The time step must be small enough to avoid numerical problems but still large enough to minimize the total CPU-time consumption. A strong influence of compartment sizes, soil properties and boundary conditions sometimes make the choice difficult. To handle the influence of shifting boundary conditions the default time step may be changed during the simulation according to values specified to parameters in the group Numerical. To avoid truncation errors you should specify a number of iterations with a number that could be represented exactly in a binary form, e.g. make use of the series 2, 4, 8, 16, 32, 64, 128, ... Remember that the relationship between compartment size and the time step is quadratic, i.e., in case a stable solution exist with 128 iterations and 5 cm thick layers you can reduce the number of iterations to 32 if you increase the thickness to 10 cm. Also remember that you normally can increase the compartment size with increasing depths in your profile because of the slower rates of water and heat flows deeper in the soil. 1. Do not confuse time step with Time resolution (the resolution of model output).