[1] Soil erosion can severely degrade landscapes, and concentrated flows such as rills and gullies can be the dominant contributor to the soil losses. This paper examines the growth, development, and spatiotemporal evolution of rills and rill networks using a soil-mantled experimental landscape subjected to simulated rain and downstream base level lowering. Rill incision and network development and extension occurred due to actively migrating headcuts formed at the flume outlet by base level lowering. The communication of this wave of degradation due to this exogenic forcing occurred very quickly in space, and resulted in nearly the same amount of bed incision throughout the network. Rill incision, channel development, and peaks in sediment efflux occurred episodically, yet these were in direct response to the downstream base level adjustments. Although flows were supply limited, most of the sediment efflux was genetically linked to headcut development and migration. The geometry of the eroded rills and the rates of headcut migration were well correlated to overland flow rate. These findings have important implications for the prediction of soil loss, rill network development, and landscape evolution where headcut erosion can occur.

[1] Experiments were conducted to examine the effect of vertical stratification in soil erodibility on the development and migration of steady state headcut scour holes in upland concentrated flows typical of agricultural fields. Packed soil beds with a preformed step were subjected to identical simulated rainstorms and clear-water overland flow rates, which resulted in predictable, actively migrating headcut scour holes with nearly identical characteristics. When an erosion-resistant layer was incorporated into the packed soil bed at a depth that exceeded this expected plunge pool scour depth, the erosion and hydraulic processes of the migrating headcuts remained unchanged. When the erosion-resistant layer was placed so as to intersect this potential headcut scour depth, the erosivity of the reattached wall jet was unable to erode this layer, and the depth of scour, the nappe entry angle, and sediment efflux all were reduced inversely proportional to the relative depth of the resistant layer. These data were successfully predicted using modified jet impingement theory for headcut scour holes and demonstrate further the effects of soil management and tillage practices on total soil losses from agricultural fields.

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