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Incorrect material flux at upper boundary


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Hi all

This question concerns how contaminant boundary fluxes are calculated in hydrological models.

I am using a constant flux boundary condition to simulate infiltration of contaminated water at the soil surface. I am putting in water at a constant rate (0.002 m/d) and constant concentration (0.0006 kg/m3). This should mean a constant material input flux of 1.2e-6 kg/m2/d.

However, because the initial background concentration is much lower (0.0001 kg/m3) there is a large concentration gradient at the boundary. This results is a large dispersive flux being calculated, although the boundary should be zero concentration gradient as well.

How do you apply a constant concentration or flux boundary condition that is also zero gradient?



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Hmm. Further to this, I just realised that a Cauchy boundary condition is supposed to specify the TOTAL flux (advective + dispersive) across the boundary, so shouldn't that work? Maybe there's a bug in FEMWATER's flux calculations...

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