An Alternative for Advection–Dispersion in Structured Soil: Predicting the Transport of Dissolved Phosphorus Using the Convective–Preferential Equation

Accurate prediction of dissolved phosphorus (P) transport to groundwater, and especially to drain tiles, is a major challenge, particularly in structured soils where preferential flow dominates subsurface hydrology. Existing vadose zone models rely on advection–dispersion formulations that insufficiently represent macropore-driven transport, leading to underestimation of dissolved P losses. This study develops a simplified convective–preferential (CP) transport equation to describe event-scale movement of dissolved P through the vadose zone to groundwater. The equation conceptualizes the soil profile as a near-surface distribution zone that acts as a linear reservoir with a transmission zone below where preferential flow is activated when imposed fluxes exceed the effective matrix conductivity. Under these conditions, P-rich event water bypasses much of the soil matrix, rapidly reaching the groundwater. The CP equation uses a small set of physical parameters, based on partitioning the flux between the matrix and preferential pathways. Model results were validated against laboratory leaching experiments, published column studies of various soil textures, and tile-drained field soils. The CP equation reproduced observed dissolved P breakthrough patterns and flow-dependent concentration in tile drains. Model results confirm that significant P transport occurs during high-intensity rainfall on saturated soils with restrictive layers on structured soils with a low-permeable hardpan.