Model results differ between constant and transient recharge (same values).

Hi,

When I run my transient model with constant recharge, and then again with the exact equivalent transient recharge using a .dat file (i.e. same value as the constant recharge for the whole model duration), the model output is slightly different when theoretically speaking I expected them to be identical. The transient version converges to a head distribution that is approximately 0.5 m lower than the constant recharge version.

What might be the problem with this?

I entered the transient recharge by creating a .dat in the format:
ID 0 100000
1 5 5

To yield a recharge of 5x10-4m/d for 100000 days. Then I used the 'Join' option to apply this recharge to a particular area and linked the above database with time levels of 0 and 100000. Checking the T-List shows that the recharge appears to be correct.

However, for some reason the model yields lower heads using this method.

Could this be an issue with the way I have entered the recharge? Or a mass balance issue? Or even something in the way Feflow interprets the transient recharge?

Thanks.

I have some further information. After exporting the budget analyzer data it seems that the constant value of transient recharge results in a smaller flux into the model (~ 2% less) than just assigning a constant non-time dependent recharge, this is despite entering identical values for both for identical recharge areas.

Any ideas why this may be the case? It's making long term scenario testing tricky since the error in recharge is more noticeable the longer the model runs.

Thanks.

I have figured out that assigning transient recharge to two non-contiguous polygons seems to result in the second polygon not actually being assigned the recharge values. For whatever reason, probably the way in which I am trying to enter the data, I can work around this by creating two seperate polygons and applying the recharge seperately.

I still have a slight problem with long term heads in a transient model. The model has identical total annual recharge as the steady state mode, except it is applied in a seasonal pattern i.e. high winter rainfall and low summer. For some reason, the model drops below the st.st heads by about 0.25m during the first few years, then as the model continues (20+ years) the heads tend towards values ~0.25m above the st.st. values. I cannot think of an explanation for this!