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Salt precipitation over porous media due to evaporation is known to affect various important processes, including evaporation itself. Many studies have shown that salt crust precipitation reduces evaporation rate, and it has been suggested that the crust constitutes a barrier to vapor flow. Nevertheless, it remains unclear how a porous medium such as the salt crust does not enable upward liquid water transfer by capillary flow. Herein, we show by various imaging methods that the salt crust grows out of specific nucleation centers, to create an elevated crust approximately 1mm above the matrix surface, most of which is disconnected from the matrix. This indicates that evaporation from a porous medium that is covered by NaCl efflorescence salt crust may occur below the crust, at the soil surface.

Plain Language Summary This paper explains the puzzling observation that salt crusts over evaporating soil surface reduce evaporation, even though capillary flow of water through the salt crust is possible. We showed, by the use of various imagining methods, that the formed salt crust is mostly elevated above soil surface with a very limited contact between the two. Therefore, most of the water has to pass the crust as vapors and that is why it reduces evaporation rate. To observe the nature and architecture of the salt crust, we used X-ray tomography, as well as pictures of the growing salt crust in the visible wavelength. In addition, field observations of salt crust under natural conditions were also preformed. The field observations indicated that the findings from the laboratory are also found in the field.