Initial boundary condition

[Zhaoyang Luo]

Hi,
Recently, I'm simulating rainfall infiltration in a 2D hillslope. I didn't know how to set a completely dry initial condition.
Can anyone provide some help for me? Thanks in advance.

[Jirka]

Water content equal to the residual water content represents a completely dry initial condition. However, that is not possible to consider since that would be equal to the pressure head equal to minus infinity. To avoid numerical problems you can perhaps use the same rule as for hCritA, which states “hCritA should be selected such so that the corresponding water content is at least 0.005 higher than the residual water content.” J.

[swhitman]

Hello Jirka,

I have a similar issue to the OP, although I am working with packed laboratory columns.

Do you have any suggestions for modeling a situation where you know that the initial moisture content is actually below residual water content?

I am trying to inverse model infiltration into a packed column that came from a very dry field site, and the only thing I can think to do so far is to maniuplate the theta r parameter to near zero and try to adjust the rest of the parameters to still fit my laboratory derived SWCC points, using a very high suction initial condition, but this seems like a sub-optimal solution.

I know that my simulation is not fitting my observed infiltration values using laboratory based soil parameters because the initial condition is too wet in the model, but I can't think of anything besides my ideas above to remedy the situation.

Thanks,

[Jirka]

You cannot have the initial condition lower than theta_r since for such conditions the retention curve is not defined. The knowledge of the retention curve (for all values of h and theta) is required when numerically solving the Richards equation. The pressure head is equal to negative infinity for the water content equal to the residual water content. For lower water contents, it is not defined. In HYDRUS, when you specify the initial condition in terms of the water content, it is converted using the retention curve into the pressure head, which is then used in the numerical solution. J.