Seasonal freezing-thawing process and hydrothermal characteristics of soil on the Loess Plateau, China

In seasonally frozen soil regions, freezing-thawing action and hydrothermal effect have strong influence on physical and mechanical behavior of shallow soil. A field experiment on the Loess Plateau in Northwest China was carried out to analyze the freezing-thawing process and hydrothermal characteristics of shallow soil considering the climate influence. The results show that the maximum seasonal freezing depth under bare ground surface in this area is from 20 cm to 50 cm. The ground temperature shows a similar changing trend with air temperature, but it has lagged behind the air temperature, and the ground temperature amplitude exponentially decreases with the increase of soil depth. The seasonally frozen soil has experienced four typical stages: unfrozen period, alternate freezing period, freezing period and alternate thawing period. The freezing-thawing process is characterized by unidirectional freezing and bidirectional thawing. The water content of shallow soil is significantly affected by air temperature, evaporation and precipitation, and the soil water content shows a “low-high-low” changing trend with the increase of depth. The soil temperature and water content interact with each other, and are often coupled. The variation trend of soil moisture with time is consistent with the change trend of the ground temperature with time in each soil layer, and the degree of consistency is higher in the near surface soil than that in the lower layer. Also, the spatial-temporal characteristics of soil moisture and temperature is that the volumetric water content and ground temperature of near surface soil have strong variability, and the range value Ka and coefficient of variation Cv of soil water content and ground temperature in different seasons show a decreasing trend with the increase of depth.