How to define the lower boundary conditions: seepage face and unit gradient in terms of soil volumetric water content?

Question

context: the models that simulate the flow of water in the soil (e.g. land-surface models)

In these models, in order to simulate percolation (/drainage) from the bottom of the defined soil column, we need to define a lower boundary condition. The two options which I see in the literature are a seepage face (SF) and a unit gradient condition.

Is it correct to define these two as the following:

• SF: percolation happens only if the last layer in the soil column is saturated.
• UG: percolation happens only if the soil water content at the bottom of the column is greater than the soil water content at the field capacity.

I have been told that since SF and UG are defined based on suction and not soil moisture, the above definitions are not technically correct.

Please let me know what you think, thanks in advance.

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more context: the model calculates the percolation from the bottom of the soil column using the 1D Darcian flow equation, triggered by a water content value (of the last layer) threshold. That is why I need to define the lower boundary based on water content.

Answer 1

It's been many months since August, but maybe someone else will find this useful.

Like a lot of questions in hydrology - the appropriate boundary condition is defined by accurately characterizing the subsurface system. If limited data is available, it's probably best to run both boundary conditions and look at the sensitivity of the assumption and then determine if the conservative answer in the condition is reasonable (maybe to be revised with more detailed characterization later).

In terms of the boundary conditions themselves:

A seepage face requires a bottom layer to reach saturation before water is allowed to transmit to a lower level or to escape a model. This would only be appropriate if there was a realistic interface at the bottom layer of the model that would promote ponding and then water were to "punch through" that layer in the event the soil directly above the layer became saturated.

A free draining boundary will assume the water can continue moving vertically at a unit gradient with the elements above. This assumption is good if the depth to a water table or some kind of low-permeability zone is quite a ways below a model area of interest.

So - without knowing more about the particulars of your analysis, I'd expect a free draining boundary condition would be an ok starting point. Again, probably worth a sensitivity analysis when in doubt.