Conventional methods for numerical simulation of the soil evaporation process do not take into account the mechanical dispersion of vapor in the porous matrix. This omission is due to the uncertainties about the flow process that generates mechanical dispersion and about the numerical value of dispersivity. In this study we assess three processes that can generate mechanical dispersion: (a) temperature variation, (b) barometric pressure variation, and (c) Stefan flow. Order of magnitude estimates show that under natural conditions, only Stefan flow can contribute significantly to the generation of mechanical dispersion. However, a simple sensitivity analysis of the effects of dispersivity on the contribution of different mechanisms to the evaporation process shows that diffusion and mechanical dispersion act in a complementary way. In simulations of a natural system, the evaporation flux increases by only 12% when dispersivity is increased from 0 m (no mechanical dispersion) to 0.078 m, although at the latter value the mechanical dispersion flux contributes up to 40% of the total evaporation flux.