Numerical simulation of flow dynamics during macropore-subsurface drain interactions using HYDRUS

Macropores, such as those created by deep-burrowing earthworms, have the potential to be hydraulically connected not only to the soil surface but also to subsurface drains. This hydraulic connection may lead to rapid movement of surface-applied chemicals to receiving waters as they bypass the bulk of the soil matrix. In this study, a numerical model (HYDRUS) that solves the three-dimensional Richards equation for both matrix and macropore domains was used to analyze previously conducted experiments that contained a single, surface-connected or buried, artificial macropore and a subsurface drain installed in a laboratory soil column. Both matrix and macropore domains were parameterized using continuous soil hydraulic functions. Simulations confirmed that surface-connected macropores were highly efficient preferential flow paths that substantially reduced arrival times to the subsurface drainage outlet, with this reduction being directly related to the length of the macropore. Surface-connected macropores need to extend at least halfway to the drain to have a noticeable effect (> 50% reduction) on the arrival time. No significant changes were observed in total drain outflows for columns with laterally shifted macropores (away from a drain) compared with centered macropores unless the macropore depth extended significantly (> 75%) into the profile. The model predicted that buried macropores became active and contributed to the total outflow only when pressure heads in the soil profile became positive. The effect of buried macropores on drain flow was investigated for a case where an initially partially saturated profile was drained. Under these conditions, the numerical simulations suggested that buried macropores could contribute up to 40% of the total outflow, which confirms laboratory observations with subsurface-drained soil columns with macropores.