Preferential flow and transport modeling in agricultural soils
Abstract
A deterministic model of solute transport in soils with preferential flow pathways is developed, analyzed, solved and validated. Model development is done using formal averaging on a succession of scales and is limited to the transport of non-sorbing solutes under steady-state flow conditions. The resulting pathway level description is mathematically formulated as a two-dimensional parabolic Partial Differential Equation (PDE). One-dimensional mono-continuum approximations are derived in the form of a system of equations and a single nonlocal transport equation. The parameters of both formulations can be obtained independently of solute transport experiments. The model is analyzed using the method of moments. Previously unpublished expressions for the third and fourth moments are presented. These expressions are used to obtain the parameters of the monocontinuum approximations. Moments are also used to compare the predictions of the new model with those of three other models. This analysis shows that the Convection Dispersion Equation (CDE) can be used to approximate the model at large times. The applicability of three Finite Difference (FD) schemes to the solution of the 2-D model equation is assessed through considerations of consistency, accuracy, stability, wave propagation characteristics, diagonal dominance, oscillations and their ability to predict the analytical moments of the model equation. The third order method of Noye and Tan (1989) is found to produce the best solution. A similar methodology is used to demonstrate that the standard second order in time and space FD method produces accurate solutions of the monocontinuum approximation. The 2-D model predicts Breakthrough Curves (BTCs) equivalently to or better than the CDE. The new model describes preferential BTCs well and consistently whereas the CDE fails when preferential flow is significant. The resident concentration predicted by the model near the inlet of soil columns is consistent with experimental results and the model explains the phenomenon of incomplete leaching observed in natural soils. The response of the new model to flow interruption and to desaturation is also in qualitative agreement with experimental results. It is concluded that the new model is a better descriptor of solute transport phenomena than the CDE.
Degree
Ph.D.
Advisors
Haghighi, Purdue University.
Subject Area
Agricultural engineering|Soil sciences
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