|Spaans, Egbert - UNIVERSITY OF MINNESOTA|
Submitted to: International Symposium on Physics Chemistry and Ecology of Seasonally Froz
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 30, 1997
Publication Date: N/A
Interpretive Summary: Time domain reflectometry (TDR) is a relatively new method for soil moisture measurement that has particular application in frozen soils, because it responds primarily to liquid water, so it has the potential to discriminate between water and ice. However, to this point there has been no way to evaluate its performance in frozen soil. Researchers had been applying calibrations developed in unfrozen soil to frozen conditions, which we felt was incorrect. We developed a new device, a gas-phase dilatometer, that allows independent measurement of the conversion of water to ice. Using it we were able to calibrate a TDR for measurement of liquid water content in frozen soil. The results supported our hypotheses that: 1) the slope of the calibration in frozen soil differs, by more than 10 percent, from that in unfrozen soil; 2) the intercept of the calibration depends on the total water content. Thus, for absolute measurement of liquid water content in frozen soil it is necessary to know the total wate content. We also demonstrated that a TDR can provide valuable information about solute behavior in frozen soil by providing accurate measurements of bulk electrical conductivity. This can be used to investigate questions regarding solute redistribution during freezing and thawing, which have potentially important water quality implications. We found that in our soil at Rosemount, MN there was no evidence for solute redistribution over the winter due to freezing and thawing. The results indicated that solute exclusion during the freezing process must have isolated solutes locally in films and unfrozen pores, rather than forcing them ahead of the freezing front into the unfrozen soil below. The results will be useful to researchers studying water movement in freezing and thawing soils.
Technical Abstract: Time domain reflectometry (TDR) is a powerful technique to simultaneously measure volumetric liquid water content and bulk-soil electrical conductivity in frozen soil. Since TDR detects liquid water indirectly it requires calibration, which is different in frozen than in unfrozen soil because the permittivity for ice is larger than that for air. This hypothesis was confirmed by means of a new procedure to calibrate TDR for water content. TDR was applied to monitor water content and electrical conductivity in situ in a silt-loam to investigate whether solutes are redistributed when the soil is subject to freezing and thawing cycles. There is no evidence of macroscopic redistribution of solutes, hence it is concluded that solutes were excluded locally into nearby liquid films.