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.
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.