Steady-state migration of barchan dunes: Field evidence from the dynamic equilibrium of sediment flux

Barchan dunes migrate downstream while largely preserving their morphology, a process thought to result from a dynamic balance between sediment influx and outflux. However, direct field evidence and the governing mechanism of this balance remain limited. Here we combine field observations and numerical simulations to examine an isolated barchan dune under a bidirectional wind regime. We show that steady-state migration is sustained when sediment entering the windward slope is balanced by losses from the horns and leeward slope. We introduce a characteristic length, L 0.5 flux , defined as the location where cumulative influx reaches half of the total, which captures internal flux redistribution and serves as a diagnostic indicator of migration. Under unidirectional winds, L 0.5 flux remains stable, reflecting equilibrium, while under bidirectional winds it shifts upwind, reducing migration rates. These results establish sediment flux balance as the mechanism underlying steady-state migration and provide a unified framework for barchan morphodynamics.