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Title: LATERAL WATER DIFFUSION IN AN ARTIFICIAL MACROPOROUS SYSTEM: THEORY AND EXPERIMENTAL EVIDENCE

Author
item CASTIGLIONE, P - UC RIVERSIDE, CA
item MOHANTY, B - UC RIVERSIDE, CA
item Shouse, Peter
item SIMUNEK, J - UC RIVERSIDE, CA
item Van Genuchten, Martinus
item SANTINI, A - UNIV OF NAPLES, ITALY

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/14/2002
Publication Date: 5/15/2003
Citation: Castiglione, P., Mohanty, B.P., Shouse, P.J., Simunek, J., Van Genuchten, M.T., Santini, A. 2003. Lateral water diffusion in an artificial macroporous system: theory and experimental evidence. Vadose Zone Journal. Vol 2:212-221

Interpretive Summary: Extensive use of agrochemicals has put soil and water quality at risk. Bypass (preferential) flow through soils via large continuous pores is reported to be one of the most important but most unpredictable flow mechanisms contributing to the transport of water and chemicals from the soil surface to groundwater. To date few simulation models of water flow in soils are available to characterize preferential flow. Our experimental research using artificial macropores provided considerable insight into preferential water flow through soils. We could clearly separate macropore flow in the larger pores from matrix flow in the smaller pores during infiltration and we could measure the integrated water exchange flux between marcopore and matrix. We found that in general the mass transfer rate coefficient for water flow between the large and small pores can not be derived from the mass transfer rate coefficient for solute. Researchers and engineers developing simulation models of solute transport through cracking soils will benefit from our experimental results.

Technical Abstract: Accurate characterization of preferential flow in macroporous and fractured systems remains a challenge in vadose zone hydrology. A range of conceptual models with different levels of complexity (e.g., equivalent continuum, dual porosity, and dual permeability) have been used to describe macropore flow/transport. In this paper, using a novel soil column design with an artificial macropore at the centre, we evaluated the dual-permeability model (DPM) where water moves in two independent domains (macropore and matrix) with inter-domain mass exchange. Concurrent measurements at high temporal resolution were made of the profile soil water contents and pressure heads, and domain-specific outflow rates. The bottom of the 75-cm long sand loam soil column was partitioned into six pie-shaped chambers, while the upper boundary was controlled using a tension infiltrometer. The matrix was hydraulically characterized using several infiltration and drainage experiments. One single macropore was added to the soil column using a 1-mm rod, after which several infiltration experiments were repeated. Matrix and macropore flows could be discriminated by contrasting the effluent fluxes from different outflow chambers. Comparison of the experimental data with model predictions showed that the invoked boundary conditions are critical in defining the initiation, continuity, and magnitude of flow and between-domain mass exchange, and that first-order approximations of water exchange between the two flow domains at the macroscopic scale could reproduce the experimental data reasonably well. Experimental results could be described especially well using a newly derived expression for the mass transfer coefficient that incorporates the hydraulic conductivity of the macropore.