Moisture profiles of the upper soil layer during evaporation monitored by NMR

Abstract

Near-surface soil moisture profiles contain important information about the evaporation process from a bare soil. In this study, we demonstrated that such profiles could be monitored noninvasively and with high spatial resolution using Nuclear Magnetic Resonance (NMR). Soil moisture profiles were measured in a column exposed to evaporation for a period of 67 days using a stationary Magnetic Resonance Imaging (MRI) high field scanner and a unilateral NMR sensor. The column was packed with medium sand and initially saturated. Two distinct shapes of soil moisture profiles that are characteristic for stage I (evaporation rate is controlled by atmospheric demand) and stage II (evaporation rate is controlled by the porous medium) of the evaporation process were followed by both MRI and unilateral NMR. During stage I, an approximately uniform decrease of soil moisture over time was monitored, whereas during stage II, Sshaped moisture profiles developed which receded progressively into the soil column. These promising results and the specific design of the unilateral NMR system make it very well suited for determining soil moisture profiles in the field.

Long-term and high-frequency non-destructive monitoring of water stable isotope profiles in an evaporating soil column

“…In this second scenario, RH EF decreased from 100 to 81 % with a mean value of 99.5 ± 0.03 % for the period DoE 0-150, i.e., in a similar fashion than fitted n values obtained in the first scenario. These values were significantly lower than those calculated with Kelvin's equation linking RH EF with soil water tension at the EF in the case of liquid-vapor equilibrium, which for the given soil retention properties (Merz et al, 2014) would range between 100 and 99.6 %. In a third scenario one could consider a combined decrease of n and RH EF to a smaller extent, for which there are no unique solutions at each time step.…”

“…4b no distinct characteristic evaporation stages, i.e., stages I and II referring to atmosphere-controlled and soil-controlled evaporation phases, respectively, could be identified. The opposite was observed by Merz et al (2014), who conducted an evaporation study using the same sand. This indicates greater wind velocity in the air layer above the soil column due to the laboratory ventilation.…”

“…100. Around DoE 100, when θ at −0.01 m reached a value of 0.090 m 3 m −3 (i.e., significantly greater than the sand residual water content θ = 0.035 m 3 m −3 , determined by Merz et al, 2014) p value < 0.01), with the exception of the period DoE 125-155 (R 2 = 0.31, p < 0.001), when atmospheric water vapor δ 2 H was remarkably high in the laboratory ( Fig. 6c and d).…”

“…Two closed acrylic glass vessels (0.12 m i.d., 0.22 m height), in each of which a 0.15 m long piece of tubing as well as a type K thermocouple were installed, were filled with FH31 sand (porosity = 0.34 m 3 m −3 , dry bulk density = 1.69 × 10 3 kg m −3 , particle size distribution: 10 % (> 0.5 × 10 −3 m), 72 % (0.25-0.5 × 10 −3 m), and 18 % (< 0.25 × 10 −3 m)) (Merz et al, 2014;Stingaciu et al, 2009). Each vessel was saturated with water of two different isotope compositions: δ 2 H st1 = −53.51 (±0.10) ‰, δ 18 O st1 = −8.18 (±0.06) ‰ and δ 2 H st2 = +15.56 (±0.12) ‰, δ 18 O st2 = +8.37 (±0.04) ‰.…”

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

“…The experimental section consists of two parts: Part one deals with drying of a sand column during evaporation using different NMR and MRI sequences, which is presented in detail in [7]. Part two focuses on unilateral NMR measurements performed on an evaporating natural soil (silt loam).…”