A Co-Tube-Radiation Method For In-Situ Determination Of Ice And Liquid Water Contents In Frozen Soil And Freeze-Thaw Model Validation

The annual freeze-thaw cycle of soil has attracted increasing attention because it can significantly impacts agricultural services in cold regions, such as water infiltration and runoff over winter and optimal time for tillage in early spring. Many previous studies have only concerned the fundamental of freeze-thaw cycle under laboratory condition because of the difficulty for the in situ measurements of liquid soil water content (LSWC) and soil ice content (SIC) in field, and the latter is more challenging. The shortage of field-based data resulted in the limitation of the wide applicability of soil freeze-thaw models developed. For this, a co-tube-radiation method was innovatively proposed for in situ determination of LSWC and SIC. The soil moisture characteristic curve (SMCC) was deduced from soil freezing characteristic curve (SFCC) determined in situ based on the similarity between the two curves, and the parameters in SMCC were estimated as well. Based on this, a freeze-thaw model was validated and evaluated by the comparison between the model-simulated data and the measured data. This study was financially supported by the Chinese-German Center for Scientific Promotion under Project No. GZ494, National Natural Science Foundation of China under Project No.31171458and the Doctoral Promotion Fund of the Educational Ministry of China under Project No.20110008110046. The major research works and conclusions of this study can be briefly summarized as follows:1. Based on the equivalence of dielectric properties between soil drying-wetting and freezing-thawing processes, dielectric mixing model and temperature effect on permittivity of liquid water in frozen soil, the in-situ determination of LSWC is feasible. Moreover, the neutron probe is proportional to amount of hydrogen in soil, which is independent of the status whether the soil is frozen. Thus, the total soil water content (TSWC) can be measured either in frozen or unfrozen soil.2. A co-tube-radiation method was innovatively proposed. Theoretical analysis and experimental results indicate that both the electromagnetic radiation from tube-based dielectric probe and neutron radiation of neutron moisture meter can penetrate through the PVC tube. By uniting the probe size of both instruments, it is possible that the two instruments can be used in a common access PVC tube. A method was used for determination of radius of sensitivity (ROS) of dielectric tube sensor based on the equivalence of dielectric properties between soil drying-wetting and freezing-thawing processes. According to the experiment results, the ROS of dielectric tube sensor is5-8cm, which is close to the ROS of neutron moisture meter (10-20cm).3. The field experiment showed that the LSWC was determined in-situ. The variations of LSWC can reflect the dynamics of freezing/thawing front over winter. Based on this, the soil freezing characteristic curve (SFCC) was also determined in-situ. Moreovers, the soil moisture characteristic curve (SMCC) can be deduced from SFCC based on the similarity between them. The parameters of SMCC can be estimated through SFCC, which were compared with those determined from pressure plate method (reference). The coefficient of determination (R2=0.837) and RMSE (131kPa) showed a good agreement between the characteristic curves from the two methods. 4. The cross-check results showed that both instruments had good interconvertibility (R2>0.95) with considerably low error (RMSE<0.02cm3cm-3). The co-tube-radiation experiment in the field indicated that the proposed method can determine LSWC and TSWC, and thus SIC. The measuring accuracy was significantly improved even accompanying with water migration during freezing-thawing process. This method can provide technical support to field management for winter irrigation and tillagement in spring and can be used to monitor the soil water dynamics throughout the year.5. The PDE model was validated by two groups of data. One group only measured LSWC and the other measured both LSWC and SIC. The filed experiment showed that the latter group can yield higher accuracy compared with the former group and the influences from the ignored items in PDE model was analyzed. Besides, it was proved from the experiment that the snowcover is very effective at insulating the soil, decreasing the frost penetrating velocity and therefore increasing moisture migration to the freezing front.