Lee M. Gordon

Simulating ephemeral gully erosion in AnnAGNPS

Ephemeral gully erosion can cause severe soil degradation and contribute significantly to total soil losses in agricultural areas. Physically based prediction technology is necessary to assess the magnitude of these phenomena so that appropriate conservation measures can be implemented, but such technology currently does not exist. To address this issue, a conceptual and numerical framework is presented in which ephemeral gully development, growth, and associated soil losses are simulated within the Annualized Agricultural Non-Point Source (AnnAGNPS) model. This approach incorporates analytic formulations for plunge pool erosion and headcut retreat within single or multiple storm events in unsteady, spatially varied flow at the sub-cell scale, and addresses five soil particle-size classes to predict gully evolution, transport-capacity and transport-limited flows, gully widening, and gully reactivation. Single-event and continuous simulations demonstrate the models utility for predicting both the initial development of an ephemeral gully and its evolution over multiple runoff events. The model is shown to recreate reasonably well the dimensions of observed ephemeral gullies in Mississippi. The inclusion of ephemeral gully erosion within AnnAGNPS will greatly enhance the models predictive capabilities and further assist practitioners in the management of agricultural watersheds.

Modeling long-term soil losses on agricultural fields due to ephemeral gully erosion

It is now recognized worldwide that soil erosion on agricultural fields due to ephemeral gullies may be greater than those losses attributed to sheet and rill erosion processes. Yet it is not known whether the common practice of repairing or obliterating these gullies during annual tillage activities exacerbates or mitigates soil losses over long time periods. Here, a numerical model is used to demonstrate the potential effects of annual tillage on cumulative soil losses from four geographic regions plagued by ephemeral gullies as compared to no-till conditions where the gullies are free to grow and evolve over time and space. Historical precipitation data and field measurements were compiled for specific sites in Belgium, Mississippi, Iowa, and Georgia, and the model simulated ephemeral gully development and evolution during the growing seasons over a continuous 10-year period. When agricultural fields are not tilled annually, the simulations suggest that gullies attain their maximum dimensions during the first few years in response to several relatively large runoff events. During subsequent runoff events, the gullies no longer erode their channels significantly, and soil losses due to gully erosion decrease markedly. When agricultural fields are tilled annually, the ephemeral gully channels are reactivated, thus causing significant soil losses each year in response to runoff events. Over the 10-year simulation, the modeling results suggest that erosion rates in these four geographic regions can be 250% to 450% greater when gullies are tilled and reactivated annually as opposed to the no-till condition. These results reveal that routine filling of ephemeral gully channels during tillage practices may result in markedly higher rates of soil loss as compared to allowing these gullies to persist on the landscape, demonstrating a further advantage of adopting no-till management practices.

Emergence, persistence, and organization of rill networks on a soil-mantled experimental landscape

Soil erosion remains a critical concern worldwide, and predicting the occurrence, location, and evolution of rills on hillslopes and agricultural landscapes remains a fundamental challenge in resource management. To address these questions, a relatively large soil-mantled experimental landscape was subjected to continuous rainfall and episodes of base-level lowering to force the development of a rill network system, and high-resolution digital technologies were used to quantify its evolution over time and space. These results show that waves of degradation and landscape incision occurred in response to base-level lowering, where headcut development and its upstream migration produced a fourth-order rill network. Stream order indices derived for this incised rill network confirm that this pattern emerges relatively early in time, and it remains relatively unchanged despite continued application of rainfall and additional base-level lowering. Using the same digital technologies, a surface drainage system was defined and mapped on the landscape prior to any soil erosion and rill development, and similar network indices also were derived. These results show that network characteristics and organization of this surface drainage system, as well as its location in space, were in very close agreement with the subsequent incised rill network following base-level lowering. It is demonstrated here that rill networks formed in this experiment are strongly conditioned by surface drainage patterns prior to any significant soil erosion and that the location of rill networks can be accurately delineated through analysis of the high-resolution digital terrain.