Dear Jirka,
I performed multistep outflow experiments with wood chip media to determine hydraulic properties by inverse modeling cumulative outflow data and also pressure head data at two different heights in a 15 cm tall flow cell. Since the flow cell is completely saturated at the start of the experiment I recorded positive pressure head readings from both tensiometers at the beginning of the experiment that become negative once water in large inter-particle pores drain.
I am trying to get three models to fit the data (the single porosity VGM, the dual porosity Durner (1994) model, and the dual permeability (Gerke and van Genuchten, 1993). The models are able to fit the cumulative outflow data well. They are, however, unable to simulate the positive pressure at saturation. Even at saturation (time=0) or a few seconds into the experiment, all models predict negative pressure head at both heights in the flow cell even though measured values indicate that the pressure head is still positive. The water in the flow cell took several hours to drain since there was a filter paper-sponge cloth combination at the lower end of the flow cell (which was taken into account during inverse modeling as a second material).
Is there any reason why the models are not able to simulate the positive pressure head? Please let me know.
Thank you.
-Vandana Subroy.