Drying of a Natural Soil Under Evaporative Conditions: A Comparison of Different Magnetic Resonance Methods

Abstract

Soils are highly complex and heterogeneous porous materials, and thus measuring water distribution non-invasively with high accuracy and adequate spatial resolution still remains challenging. The first few centimeters of a soil surface control the vapor flux to the atmosphere justifying the need for high spatial resolution measurements of moisture content. The objective of this study was to compare and assess the feasibility of various high-resolution magnetic resonance (MR) methods to characterize an unsaturated porous system. We employed (1) a spin-echo, (2) three types of single-point imaging and (3) a unilateral three-magnet array to monitor T₁ and T_2,app relaxation time spectra and the effective moisture saturation (Θ_MR) of a silt loam under progressing desaturation with focus on an emerging unsaturated surface layer, which is predicted by theory. During the first stage of drying where evaporation occurred at the soil surface, all methods showed homogeneously distributed moisture. A decreasing Θ_MR and a shift in the T₁ and T_2,app relaxation time spectra to shorter values indicated the commencement of stage 2 evaporation coincided with an increasing unsaturated layer. At low water contents, the most suitable method to determine the extent of a desaturated surface zone with high accuracy was found to be single--point ramped imaging with T₁ enhancement. As a simple and low-cost device the unilateral three-magnet array was feasible to monitor the drying process until the dry surface layer developed.