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.
