Based on batch experiments conducted on soil samples in the laboratory, the relationship between dielectric constant and water content based on electrical capacitance measurement was investigated. Factors that may affect the relationship, such as measurement frequency, electrode array methods, and soil particle sizes, were analyzed. A model fitting the relationship between dielectric constant and water content was proposed. The results clearly indicate that even with the use of rod-like sensors, the measured electrical capacitance with 1 kHz frequency can be used to construct a good relationship between dielectric constant and water content. Although the electrode array methods (parallel or non-parallel electrode pair) and soil particle sizes do affect the obtained absolute values of dielectric constant to some extent, the relationship between dielectric constant and water content remains unchanged. A clear characteristic is the increase reduction in the dielectric constant at near saturation condition, and the low increase speed when water content is small. The proposed modified VG model incorporated this characteristic into it and fitted the data better than the reported models. These results suggest that the electrical capacitance-based dielectric constant measurement can be extensively used and would be useful, especially in the situations where parallel sensor requirement is difficult to satisfy.

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The process of soil freezing and thawing refers to the alternating phase change of liquid water and solid water in the soil, accompanied by a large amount of latent heat exchange. It plays a vital role in the land water process and is an important indicator of climate change. The Tibetan Plateau in China is known as the “roof of the world”, and it is one of the most prominent physical characteristics is the freezing and thawing process of the soil. For the first time, this paper utilizes the spaceborne GNSS-R mission, i.e., CYGNSS (Cyclone Global Navigation Satellite System), to study the feasibility of monitoring the soil freeze-thaw (FT) cycles on the Tibetan Plateau. In the theoretical analysis part, model simulations show that there are abrupt changes in soil permittivities and surface reflectivities as the soil FT occurs. The CYGNSS reflectivities from January 2018 to January 2020 are compared with the SMAP FT state. The relationship between CYGNSS reflectivity and SMAP soil moisture within this time series is analyzed and compared. The results show that the effect of soil moisture on reflectivity is very small and can be ignored. The periodic oscillation change of CYGNSS reflectivity is almost the same as the changes in SMAP FT data. Freeze-thaw conversion is the main factor affecting CYGNSS reflectivity. The periodical change of CYGNSS reflectivity in the 2 years indicates that it is mainly caused by soil FT cycles. It is feasible to use CYGNSS to monitor the soil FT cycles in the Tibetan Plateau. This research expands the current application field of CYGNSS and opens a new chapter in the study of cryosphere using spaceborne GNSS-R with high spatial-temporal resolution.

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