Particle movement in unsaturated zone

I have a model where we are using particle tracking to design environmental interception scenarios. The model is unconfined with a phreatic top layer and dependent layers below. The top five layers out of 19 are above the 'water table'. Our particle starting locations are at the ground surface. We have been using this model and strategy for years with some success. Recently we have noticed considerable lateral movement of particles in the top layer rather than traveling vertically downward to the 'water table'. We have not seen this before.

Is this s numerical artefact? There should be very little lateral flow as there is no water.

Dan

Dear Dan,

I would not say that there is not water... Although the first five layers are above the phreatic level, you have imposed a residual water in the Problem Settings dialog. Particle tracking is calculated based on Darcy velocities, therefore only if the lateral velocity is negligible the flow as well.
It seems to me (without knowing layer thickness) that you may have a considerable unsaturated zone. You may try the Richards approach for a better approximation of the fluxes in the vadose zone.

Best regards,
Carlos