Integrating Streambank Erosion with Overland and Ephemeral Gully Models Improves Stream Sediment Yield Simulation

Abstract

HighlightsSimulation of overland (AnnAGNPS) and ephemeral gully (REGEM) erosion underestimated watershed sediment yield.Three methods were used to disaggregate contribution of streambank sediment loads to daily streamflow events.A rainfall-based method, with scaling factor and threshold term, had good performance and low bias in all watersheds.Calibrated model results indicated that most sediment yield was from streambank sources (78% to 93%).Abstract. Models used to guide watershed management must account for sediment from all erosion sources (interrill and rill, ephemeral gully (EG), and streambank), each of which operates at different spatial and temporal scales. Our hypothesis was that the use of separate models to explicitly simulate sediment yield contributions for each of these three sources would improve model agreement with measured watershed sediment yield data. We tested this hypothesis using AnnAGNPS (overland flow/erosion model), REGEM (EG erosion model), and field-measured streambank erosion disaggregated to an event basis using three methods in three watersheds (North Fork, Main Stem, and Irish Creek, Kansas). AnnAGNPS alone and in combination with REGEM underestimated sediment yields and had a poorly calibrated performance in all three watersheds, which reinforced the need to include sediment loads from stream sources. The three streambank sediment disaggregation methods each included two calibration terms: a scaling factor and a threshold term, which disaggregated 2-year-total streambank erosion measurements into event-based values based on rainfall, streamflow, or stream power. All three methods gave “very good” sediment-yield calibration results (event-based Ef = 0.6, PBIAS near 0%) for Main Stem and Irish Creek but gave “satisfactory” results only for the rainfall-based streambank disaggregation method in the evaluation watershed (North Fork). Calibrated model results indicated that most of the measured outlet sediment yields in the study watersheds were from streambank sources (78% to 93%), with less from overland erosion (7% to 22%) and little from EG erosion (0% to 7%). These methods could be equally effective in scaling and disaggregating stream-based sediment contributions without measured streambank erosion, but the calibration terms would lose physical meaning in reference to measured streambank values. The skill demonstrated by all three streambank disaggregation thresholds may provide a foundation for building simple, process-based streambank sediment yield simulation models. Keywords: Ephemeral gully erosion, Interrill and rill erosion, Modeling, Streambank erosion, Watershed.