MIKE KIRKBY¹ & LOUISE BRACKEN²

Abstract In semiarid areas, little locally generated overland flow and even less locally eroded sediment reaches channels during floods, due to poor connectivity in both natural and disturbed landscapes. Where anthropogenic disturbance or climate change alter runoff and erosion rates in source areas, there are short and long term adjustments to both hillslope and channel morphology. Abandonment of arable cultivation is used here as an example of such a change, explored though a one-dimensional simulation model which explicitly represents spatial differences in runoff generation and the accumulation of runoff downslope across the frequency distribution of rainstorms. Discharge and topography are used to forecast sediment transport and the consequent evolution of hillslope form. The model allows for dynamic interaction with slope-base streams, and computes the stability with respect to the formation of ephemeral gullies on the hillside. This model is run until the slope form is in balance with the processes, and a step-change is then applied to represent the short term impact of land abandonment. An initial sharp increase in sediment loss is followed by a slow recovery as the hillslope begins to evolve towards a new stable form. The effect of possible remedial measures is explored showing that the most effective treatments are those which increase infiltration, and that direct reductions in sediment transport generally lead to erosion downslope.

Key words connectivity; erosion mitigation; modelling; semiarid; soil erosion