Convergence problem in Richard's case with recharge

I'm run a richard's case.

And my condition involve recharge(some >=0； some <0).    [b](* ps: Because this area didn't have enough well data, so the negative recharge is replace well) [/b]

And my richard's parameters are following :

Property            Sr        alpha      n
Gravel              0.0349  5.79  2.2355
Sand                0.045  14.5  2.68
Clay                0.068    0.8  1.09

Actually i'm not sure the gravel parameters(right/wrong?)
(reference:https://www.researchgate.net/publication/270118948_Evaluation_of_Hydraulic_Parameters_Obtained_by_Different_Measurement_Methods_for_Heterogeneous_Gravel_Soil)

[b]My problem is :

I set two layer for this case(aquifer, aquitard), and get convergence result.

However I discrete first layer to 10 layer(parameter be the same), get a result that no convergence(very high head).[/b]

Is my richard's parameters set wrong?

In my comprehension, the higher 'alpha' will cause water drain easy, so the recharge water will easy to drain to aquitard layer which numerical simulation easy divergence part,
therefore I try to let all layer 'alpha' be 0.0008, and get convergence. But it's very unreasonable.

thanks

Convergence in the non-discretised case may actually be a numerical artifact (see attached figure).
Negative recharge values may work in gravel only if corresponding area is consistently below water table. Otherwise, water cannot simply go upwards beyond capillary fringe (that is only few cm in gravel). You may want to consider other BCs located at the bottom of the model to simulate area-averaged pumping.

Further stabilisation can be done by
- applying CVFEM (tickbox in Problem Class => Unsaturated Flow menu)
- switching to Modified Van Genuchten and reducing delta parameter value as described at
[url=http://www.feflow.info/uploads/media/Frind.pdf]www.feflow.info/uploads/media/Frind.pdf[/url]

TO: Durchlassigkeitsbeiwert

Thanks your explain.

Why the recharge in gravel only can work when area below water table?

[quote author=Jimmy Lin link=topic=22183.msg29509#msg29509 date=1620707863]
Why the recharge in gravel only can work when area below water table?
[/quote]
I was referring specifically to [b]negative[/b] recharge (i.e. water being extracted from the system)
Simply drying the surface above gravel will not force water to flow upward (neither in reality, nor in properly converging model)
In contrast, in fine sediments capillary fringe can extend metres above water table. In such cases, when drying surface is within such fringe it can source water from groundwater making negative recharge values a feasible option.

Thanks you so much.

I took your reference.

And i specially adjust the layer thickness of negative recharge zone, that can avoid the water below the zone.

And this solve the convergence problem.