Rill and gully development during an extreme winter runoff event in Norway

Erosion on agricultural fields in Norway has earlier been reported to be mainly sheet and minor rill erosion. Exceptional rainfall events during January and February (230% of 30 years monthly mean) in 1990 led to extreme runoff and widespread erosion also on areas normally not considered to have a high erosion risk. This study presents the results of a field survey in three counties where rills and ephemeral gullies were measured after this event. Shifting weather conditions with freezing and thawing had given frozen subsoil. Topsoil conditions varied from ice- and snow-covered surface to thawed surface with frozen subsoil. In one of the areas with a high clay content, only the upper topsoil layer was unfrozen during this event. Extended sheet erosion and smaller rills occurred. In the southernmost locations, more of the topsoil profile was unfrozen. Combined with a high silt/sand content, more severe rilling and ephemeral gullies developed. Gullies developed down to the depth of the drainpipes in all locations. This erosion equals soil losses of more than 100 tons ha−1 or 8–9 mm soil lost from the entire field area. Different mechanisms for gully development were registered. Gullies developed as enlarged rills with headward migration and sidewall sloughing. Knickpoints in the slope could also lead to gully development. Concentrated water flow entering the field from farmyards, roads, neighbouring fields and woods could be the starting point for uncontrolled rilling and ephemeral gullying. The combination of frozen subsoil, saturated soil with low strength and intense rainfall led to gully development also on areas with gentle slopes, especially sandy soils. Management practices like crop cover, tillage and lack of surface water control highly influenced the development of gullies.