Quoted from: https://www.cbrfc.noaa.gov/present/rdhm/RDHM_User_Manual.pdf

When the frozen ground model is run, the nodes (depths) where temperature computations are made are prescribed by the program in a way such that the numerical solution of the heat transfer equations is accurate. The heat transfer computational depths near the surface will tend to be closer together because the temperature gradients are higher near the surface. For conversion from SAC-SMA states to physical layers or from physical layers to SAC-SMA states the program adjusts the computational layer depths so that a selected set of layers are associated with the SAC-SMA upper zone (could be one or more) and a selected set of layers are associated with the SAC-SMA lower zone. If there is no frozen water, then all layers that correspond to a given SAC zone (upper or lower) will be assigned the same soil moisture value after SAC water balance calculations are made. If there is frozen water, then the allocation of water to layers in the each zone will vary depending on the amount of frozen water in that layer.