unsaturated flow across a river basin

Hi,
I would like to use FeFlow to model subsurface and groundwater flow across a river basin having a drainage area of about 500 km2. I would like to simulate both groundwater flow in the saturated zone and specially the flow in the unsaturated zone, in order to compute the fluxes from groundwater to the surface and vice versa. Unfortunately, I couldn't find any example or study in literature at a similar scale, where Feflow is used to solve a unsaturated flow problem. Perhaps, could anyone indicate me some paper or example that could be useful to my work?
However, I started trying with a unsaturated steady flow model having the 1st slice coincident with the topographic surface and a total depth of 200 m, all layers parallel to the surface. After consulting geological studies made in that area, I set Ks = 3x10-7 m/s in the first 100 m from the top and Ks = 10-8 m/s in the lower layers, and also tried different vertical discretizations. In order to allow water to exit the model through the surface, I set a 3rd kind boundary condition on all the superficial nodes with hR equal to the elevation, so that, when a node gets saturated (hR is bigger then elevation), an outflow is expected. Inflows are instead coincident with the mean annual recharge (300 mm/yr is the value I'm using). Hence, the inflow transfer rate is equal to zero, while I tried different values for the outflow transfer rate. The problem I have is that water doesn't seem to leave the model, because all the resulting configurations have saturation equal to one in all nodes except one or two nodes in the middle of the basin where saturation is next to zero. I also tried using constraints for boundary conditions but I couldn't get any improvement but convergence got more difficult, instead. Do you think my strategy for boundary conditions is a good one or is there something wrong in my model, due to some aspects of Feflow that I probably ignore?
Afterwards I would also like to perform some transient simulations but first of all I would like to obtain a good steady-state simulation.
Thanks!
Alice

Hi Alice
I don't understand if you are trying to simulate the exchanges between the river and the groundwater reservoirs or if you are trying to simulate the exchanges between the groundwater reservoirs and the soil system.
To help me to understand you should give me informations about the water table you are modelling.
If your water table doesn't touch the surface, it's impossible that water can leave the model via 3rd kind B.C. unless the nodes on which you set the 3rd kind B.C. represent a draining stream bed.

I modelled a river basin of the same dimension: the model simulates only saturated flows, but the exchanges with the rivers are obtained coupling a hydrodinamic surface water model (MIKE11) with the groundwater model (feflow) via 3rd kind B.C.. In my model the water exchanges (through the unsaturated zone) between the groundwater systems and the soil system are computed by MACRO (LARSSON & JARVIS, 1999) and then applied to feflow as net water inflow on the top slice. MACRO calculates coupled unsaturated-saturated water flow in cropped soil basing on pedologic, climatic and cultivation data.

If you want you can give me your e-mail address so I can send you my last publication on that subject (pdf 4500 kB).

Bye
Gianmarco

Hi Gianmarco,
Thanks for your answer.
I'm trying to simulate both exchanges between groundwater and soil system and exchanges between the river and the groundwater reservoir, and also exchanges between atmosphere and soil/groundwater, by setting "inflow on top" equal to infiltration and modeling both saturated and unsaturated flow.
Unfortunately we don't have much information about the water table, because we are studying a mountain basin where there are no wells. However, we know that water table may touch the surface in some zones and stay under the surface in other zones. Obviously I would like water to leave the model right where the water table touches the surface, i.e. through superficial nodes that become saturated.
I found it so strange that nobody seems to have used feflow to create a 3d unsaturated-saturated flow model.
Anyway I'm really interested in your publication, you can send it here:
alice.cusi@unimore.it

Thank you again,  bye!
Alice

Hi Alice,

I think a key question you need to ask is [b]why [/b] you would need to explicitly simulate the unsaturated flow processes across the basin.

To me this sounds like a problem that would be appropriately handled using a saturated flow model.  You can simulate water table exchanges using in/outflow distribution and a seepage face on slice 1 (=ground surface).  Alternatively, you could use the equation functionality in in/outflow to calculate net recharge/evapotranspiration dependent on head an a reference elevation (ground surface).  There is a video tutorial of how to set this equation up on the feflow website.

If you have processes occurring in the unsaturated zone that affect recharge, then perhaps a separate model is required to precondition recharge - but it is unlikely this would need to be coupled to the groundwater flow model.

The main limitation with unsaturated flow modelling (in my understanding) is that it requires intensive mesh refinement to allow stable solution of Richard's equation.  Hence, performing such a simulation in 3D over 500 km2 domain would be impractical (or impossible?)

Modeling large basins having significant topographic relief can be difficult with saturated-only modes (mass balance problems).  I've successfully used variable saturation mode with a very smooth Sr curve and linear Kr curve.  I use the modified VG model and set A to the inverse of layer (or element) thickness.  This works, but thick layers in the unsat zone will cause mass-balance problems...more layers must be used.