Matured gullies act primarily as sediment pathways rather than sources in catchment-scale sediment budgets

Abstract

Gullies supply and transport sediment, yet their contribution to catchment-scale sediment budgets varies and remains difficult to quantify, particularly in data-limited catchments. We examined the role of gullies in the sediment budget of a sub-humid catchment in the Ethiopian Omo-Gibe using the SIMWE (SIMulated Water Erosion) model, morphometric analysis of field-inventoried gullies, and flow- and sediment-monitoring. A total of 145 gullies were inventoried to analyse their morphology and spatial distribution. Flow and sediment dynamics were simulated using the two-dimensional, process-based SIMWE model. The model was calibrated against seasonal, event-based flow and sediment data. The results indicate that rainfall variability, vegetation cover, and agricultural practices affect both the SIMWE model and the sediment rating curve parameters. Additionally, incorporating seasonally differentiated model parameters improved decadal sediment simulations (2015–2025), highlighting their role in capturing intra-annual catchment-scale erosion processes. However, simulated decadal sediment yield differed by 37% from reservoir inlet bathymetric estimates, reflecting uncertainties arising from temporally constant model infiltration assumptions and limitations in the validation data. Furthermore, the morphometric analysis shows that most gullies in the catchment exhibit V-shaped cross-sections and contribute only 1–1.4% to total sediment yield, suggesting that the gullies primarily act as sediment pathways rather than dominant sources during this period. These findings offer new insights into sediment connectivity and emphasise the need to account for temporal variability when parameterising models to improve sediment budget assessments and inform targeted conservation measures in gully-prone catchments. Future research using coupled hydro-geomorphological models may incorporate spatio-temporal variability in model parameters and infiltration rates.