Numerical Simulation of Flow Dynamics during Macropore–Subsurface Drain Interactions Using HYDRUS

Abstract

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.

Evaluation of Macropore Flow and Transport Using Three‐Dimensional Simulation of Tension Infiltration Experiments

“…Data on infiltration volumes and dye depth were necessary for the model to capture the macropore‐dominated flow behavior observed in the field. Water or solute flux data in the subsurface would also have been beneficial for macropore parameterization, as has been demonstrated in previous studies of preferential flow to tile drains or drainage lysimeters (e.g., Akay et al, 2008; Gerke and Köhne, 2004; Kung et al, 2005). On the other hand, SWC data had the least value for constraining model input parameters in the current study.…”

“…The van Genuchten–Mualem (VG) model (van Genuchten, 1980) was used to describe the constitutive relations between capillary pressure, water content, and hydraulic conductivity for both matrix and macropore domains following previous work (e.g., Akay et al, 2008; Allaire et al, 2002; Rosenbom et al, 2009). The water retention relation for the VG model is given by where S e ( h ) = [θ( h ) − θ r ]/(θ s − θ r ) is the effective saturation, h is the pressure head, α, n , and m are empirical fitting parameters, θ is the volumetric water content, θ s is the saturated water content, and θ r is the residual water content.…”

Evaluation of water movement and water losses in a direct-seeded-rice field experiment using Hydrus-1D

“…The correspondence between measured and simulated percolation fluxes during both early and entire growing periods is very good. The difference between simulated and observed values may be caused by macro-pores, earthworm burrows, roots, soil cracks, and lateral leakage (Akay et al, 2008;Janssen and Lennartz, 2009;Sander and Gerke, 2009;Wang et al, 2011).…”

Analysis of the Role of Tortuosity and Infiltration Constants in the Beerkan Method

“…[1] and [2]) adopted in the analytical BEST algorithm are not able to capture the presence of macropores in the investigated porous medium (e.g., Kodesová et al, 2006;Akay et al, 2008). Bimodal forms of the soil hydraulic properties would certainly be more suitable to describe possible effects of macroporosity (e.g., Durner, 1994).…”