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Modeling Transient Water and Solute Transport in a Biporous Soil
Author(s) -
Zurmühl Torsten,
Durner Wolfgang
Publication year - 1996
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/95wr01678
Subject(s) - hydraulic conductivity , richards equation , saturation (graph theory) , mechanics , soil science , tracer , boundary value problem , flow (mathematics) , dispersion (optics) , thermodynamics , soil water , hydrology (agriculture) , environmental science , mathematics , geotechnical engineering , geology , physics , mathematical analysis , combinatorics , nuclear physics , optics
In this study we investigate numerical simulations of one‐dimensional water flow and solute transport in a soil with a nonuniform pore‐size distribution. Water transport was modeled by treating the soil as one domain by applying Richards equation, while using alternatively a unimodal and a bimodal model for the hydraulic properties. The retention curves were fitted to a set of measured data; the relative conductivity functions were estimated by Mualem 's (1976) model. Contrary to the unimodal case, the bimodal conductivity curve shows a steep decrease in water content θ near saturation. Simulated water regimes under transient boundary conditions differed strongly for the two cases. The use of the bimodal functions yielded a preferential flow characteristic which was not obtained using unimodal functions. For both hydraulic regimes we modeled solute transport comparing four different variants of the convection‐dispersion equation. For the classical one‐region model we found that the breakthrough curve of an ideal tracer was not affected by the dynamics of the water flow. For the two‐region approach, where the water‐filled pore domain is divided into a mobile region θ m and an immobile region θ im , three different conceptual treatments of θ m under transient conditions were investigated. For the case where θ im was kept constant, the different hydraulic regimes again caused only minor differences in solute transport. The same was true for the alternative case where the ratio θ m / θ was kept constant. However, for the third case, where θ m was treated as a dynamic variable which changes with the actual water content in a way that depends on the shape of the hydraulic conductivity function, the transport simulation based on the bimodal hydraulic model reflected enhanced preferential transport at high‐infiltration rates.