Essentially, if we consider gravity in Darcy's law, we should also consider the corresponding term in the energy balance for consistency.
Subtracting the momentum balance multiplied by the velocity gives the balance of internal energy (non-conservative because only total energy is conserved):
This shows that there are conversion terms resulting in modification of the internal energy. Even if we assume that one of these is small, or both of these are small, we don't arrive at the implemented equation. If we don't have the gravity/potential energy term in the original balance, there would be an additional gravity term popping up here that mingles with the other energy conversion terms.
The gravity term vanishes if the fluid only moves perpendicular to the gravity field. However, for scenarios with strong upward/downward movement, this should have an effect.
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