Freezing front

At the freezing front the hydraulic conductivity changes drastically and therefore needs to be adjusted. Two different calculations are made in the model to reduce the hydraulic conductivity in the low-flow domain under partially frozen conditions. The first procedure affects the boundary conductivity whereas the second one reduces the hydralic conductivity of a partially frozen soil layer directly.

Normally an upward water flow towards a partially frozen soil layer is calculated based on a conductivity which is the linear interpolated value at the boundary between the adjacent layers. This interpolation procedure for obtaining the boundary conductivity between two layers may optionally be replaced by a procedure in which the boundary conductivity is selected as the minimum conductivity of the two layers (see switch k-estimate). This will normally substantially reduce the flow towards the layer where freezing takes place, such that the clear tendency to overestimate redistribution during freezing will be reduced (Lundin, 1990).

In addition to the alternative interpolation procedure an impedance factor is considered when the hydraulic conductivity of a partially frozen layer, k_wf, is calculated:

                                                           (1.35)

where Q is the thermal quality, c_fi is an impedance parameter and k_w is the hydraulic conductivity of the layer calculated from the unfrozen water content without accounting for occurrence of ice (see “Soil hydraulic properties”). See viewing function Low-flow domain hydraulic impedance function.