Budget Analysis in unsaturated model

Hi,

I have a 2D vertical unsaturated flow slice model in which I am using 3rd type transfer boundary conditions to simulate transpiration from both the unsaturated and saturated zones. The model appears to be running fine, however when I run a budget analysis on the transfer nodes I get significantly different flux rates and water volumes (as high as two orders of magnitude) exiting the system for identical simulation runs. Does anyone have any thoughts on this? Could it be due to budget calculations for the unsaturated zone?

Any thoughts would be greatly appreciated.

Thanks

In dry soil, very small changes in saturation correspond to huge changes in (pressure) head. If the model is not perfectly stable, small changes induced for example by different orders of summation when using a multi-processor environment might lead to larger head differences, and thus larger differences in the budget. However, this is just an idea - it is hard to find 'the' solution without seeing the actual model.