Integration time step must be chosen to avoid numerical instabilities in the simulation. With Euler integration one must normally choose the simulation time step equal to the shortest step necessary for the most variable condition. This may result in inconceivably long execution times, if long term simulations are made, even for a moderate compartmentalisation of the soil. Conditional changes of the time step are made during the simulation to avoid such execution times. A base time step is given initially for the simulation, but during conditions of high infiltration rates the time step is substantially decreased. Water flow rates into the top soil layer and into a layer slightly below top soil are used as tests. The occurrence of frost in the soil also decreases the time step.
In addition to conditional changes in integration time step, conditional bypasses are made to cut down execution times. If the changes in some state variable have been below a prescribed limit no flow recalculation is made. This procedure is used for water and heat flow equations separately. Since frost conditions strongly influence both water and heat flows, recalculation of both are made if any change exceeds the limit for either water or heat. Recalculation is made of flows for a number of the upper soil layers. At regular intervals the whole soil profile is updated.
Three classes of input data may be distinguished. Driving variables are the climatic data which govern the model. Initial values are required to define a starting point at a specific time and physical parameters are constants needed to express relevant properties for the different processes in the model. However, some of these properties may be varied with time and this could either be done by using time dependent functions for some of the parameters or by selecting a new value of a certain parameter to be valid at a specified date (CHange parameter option)