Abstract
Forest wildfires substantially alter soil surface conditions, producing ash-rich layers that can be rapidly mobilized by post-fire runoff, impacting downstream water quality. However, rill erosion processes on ash-dominated post-fire hillslopes remain poorly characterized, particularly regarding the mobilization and transport of ash-rich sediments under varying flow conditions. To investigate these dynamics, we conducted controlled concentrated-flow experiments on burned hillslopes affected by the 2021 Lick Creek Fire and the 2022 Moose Fire. Eight rills per site were subjected to 20–50-min inflow experiments across a range of discharge rates (7.5 and 15.3 L min−1 at Lick Creek; 3, 7, 15, 22, and 30 L min−1 at Moose). Surfaces were dominated by ash cover (65–68%) with no exposed mineral soil, representing early post-fire conditions. Increasing inflow rapidly established quasi-steady hydraulic conditions; however, export of fire-derived sediment remained strongly time-dependent and supply-limited. Fire-derived sediment flux declined from 2.1 to 4.6 × 10−3 kg s−1 during the initial phase to 0.5–0.7 × 10−3 kg s−1 under quasi-steady conditions (reductions of ∼76–85%), whereas runoff increased only from 9.5 to 12.7 to 10.2–12.4 L min−1 (∼2–7%), demonstrating a decoupling between hydraulic forcing and sediment transport. Incremental export of ash-rich sediment decreased exponentially with cumulative runoff across all inflow treatments (R2 = 0.51–0.97), indicating that increasing hydraulic forcing primarily accelerated depletion of a finite, highly mobile surface source rather than sustaining sediment transport. Laboratory fingerprinting revealed persistent selectivity, with exported sediments enriched in organic matter, dominated by fine particles (<75 μm), and characterized by geochemical signatures consistent with ash source material, including elevated concentrations of base cations, nutrients, and selected trace elements. These results indicate that post-fire rill systems rapidly stabilize hydraulically, while export of fire-derived sediment becomes progressively supply-limited through depletion of highly mobile ash-rich surface material, generating short-lived pulses of fine, geochemically enriched sediment with implications for downstream water quality and post-fire erosion modeling.