Spatially distributed simulation of water balance and sediment transport in an agricultural field

•Water balance and soil erosion within a clayey field were simulated with FLUSH.•The FLUSH model is a field-scale, 3D distributed hydrological and erosion model.•The water balance included tillage layer runoff, drainflow and groundwater outflow.•Hourly sediment concentrations in tillage layer runoff and drainflow were assessed.•Spatial distribution of erosion in the field area was investigated with the model.

Runoff and sediment transport are distinctively three-dimensional (3D) processes and occur through overland, tillage layer and subsurface pathways. The objective was to quantify water balance and sediment concentrations in runoff waters and to assess sediment loads via surface runoff and drainflow in a clayey, subsurface drained field section using the FLUSH model. The model can simulate field scale two-dimensional overland flow and 3D unsaturated and saturated subsurface flow, including preferential flow in macropores. The erosive processes, comprised of hydraulic and raindrop splash erosion, occur in the overland domain while suspended sediment is conveyed from the field surface to subsurface drains via preferential transport in macropores in the subsurface domain. The study site, located in southern Finland, is a clayey, subsurface drained field section with an area of 12 ha and an average slope of 2.8%. The growing seasons and the following autumns of two years with distinctly different rainstorm characteristics were modelled. The simulated sediment loads via tillage layer runoff and drainflow were 85 and 117 kg ha−1, respectively in 1988, and 63 and 189 kg ha−1 in 1984. Despite the high precipitation in October 1984 (143 mm), erosion in the field area was low due to minimal surface runoff and consequently minimal hydraulic erosion. The suspended sediment at the site was generated by raindrop impacts and the eroded material was transported to the open ditch through tillage layer flow and subsurface drainflow. The model was able to reproduce the measured sediment concentrations in the main open ditch into which both tillage layer runoff and drainflow are discharged from the field. Spatially distributed erosion simulations facilitate the detection of net erosion and deposition locations in the field and could be used to design intensive measurement campaigns and guide erosion control practices in the future.