Hydrological functioning of a field combining surface and subsurface
drainage: from the water balance to the soil water pathways.
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.