Transition of stage I to stage II evaporation regime in the topmost soil: High-resolution NMR imaging, profiling and numerical simulation

• Two natural porous media were evaporated in the lab.
• Moisture profiles were investigated by MRI and a unilateral NMR sensor.
• An increasing dry surface layer at stage II evaporation could be observed.
• The unilateral NMR sensor shows promising results for direct field measurements.

Bare soils are natural porous media where moisture and transport properties may change considerably. Under very dry conditions, it is predicted that capillary continuity from deeper soil layers to the surface ceases and evaporation decreases drastically because it is only sustained by vapor transport through an increasing dry surface layer (stage II). In this study, we firstly confirm this effect by investigating the drying of a lab-scale sand column using various MRI sequences as well as a unilateral NMR sensor (NMR-MOUSE). Proofing the convenience of the unilateral sensor, we take a step forward by monitoring moisture development of a natural soil under controlled ambient conditions. Finally, the experimental results clearly validate the prediction of a coupled water, vapor and heat flow model regarding the onset of stage II evaporation and the subsequent receding secondary evaporation front.

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