Soil Crack Width Controls Preferential Flow Velocity Through Drag Partitioning

Abstract Preferential flow within soil cracks influences land surface hydrological processes, yet direct quantifying preferential flow velocity (PFV) remains challenging. Here, we develop a method for high‐resolution monitoring and quantifying PFV in soil cracks using optical frequency domain reflectometry (OFDR). Building on the flow characteristics of preferential flow, we further establish a quantitative framework that links crack width to PFV through a physically based drag‐partitioning mechanism. Laboratory experiments and field slope monitoring enable continuous and accurate measurement of PFV and reveal a coupled evolution between crack width and PFV during flow flushing. This coupled behavior is interpreted from a dynamical perspective based on redistribution of viscous and form drag. By mechanistically relating PFV to readily measurable crack width, the proposed framework provides a parameterization pathway for improving hydrological models. Incorporating crack‐scale PFV enhances the representation of infiltration and other surface hydrological processes.