Macropore component assessment of the root zone water quality model (RZWQM) using no-till soil blocks

In structured soils, macropores can contribute to rapid movement of water and solutes through the profile. To provide insight into these processes, model assessments should be performed under a variety of conditions. We evaluated the macropore component of the RZWQM using undisturbed soil blocks with natural macropores. To accomplish this, atrazine, alachlor, and bromide were surface-applied to nine 30 X 30 X 30 cm blocks of undisturbed, no-till silt loam soil at three water contents (dry, intermediate, and wet). One hour later, we subjected the blocks to a 0.5-h, 30-mm simulated rain. Percolate was collected and analyzed from 64 uniform size cells at the base of the blocks. After percolation ceased, the soil was sectioned and analyzed to determine chemical distribution. We tested the chemical sub-component of macropore flow using these blocks following hydrologic calibration, while a separate set of blocks was used to calibrate selected chemical parameters. Parameterization of the macropore component included measuring the effective macroporosity (50% of percolate producing macropores) and calibrating the effective soil radius (0.6 cm). The effective soil radius represents the soil surrounding the macropores that interacts with macropore flow. This parameterization strategy resulted in accurate simulations of the composite chemical concentrations in percolate (i.e., all simulated chemical concentrations were within a factor of 2.0 of the average observed value). However, observed herbicide concentration in percolate decreased with cumulative percolate volume, while simulated concentrations increased. Model modifications, such as incorporating a dynamic effective macroporosity (effective macroporosity increase with increasing rainfall) and chemical kinetics in macropores, may improve simulations.