Integrating streambank erosion with overland and ephemeral gully models improves stream sediment yield simulation

Authors: Mankin, Kyle; MODALA, N - Texas A&M University

Submitted to: Journal of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2022
Publication Date: 8/1/2022
Citation: Mankin, K.R., Modala, N.R. 2022. Integrating streambank erosion with overland and ephemeral gully models improves stream sediment yield simulation. Journal of the ASABE. 65(4):763-778. https://doi.org/10.13031/ja.14840.
DOI: https://doi.org/10.13031/ja.14840

Interpretive Summary: Many watershed models simulate only overland erosion sources from crop fields but not ephemeral gully erosion or streambank erosion sources. This is important because model results can be used to guide management and funding priorities. Our hypothesis was that better models that include all sediment sources will produce better simulation of watershed-outlet stream sediment measurements. We tested this hypothesis using AnnAGNPS (an overland erosion model), REGEM (an ephemeral gully erosion model), and several modeling approaches that split measured 2-year-total streambank erosion into individual events in three Kansas watersheds. Model results that did not include streambank erosion gave poor results. Calibrated models that included streambank sources performed very well. Results indicated that most of the measured watershed-outlet sediment came from streambank sources (78 to 93%), with less from overland erosion (7 to 22%) and little from ephemeral gully erosion (0 to 7%). The streambank modeling approach shows promise for building simple, process-based streambank sediment yield simulation models.

Technical 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 use of separate models to explicitly simulate sediment yield contributions for each of these three sources will 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 poor 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 disaggregrated 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 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.