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Application of an Approximate Analytic Method of Computing Solute Profiles with Dispersion in Soils¹

ABSTRACT

The calculation of mean solute penetration (or solute peak position), e.g., using the method of Rose et al. (1982), provides a Lagrangian (or moving) coordinate system about which solute dispersion takes place. An approximate analytic equation, useful in calculating the convection and dispersion of applied N fertilizer was then developed as follows: the solution of the convection/dispersion equation for a step change in concentration under constant flux boundary conditions was assumed to apply approximately under non-steady conditions, using an equivalent constant flux; then an equation describing convection/dispersion of an initially rectangular concentration pulse (which approximates the initial concentration profile following fertilizer application) was constructed by superposition.

This model gave a useful interpretation of the field monolith lysimeter data of Chichester and Smith (1978), who used ¹⁵N-labelled N fertilizer to study the N balance. Such discrepancies as remained require separate investigation, but are likely to be due to assumptions on the theory not being met in the permanently fissured bedrock lower in the profile.

Key Words: N-labelled fertilizer • NO₃ leaching • NO₃ dispersion • monolith lysimeters • deterministic model • Lagrangian coordinates • preferential solute movement

¹ Contribution of the USDA, Agricultural Research Service, and the School of Australian Environmental Studies, Griffith University, Brisbane, Australia, 4111, in cooperation with the Texas Agric. Exp. Stn., Texas A&M University, College Station, TX 77840.

² Visiting Scientist (from Sch. of Australian Environ. Studies, Griffith Univ.), Soil Scientist, Hydraulic Engineer, and Soil Scientist, respectively, USDA-ARS, Grassland, Soil, and Water Res. Lab., Temple, TX 76503.

Received for publication June 12, 1980.