I am having mass balance issues in my model when I use the Richards Equation based solution that are not present when I use the gravity drainage solution.

In my model, there are some areas that are intermittently ponded.  These ponded areas are underlain by a fixed position groundwater table about 10m below ground surface.

Example of a ponded interval in first image.

I show below results first from a model run that starts as a precipitation event generates runoff and fills the depression.  I then extract the detailed uz water balance from a cell within the ponded area.

t looks to me like an issue with the UZ-SZ coupling, which is affecting recharge.

Here's what I think might be going on (2nd image, plot):

This is for a simulation that uses REq, and starts up at the start of a storm event, so it ponds immediately.

For the first couple of days, increases in UZ storage are keeping up with infiltration, so wb error is fairly low.

As the soil wets up beneath the ponded area, and the moisture  starts to reach the SZ, increases in storage slow down.  What should also be happening here is that the sz recharge should start to make up the difference.  I'm not sure yet why it's not, but this IS what happens when I use the gravity drainage option.

Finally you see and inflection point where uz changes in storage really slow down.  I think this is where the vertical seepage front is nearly saturated and intercepting the SZ.  At this point the imbalance between infiltration and UZ dS is more sever, causing error to rise.  Again, I think that recharge should rise here.

In contrast, the second plot is from the same simulation that uses the gravity drainage option.

Notice that sz recharge keeps up with infiltration, and wb error stays fairly low and constant.

Is the fixed water table position causing a mass balance error when I used the REq based solution??