Variations in the top-layer soil freezing/thawing process from 2009 to 2018 in the Maqu area of the Tibetan Plateau

Considerable progress has been made in understanding the variation rules of the freezing/thawing period and soil water-heat exchange. With the use of meteorological and soil observation data, this study highlighted the importance of vegetation cover and soil moisture for the beginning of the freezing/thawing period, and an improved calculation equation of the soil moisture content was presented, which could be used to simulate the variation in soil moisture during the freezing/thawing period. Our analyses showed that the temperature changes at the beginning of the soil freezing month (December) and the soil thawing month (February) were significantly different from 2009 to 2018. The rate of air temperature increase in December reached 0.38 °C/a. However, the air temperature in February changed little. As a result, the beginning of the soil freezing period was clearly lagged, while the beginning of the soil thawing period fluctuated little. The total soil thawing time increased by 25 days over 10 years. Furthermore, it was shown that the surface vegetation cover and soil moisture affected the beginning of the freezing/thawing time. In general, the soil freezing/thawing state at sites with low vegetation cover and soil moisture content was more likely to change. To further study the freezing/thawing mechanism, the relationship between the soil temperature and soil moisture during the freezing/thawing period was expressed as a fitting function of the relationship between soil water and heat. It was shown that the relationship between the soil moisture and soil temperature followed the new fitting function in both warm and cold years.