MACROPORE COMPONENT ASSESSMENT OF THE ROOT ZONE WATER QUALITY MODEL (RZWQM)USING NO-TILL SOIL BLOCKS

Authors: Malone, Robert - Rob; Shipitalo, Martin; Ma, Liwang; Ahuja, Lajpat; Rojas, Kenneth

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: Water and chemicals are often transported through soil via preferential flow paths sometimes called macropores (worm holes, root channels, cracks, etc.). Consideration of macropore flow is important when studying water quality because chemicals are transported to groundwater much faster and in higher concentrations than if macropores are not present. Therefore, simulation of macropore flow is important to accurately predict the environmental impact of different management practices. One of the few contaminant transport models available with macropore flow capability is the ARS Root Zone Water Quality Model (RZWQM). The performance of the macropore flow component of RZWQM is relatively untested, therefore, we investigated the performance of RZWQM compared to actual data from soil blocks with natural macropores. Results indicate that the model accurately simulates chemical movement through macropores when correctly parameterized d(8 of 9 simulated chemical concentrations were within a factor of 1.5 of observed values). This research will primarily benefit researchers studying environmental quality issues.

Technical Abstract: In structured soils macropore flow 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. Therefore, we evaluated the macropore component of the Root Zone Water Quality Model (RZWQM) using undisturbed soil blocks with natural lmacropores. To accomplish this, atrazine, alachlor, and bromide were surface-applied to nine 30 by 30 by 30 cm blocks of undisturbed, no-till silt loam soil at three water contents (dry, intermediate, wet). We then subjected the blocks to a 0.5-h, 30 mm simulated rain shortly after chemical application. Percolate was collected from individual cells (64 total cells) at the base of the blocks and, 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 whereas 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 (1.7 cm). The effective soil radius is the radius of soil surrounding macropores that interacts with macropore flow. This parameterization strategy resulted in accurate simulations of the composite chemical concentrations in percolate (i.e., 8 of 9 simulated chemical concentrations were within a factor of 1.5 of the average observed value). Observed herbicide concentration in percolate, however, decreased with cumulative percolate volume while simulated concentrations increased. Model modifications such as simulating an increase in effective macroporosity with increasing rainfall may improve simulations.