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# Simple Permafrost model

Starting with **WaSiM 8.5.0** permafrost can be considered explicitely in WaSiM.

Features:

- new standard grid which defines permafrost soils by assigning a valid so called thaw coefficient
*α*(greek alpha), which is a linear scaling coefficient for the thaw depth (may be soil type dependend → thus definable per cell) - three variable grids should be provided (in order to write and analyse grids later):
- one grid that holds the number of consecutive snow free days (
*n*)_{sf} - one grid that holds the number of consecutive snov coevered days
- one grid that holds the thaw depth

- one grid that holds the number of consecutive snow free days (
- Thaw depth is calculated using an empiric approach with α (alpha) as a parameter:
*T = α √ n*_{sf} - If a closed snow cover appears, the thaw depth is reset to zero. Some parameters have to be set in the control file to make the model rubust against short time snow covers:
- number of days with snow cover after which the soil is assumed to be froozen again
- maximum snow water equivalent for the interval to be counted as snow covered (then, the snow-cover-days grid will be incremented by the length of an interval)

- the value of thawdepth is used in the soil model to manipulate the hydraulic conductivity, which is set to 10
^{-12}m/s for frozen layers and to the value defined in the soil table (and controlled by the van-Genuchten parameters) for unfrozen layers.

Note: This module is a simple module based on empiric parameters. It may not give the expected results and needs a calibration. If freezing and thawing of the soil should be modelled more exact and physically based, the use of the physically based heat transfer model is strongly recommended

Details on calculating the thaw depth of permafrost soils