Magnetic Resonance Monitoring and Numerical Modeling of Soil Moisture during Evaporation

Abstract

Evaporation from bare soil surfaces can be restrained to a great extent with the development of a dry layer at the soil surface where capillary hydraulic conductance ceases and water flow proceeds only by gas phase transport. Model calculations and preliminary experiments with model porous media have shown that this surface layer can be very thin. An accurate characterization of these processes is required, which is provided by noninvasive magnetic resonance (MR) methods. The evaporative drying of a silt loam and a sandy loam was monitored at high spatial resolution in laboratory experiments. The MR data were used to assess the performance of two numerical models: (i) the Richards equation, which considers isothermal liquid water flow, and (ii) a coupled soil water, heat, and vapor flow numerical model. The experimental results reveal two distinct drying regimes for both soil types where, at the onset of the second evaporation stage, a dry surface zone developed with increasing thickness over time. This layer revealed that water loss inside the soil coincided with a relatively low evaporation rate as the liquid continuity to the soil surface vanished. The modeling results clearly demonstrated the need to consider heat and vapor flow. It was shown, as a proof of principle, that MR relaxation time spectra may serve as a proxy to follow desaturation processes where spatially resolved transverse relaxation can reveal a secondary evaporation front.

Magnetic resonance sounding measurements as posterior information to condition hydrological model parameters: Application to a hard-rock headwater catchment

“…This downside is emphasized when hydraulic conductivity estimates rest on empirical relationships that might spatially vary over the catchment. Nonetheless, the coupling of MRS data with hydrogeological models is a powerful strategy to tackle non-unicity issues as reported by a few studies to model pumping tests (Herckenrath et al, 2012;Vilhelmsen et al, 2014), the capillary fringe (Costabel and Günther, 2014), the evaporation in the first centimeters of soils (Merz et al, 2018), or the water movement in variably saturated media (Legchenko et al, 2020).…”

Reviews and syntheses: Gaining insights into evapotranspiration partitioning with novel isotopic monitoring methods

“…We will shortly present the widely used model variation for water bound to the soil media (For an in-depth review, the reader is kindly referred to Horita et al, 2008). The only significant difference to the original model is the evaporating front vertical coordinate (zEF), which may not correspond to that of the soil surface depending on the evaporation stage (Or et al, 2013;Merz et al, 2018). The isotopic ratio of evaporation, RE, is expressed as the ratio of E and E , i.e., the water vapour flux density rates [L 3 L -2 T -1 ] in rare and abundant isotopologues, respectively, originating from the EF:…”

Reviews and syntheses: Gaining insights into evapotranspiration partitioning with novel isotopic monitoring methods

“…cryogenic vacuum extraction, is also associated with uncertainty (e.g. Orlowski et al, 2016a, b;Millar et al, 2018). The choice of an appropriate method for sampling xylem water is also crucial for a correct determination of δ E .…”