Solute Transport Parameters

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Soil and Solute Specific Transport Parameters are specified in the Solute Transport Parameters dialog window .




The following Soil Specific Parameters (left part of the dialog window) are specified for each soil material:


Bulk.d.

Bulk density, ρ [ML-3]

Disp.L.

Longitudinal dispersivity, DL  [L]

Disp.T.

Transverse dispersivity, DT  [L]

Frac

Dimensionless fraction of adsorption sites classified as type-1 sites, i.e., sites with instantaneous sorption when the chemical nonequilibrium option is considered [-]. Set this parameter equal to 1 when equilibrium transport is considered. Frac becomes the dimensionless fraction of adsorption sites in contact with mobile water when the physical nonequilibrium option is considered [-]. In that case Frac should be set equal to 1 when all sorption sites are in contact with mobile water.

ThImob

The immobile water content. Set equal to 0 when the physical nonequilibrium option is not considered.


The following Solute Specific Parameters (right part of the dialog window) are specified for each solute:


Diffus.W        Molecular diffusion coefficient in free water, Dw [L2T-1]

Diffus.G        Molecular diffusion coefficient in soil air, Da [L2T-1]


Nonequilibrium solute transport:

HYDRUS codes can simulate either nonequilibrium chemical or nonequilibrium physical solute transport. It cannot handle both physical and chemical nonequilibrium transports simultaneously. Chemical nonequilibrium transport is implemented using either a one-site or two-site sorption model, with one of the sorption sites having kinetic and other instantaneous sorption. Physical nonequilibrium model uses mobile-immobile water concept. HYDRUS decides which of the two nonequilibrium models to use based on the input solute transport parameters. When the immobile water content (ThImob) is specified with a nonzero value, then HYDRUS uses a physical nonequilibrium model and the “Frac” parameter is interpreted as a dimensional fraction (between 0 and 1) of the sorption sites in contact with mobile water. When the immobile water content (ThImob) is specified with a zero value and the “Frac” parameter is not equal to 1, then HYDRUS uses a chemical nonequilibrium model. Then the “Frac” parameter is interpreted as a dimensional fraction (between 0 and 1) of the sorption sites classified as type-1, i.e. sites with instantaneous sorption. The rest of the sorption sites (1-Frac) is classified as type-2, i.e. sites with kinetic sorption. For both models, the “Alpha” parameter (in the next dialog window - Solute Reaction Parameters) represents the firs-order mass transfer coefficient; either for nonequilibrium adsorption when chemical nonequilibrium is considered or for exchange between mobile and immobile regions when physical nonequilibrium is considered.