Modelling soil erosion in a clayey, subsurface-drained agricultural field with a three-dimensional FLUSH model

Soil erosion is an important environmental issue in agricultural areas of northern Europe where clayey soils are prevalent. Clayey soils are routinely subsurface drained to accelerate drainage which creates an additional discharge route for suspended sediment. Previously, assessment of the sediment load from clayey fields has been difficult, because process-based models were only able to simulate sediment loads via surface runoff. A new distributed, process-based erosion model was developed and incorporated into the FLUSH modelling system to fulfil this void. The model facilitates simulation of spatially distributed soil erosion on the field surface and sediment loads via surface runoff and subsurface drainflow. Soil erosion on the field surface is simulated with the two-dimensional sediment continuity equation coupled with hydraulic and rain drop splash erosion, sediment settling, and transport capacity processes. Subsurface sediment transport in macropores is described with the three-dimensional advection–dispersion equation. The model was applied to a clayey, subdrained field section (∼3.6 ha) in southern Finland. The results demonstrated the capability of the model to simulate soil erosion and sediment transport in terms of the match between the measured (2669 kg ha−1) and modelled (2196 kg ha−1) sediment loads via surface runoff and the measured (2937 kg ha−1) and modelled (2245 kg ha−1) loads via drainflow during the validation period of 7 months. The model sensitivity analysis pointed out the importance of the flow model parameters in simulation of soil erosion through their control on the division of total runoff into surface runoff and drainflow components. The key parameters in the erosion model were those that affected hydraulic and splash erosion rates. The model application in the experimental field suggested that both hydraulic and splash erosion were the factors behind the sediment losses during the growing season and early autumn, whereas high sediment loads in late autumn were caused by hydraulic erosion due to overland flow in tilled soil.