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.