In a 'phreatic' model, FEFLOW reduces conductivity with saturation of each element. With thick layers (in you two-layer model), saturation for the large elements will be relatively large, thus conductivity reduced only moderately (but on the entire layer!!). The more you refine the model, the more likely it will be that you have dry layers, where minimum conducitivity is determined by residual water depth. Thus with thinner layers the conductivity contrasts in the model become larger, and model convergence will be harder to obtain - especially with groundwater recharge being applied to the uppermost 'dry' layer with low conductivity. However, at the same time you will also approximate the water level better, getting away from 'randomly' scaling down conductivity in the entire aquifer depending on the position of the water table within the layer.
So what you are looking for is a good balance of vertical discretization and a suitable residual water depth (as long as you use 'phreatic' mode) that allows recharge to pass through the layer(s) above the water table. This behaviour of the model shows one of the most important difficulties in groundwater modelling: Often the role of the UZ zone is not negligible, but vertical discretization does not allow for real unsaturated flow. Some ways to deal with it may be:
- Find a good balance of discretization and residual water depth
- Apply recharge in the aquifer rather than on top (only possible by using the source/sink parameter, requiring a change of the quantity by taking into account the element thickness)
- Use a simplified unsaturated model rather than 'phreatic' (but this requiring more experience on suitable parameters)
