4.3 Conceptualization of the monitored field dynamics
Rainfall-runoff events for intense drainage and low drainage periods can
be described in four stepsĀ (FigureĀ 9). During the intense drainage
period: (1) Before the rainfall event: a saturated zone is present. The
subsurface drain already flows. The drained water is the pre-event soil
water, composed by previous rainwater. In-between the tile drains, the
soil moisture above the saturated zone is between the field capacity and
the saturation. (2) Surface runoff initiation and rise of the saturated
zone level due to an increase of the water content in the topsoil. The
rain water is rapidly transferred to the tile drains through the soil
macroporosity. The rainfall event water increases the pressure on the
pre-event soil water; thus the subsurface discharge increases. The
subsurface runoff discharge increases progressively and contains both
pre-event soil water and event rain water. (3) Drainage of the saturated
zone: the upper limit of the saturated zone begins to drop. If close
enough from the surface, the saturated zone can overflow and contribute
to the surface drainage system. The subsurface runoff still contains
pre-event soil water and event rain water but the proportion of
pre-event soil water increases. (4) End of the event: the saturated zone
is progressively drained by the subsurface system. Surface runoff stops
within a few hours. Subsurface runoff decreases progressively to its
pre-event discharge.
During the low drainage period: (1) Before the rainfall event: soil
moisture is very low and cracks are visible at the soil surface. (2)
Runoff initiation and formation of local saturated zones: the surface
runoff is reduced by the crop cover. Only intense rainfall events can
generate surface runoff. Moreover, cracks are the preferential pathways
for rainwater from the soil surface to the subsurface drain. Therefore,
subsurface drainage can begin before or at the same time as the surface
runoff. A saturated zone is formed deep in the soil. (3) Drainage of the
saturated zone: the saturated zone is quickly drained by the subsurface
drains. Cracks collapse and are clogged by the soil particles eroded
from the surface soil, or form the sides of the cracks. (4) End of the
event: runoffs stop quickly after the end of the rainfall. The saturated
zone is not present anymore after the end of the event. The size and
width of some cracks have been reduced.