water table does not replicates the topography... and it should!

Is Feflow capable of doing a water table that replicates de topography in a medium with large variations of slope (mountains like the Andes or the Alpes, for exemple), and where the aquifer is composed consolidated rocks?

I have try with saturated models (and all of it´s variations), unsaturated models (and all of it´s unsaturated parameters from Van Genuchten), and nothing!

Any ideas ?

Start with a conceptual model to understand why, in some circumstances, the water table follows the shape of the ground surface. Then based on your conceptual model, identify the features of a numerical model that would capture the key processes identified.

Suggestion: focus on how recharge and discharge influence water table geometry and how this relates to topography.

If you wish that water table undulations replicate exactly the topography then apply a confined model in the Problem Settings and use a Dirichlet BC with a pressure value of 0 kPa assigned to the entire top slice. In the Data Panel you have to add a Hydraulic-head BC (Pressure): Right click on Fluid flow beneath Boundary Conditions (BC) and then on Add Parameter.

Hi Bjorn

I don´t want to replicate exactly the topography, I would like the model to come up with a solution that would do an aproximation of the topography.

When I run my steady state models (not forcing them with fixed Dirichlet BCs) the water table is always a flat line. Even though the model starts completely saturated and the recharge is very important, it ends up with a flat line.

Is there a way to have ondulations of the water table without forcing the model with fixed BCs?

If you do not wish to ad-hoc postulate that water table undulations replicate exactly the topography you may assign a Seepage Face BC on the entire Slice 1. Run a couple of simulations by solving the Richards Equation and vary a homogeneous recharge systematically. All other settings should not change within the group of simulations. You will see, the lower the recharge the less water table undulations replicate the topography. In contrast, the higher the recharge the closer the approximation of the topography and the stronger your results resemble the results by early pioneering studies (Toth 1962, 1963).

I just tryed that and that water table is still flat. When i increase the recharge too much it makes a second water table on the top of the model (flat as well).

Maybe is related to the Van Genuchten parameters? Im using Van Genuchten Modified:

alpha = 0.01,
n = 1.964
m = 0.509
sigma = 3.4

all the K = 1 m/d

all other values are set by default.

or maybe the software is uncapable of representing a water table similar to the topography in a strady state model where the topographic gradients are too high?

Hi,

Just few comments based on the previous discussion:

1) Are you running a Richards-based model with a steady-state solution? If so, I would highly recommend to use the transient mode. It is very hard to solve the non-linearity of the Richards' Equation under a steady-state solution.

2) It seems to me that model is solving fine. The fact that you see a "second" water table at the top, it reflects the typical wetting front propagation in an unsaturated medium. Ideally over the time, if recharge decreases, you may see the movement of this "second" phreatic level down to the regional water level. If the recharge is very large, eventually you will see that model becomes completely saturated after several iterations, therefore it will reach a topography-like phreatic surface.

3) Material properties: Are you sure that such van Genuchten values correspond to the assigned conductivity? As indicated in item 2, it seems to me that water is moving very slow. A higher conductivity necessarily does mean fast water movement, if van Genuchten properties are not the adequate. So, please make double-check.

4) You may try to decrease the non-linearity by means of using the Modified Van Genuchten relationship and assuming a linear form (i.e. what you named before as "sigma").

If the water table always comes out as a flat line, then it usually means that the balance between recharge and hydraulic conductivity is wrong.

Increase your recharge. Or decrease your hydraulic conductivity. Or both. Now water will build up under hills. Make sure it can discharge at seepage/spring/river locations in valleys.