Partners and International Organizations
(English)
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ERFL-LPAS (CH), Risö National Laboratory (DK), Finnish Meteorol. Inst. (FIN), BNSC-RASDU (GB), University of the Aegeau (GR), Agricultural University Wageningen (NL),Vrije Universiteit Amsterdam (NL), SLU (S), Uppsala University (S)
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Abstract
(English)
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In frozen soil water usually infiltrates through preferential pathways. This process is governed by various thermal and hydraulic processes that are coupled in a complex manner. Therefore, it is difficult to describe such events numerically and next to impossible to predict them for given initial and boundary conditions.
Our project goals within the WINTEX programme aimed at (i) development of dye tracer techniques for visualizing and quantifying water infiltration in frozen soils, and (ii) testing suitable models for describing water infiltration in frozen soil and parameterization of the pertinent soil physical properties. The experimental approach was divided in three different activities: 1. Packed sand columns and an undisturbed soil monolith were percolated under frozen conditions with a multi-tracer solution (dyes and mobile salt). 2. Low-temperature scanning electron microscopy in combination with X-ray analysis was used to investigate the small-scale spatial arrangement of the various phases in unsaturated frozen soil samples. 3. The results of the cold chamber experiments were used to test an existing numerical model that is capable for describing the water, energy, and solute dynamics in frozen soils.
Four infiltration experiments with different initial water content and dye tracers were performed using the sand columns, and one experiment with a loamy-sandy and well-structured undisturbed soil monolith. At the beginning, the columns and the monolith were frozen from the top to the bottom to a final temperature of -5 oC. Then, the dye and salt solution was irrigated for 5 h at a rate of 5 mm per hour and a chamber temperature slightly above freezing. For the sand columns, a second irrigation was conducted few days after the first one. Subsequently, the samples were frozen again before they were vertically sectioned. The stained profiles were photographed and the pictures interpreted by digital image analysis and/or fluorescence imaging. Both techniques produce two-dimensional dye concentration maps with a high spatial resolution. These techniques were shown to be useful tools yielding conclusive results also for natural soils exposed to frozen conditions. A necessary prerequisite is an accurate extraction method for obtaining dye concentrations of the calibration samples. These samples are used to relate their concentrations and the spectral signal values of the corresponding image area. In case of the digital image analysis more such samples (at least 10 to 15) are needed than for the fluorescence imaging (2 to 3 samples under optimal conditions).
Water infiltration in the sand columns was rather homogeneous and strongly affected by the initial water content. In case of the two initially wet columns (initial water content = 0.27) water infiltrated only during the first irrigation cycle, with the infiltration front being at about 16 cm depth after an
infiltration of 25 mm of water. Infiltration during the second irrigation was almost entirely impeded because of the high ice content near the surface resulting from refreezing of infiltrated water. In case of the two initially dry columns (initial water content = 0.11) all water applied during the two irrigation days infiltrated, i.e. 50 mm in total. It is interesting to note that the infiltration front reached about the same depth in both the dry and the wet columns. The dye pattern showed that most of the dye applied during the first irrigation day was displaced to 12 to 16 cm depth with the exception of some highly concentrated spots in the region above.
The infiltration pattern in the loamy-sandy soil monolith was quite different from that observed in the sand columns. Here, water and dissolved dyes percolated mainly preferentially along cracks and rotted roots down to a depth of about 30 cm.
The experiments with the packed sand columns were simulated with the heat, water, and solute transport model SOIL. For obtaining a reasonable representation of the measured dynamics, it was necessary to assume two water conducting flow domains:(a) the liquid water phase adsorbed by the soil matrix, and (b) the initially air-filled voids. The observed heat, water, and solute dynamics was simulated fairly well by fitting five key-parameters.
Some sandy and loamy samples were mixed with a CaBr2 solution before they were frozen and investigated with a low-temperature scanning electron microscope that is equipped with an electron-dispersive X-ray (EDX) analyser. The micrographs clearly showed the network of air-filled voids in the sand that enables effective water migration also under frozen conditions. In contrast, parallel ice lenses were formed in the loamy samples, which most likely reduce water transport to a negligible order of magnitude. The EDX analysis of the elemental composition yielded acceptable qualitative but not quantitative data. One reason for this problem is the very rough surface of the investigated samples.
As a general conclusion of this project it can be stated that the applied methods contain a not yet fully used potential for a further and better understanding of the frozen soil dynamics on both scales, the smaller pore scale and the larger column or even field scale.
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