Characteristics of water–heat variation and the transfer relationship in sandy loam under different conditions

The soil freezing process affects the distribution of soil water and heat, influencing decisions on the planting of spring crops. To reveal the redistribution characteristics of soil moisture and temperature and their response factors in the unidirectional freezing process, an indoor unidirectional freezing test was carried out. Three levels of initial moisture content (18.84%, 23.43%, 28.78%), three freezing temperatures (−5 °C, −10 °C, −15 °C), and three dry densities (1.33 g·cm−3, 1.45 g·cm−3, 1.58 g·cm−3) were set up. Firstly, the distribution of soil cold structure and the variation in the freezing depth curve during the freezing process were observed under different initial conditions. Additionally, the variation characteristics of soil temperature over time and space scales were analyzed, and the freezing process was divided into the rapid freezing stage and the steady freezing stage according to the rate of soil temperature change. Furthermore, the redistribution effect of soil water under the action of potential energy gradient and the characteristics of water migration in different periods were explored. On this basis, the transfer functions of soil temperature at time and space scales were constructed in accordance with regression theory. On the premise of significance test, the effects of soil moisture content and dry density on soil temperature over time and space scales were explored via function derivation and parameter analysis. With the increase of moisture content, the density of soil temperature contours decreased. However, as the dry density increased and the freezing temperature decreased, the density of soil temperature contours increased. As the soil freezing temperature decreased, the degree of soil freezing increased, the pore structure was blocked by frozen ice, and the amount of soil water migration decreased. At the same time, due to increasing soil dry density, the amount of soil water migration and migration rate also decreased. The parameter analysis showed that on the time scale, the change rate of soil temperature decreased as the moisture content increased, whereas it increased as the soil dry density increased; similarly, on the spatial scale, the soil moisture content was negatively correlated and the dry density of soil positively correlated with the soil temperature change rate. The results of this study are of great relevance for understanding the intrinsic relationships between soil moisture content, temperature and dry density in frozen soil and scientifically guiding the development of water and soil resources in cold regions.