarvis, 2007;
McDonnell, 1990; Richard & Steenhuis, 1988). Matrix flow is generally
slower than macropore flow, which is qualified as preferential flow.
Macropores can have biological or structural origin and are
distinguished from the rest of the porosity by the heterogeneity of
their distribution (vertical and lateral), their large diameter and
their strong connectivity. Geochemical tracing experiments and
observation of water flow paths through the use of brilliant blue has
highlighted the role of macropores in the hydrological functioning of
drains (Stamm et al., 2002). This study showed that macropores enhance
the connectivity between the surface and the drains. Using bromide
tracing, Everts & Kanwar (1990) measured, during two irrigation
experiments, that 29% and 20% of the total volume flowing out of the
drain was from a preferential flow. Stone & Wilson (2006), using
chloride tracing, measured a preferential flow contribution of 11% and
51% during two rainfall events. After two years of isotopic monitoring,
Leaney et al. (1993) estimated that the share of preferential flow at
the drain outlet was at least more than 80%. These studies underline
the difficulty that persists in predicting the share of preferential
flow reaching the drain during a rainfall event. A few studies have
therefore looked for factors influencing the functioning of macropores.
For example, Grant et al. (2019), by observing with brilliant blue the
water pathways taken in two types of soils, have highlighted the
influence of soil type on the flow of water through macropores: the
preferential flow in a clayey soil is greater than in a sandy-silt soil.
In addition to the soil properties, the functioning of macropores seems
to be linked to the hydric state of the soil. Following blue-glow
tracing experiments combined with high temporal resolution soil moisture
measurements, Weiler & Naef (2003) showed that water circulation in
macropores was dependent on the water content of the different horizons.
The authors explained that the reason for this difference is due to
exchanges between matrix and macropores. Moreover, they showed that the
circulation of water in the macropores could either start at the level
of a saturated horizon or from the soil surface when the rainfall
intensity exceeded the infiltration capacity of the soil. Smith & Capel
(2018), by monitoring the specific conductance of water at the drain
outlet, have shown that even a light rainfall (< 5 mm) can
lead to a preferential flow of rainwater through the macropores.
However, most of the studies concerning water flow in a drained context
have focused on a few rainfall events without accounting for the
seasonal variability of drain operation. In order to improve
agricultural practices to reduce dissolved and solid exports from
drains, it is preferable to determine the type of water flow in drained
soils throughout the year, particularly with regard to variations in the
hydric state of the soil.
Studies dealing with the different aspects of the hydrological
functioning of subsurface drains reveal that:
- The hydrological functioning of field combining surface and subsurface
drainage is rarely studied
- Most of the studies concerning subsurface drainage are conducted at
the scale of the runoff event but few are conducted over longer
periods or even over a full hydrological year.
- Uncertainties remain on the distribution between matrix and
preferential flow according to the hydric stat of the soil.
In order to answer the questions raised by these gaps, the objectives of
this study are:
- To establish the water hydric balance of a field with surface and
subsurface drainage.
- To understand the influence of soil hydric conditions on the
functioning of surface and subsurface drains under natural conditions
over the course of a full hydrological year.
- To understand the influence of soil hydric conditions on water
pathways in the soil.
To meet these objectives, we propose to quantify the surface and
subsurface drainage flows of an agricultural field by measuring at high
temporal resolution the flow rates at the outlet of drains over a whole
hydrological year. In parallel, we also propose to monitor the soil
hydric state by tensiometric and piezometric measurements with high
temporal resolution. Finally, the study of water pathways in the soil
according to the hydric state of the soil will be addressed through
isotopic tracing of water during a winter runoff event and a spring
runoff event.