The effect of tillage-induced roughness on runoff and erosion patterns

In agricultural areas, tillage-induced roughness may have large impacts on runoff patterns and therefore on the effective slope, local contributing areas and erosion patterns. A model was developed to create a runoff pattern with flow in the direction of the plough-lines for all tilled fields within a catchment. The model needs a digital elevation model (DEM), a landuse map and the major tillage orientation per tilled field as input. Optionally, the flow direction of roads and/or channels can be superimposed on the drainage network. The model calculates flow directions along parcel borders and creates headlands along borders that are not parallel to the tillage orientation. The model also calculates the slope gradient in flow direction. The created tillage-controlled runoff pattern can be combined with the topographic runoff pattern, if decision rules are available to choose between topographic or tillage direction. The tillage runoff pattern can be very different from the topographic pattern, leading to totally different patterns of slope and contributing area. The use of the tillage-controlled runoff pattern rather than the topographically controlled runoff pattern in a deterministic, event-based model (LISEM), results in a much better agreement of the predicted runoff and erosion pattern with field observations. However, accurate model predictions can only be obtained if the input DEM is sufficiently accurate to represent local topographic variations in the fields.