3.1 Runoff and rainfall seasonality
During the study period, the total runoff volume was 145.4 mm. Surface
and subsurface drainage have contributions of the same magnitude with
82.0 mm runoff from surface drainage and 63.4 mm runoff from subsurface
drainage. 88.0% of the surface runoff flow and 96.1% of the subsurface
runoff flow occurred between November 2019 and March 2020. During this
period, surface and subsurface drainage response to a rainfall event are
different. The surface runoff flow only occurs intermittently, after a
sequence of rainfall events whereas the subsurface runoff is continuous
from early November to mid-January and the end of January to mid-March.
This period is defined as an intense drainage period (Zimmer, 1988). In
contrast, the period from April to August can be defined as a low
drainage period.
The hydrological year is characterized by 108 rainfall events (Figure
4), and only 41 have led to a runoff. During the intense drainage
period, 39 of the 53 rainfall events induced runoff. During, the low
drainage period, 28 rainfall events occurred but only two generated
runoffs. Moreover, two periods could be defined from the soil hydric
state (Figure 5.): the whole soil is constantly saturated until
mid-March and partially saturated only during storm events after this
date.
The seasonality of the runoff events is consistent with the seasonality
of the effective rainfall events (Figure 2) and thus with the variations
of the soil hydric state (Figure 5). In the following, a focus on two
rainfall events representative of the intense and low drainage periods
will be performed. The aim is to provide the reader a detailed
description of surface runoff and tile drain flows dynamics during these
two periods