Hydro-sedimentary Dynamics of a Drained Agricultural Headwater Catchment: A Nested Monitoring Approach

Soil erosion and sediment transfer are extensive in lowland agricultural catchments. In these environments, high-frequency datasets of both water discharge and suspended sediment concentrations are often lacking. In particular, the impact of tile drainage networks on sediment fluxes in these catchments is poorly understood. Our research quantified sediment fluxes between 2013 and 2016 at five nested stations including a tile drain outlet across a small (25-km(2)) agricultural catchment (located in the Loire River basin, France) representative of lowland cultivated environments of the middle part of this basin. Sediment fluxes varied from 1 to 38 t km(-2) yr(-1) across the monitored subcatchments. Most of sediment fluxes (79 +/- 9%) were measured during flood events (n = 44), mostly occurring in winter (75%) and spring (20%). Seasonality controlled most of the variations of sediment fluxes. Sediment transfers in tile drains occurred episodically. Flows were measured during 11.4% of the monitoring time. A mean time lag of 80 min was recorded between the peak discharge at the drain outlet and the downstream river station. At the event scale, sediment fluxes exported from the tile drain varied between 1.1 x 10(-4) and 2.5 t km(-2) and remained of the same order of magnitude as those measured at the downstream river station, although the latter were on average 53% lower. Results suggest significant storage of sediment material in the river channel and its export during the most intense flood events. Our research emphasizes the need for continuous monitoring of water and sediment transfers in agricultural catchments equipped with tile drains.