Book/Dissertation / PhD Thesis

FZJ-2017-03771

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Drying front formation in topmost soillayers as evaporative restraint - Non-invasive monitoring by magneticresonance and numerical simulation

Merz, S. (Corresponding author)FZJ*

2017
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag Jülich
ISBN: 978-3-95806-234-4

Please use a persistent id in citations: http://hdl.handle.net/2128/14806  urn:nbn:de:0001-2017070506

Abstract: Evaporation from bare soil surfaces is a highly dynamic process and one major component of the globalwater cycle. Due to the nature of soils as complex porous materials, the process of water loss to the atmospheric boundary layer causes a considerably moisture variation in space and time on larger scales driven by the interplay between internal flow processes and the atmosphere. Particularly the top few centimeters of a soil constitute the source for vaporized water to be dissipated by the atmospheric boundary layer. Prediction of soil moisture changes on large scales is of various interests with regard to water resources management (e.g. in agriculture). To be able to describe the processes driven by evaporation on large scales, small scale measurements with high spatial resolution of moisture in the top most soil layerare mandatory. Within this PhD work, different Magnetic Resonance (MR) concepts/apparatus and sequences were applied and assessed to monitor changes in the moisture content of evaporating soils on various scales in high resolution non-invasively while focusing on their field applicability. Particularly focus was spent on the origin of the dry surface layer as it is predicted by theory. Since the concept of single sided MR is of great potential for direct field applications a unilateral single sided sensor and at hree magnet array were employed and the results were compared to measurements performed on stationary MR magnets of different concepts. During the first phase, the moisture development of pure sand exposed to evaporation was studied. A column consisting of medium sand was evaporated and monitored using different echo based and single point imaging methods employed at a 200 MHz stationary MR magnet. The obtained vertical moisture profiles were compared to the results of a 13.4 MHz mobile unilateral single sided MR sensor (NMRMOUSE)and the pros and cons of each concept were evaluated. As a next step, by proving the convenience of the unilateral sensor, a mini lysimeter consisting of a silt loam was exposed to evaporation underdefined irradiative conditions and soil moisture was monitored using the NMR-MOUSE. The results validate the predictions of a coupled heat, water and vapor flow model. Since soils naturally possess paramagnetic impurities and the clay content can vary considerably, MR measurements become challenging in terms of a decreasing signal to noise ratio (SNR) together with significantly accelerated signal decay what gets particularly tricky with progressing desaturation. These a forementioned inherent soil properties justify the need for feasible MR pulse sequences which ideally reduce signal decrease by relaxation acceleration due to internal and external magnetic field gradients and short detection times to reduce signal loss. ...

Note: Universität Bonn, Diss., 2017

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Contributing Institute(s):

1. Agrosphäre (IBG-3)

Research Program(s):

1. 255 - Terrestrial Systems: From Observation to Prediction (POF3-255) (POF3-255)

Appears in the scientific report 2017

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