Rules of the dynamic water-heat-salt transfer in soil of Horqin meadow during freezing and thawing period

To elucidate the dynamic water-heat-salt transfer in soil during freezing and thawing periods, data on weather and soil in Horqin meadow were analyzed and changes in the temperature, water content, and salt content of the soil in the meadow during these periods were investigated. Data were collected by the Agoura Eco-hydrological Experiment station at Zuoyihouqi, Horqin from October 2013 to May 2014. Results indicated that the effect of temperature on the soil profile temperature decreased with increasing soil depth. In addition, changes in soil profile temperature lagged behind temperature variations and were also affected by the amplitude of temperature variation. Moreover, these changes showed no significant pattern. When the rate of temperature increase exceeded that the rate of cooling, the rate of soil thawing consequently increased compared with that the rate of freezing. Freezing begins from the soil surface and proceeds deeper beneath the surface, and freezing temperature was negatively correlated with the salt content of the soil. Linear interpolation of the temperature was adopted to accurately determine the maximum frost depth in the meadow on 9 March 2014. The computed maximum frost depth was 104 cm. Because of the effects of underground heat from warm soil and surface temperature, the thawing begins from two directions, namely, bottom-to-surface and surface-to-bottom. The groundwater depth is shallow and is affected by soil freezing and thawing with significant ascending or descending trend. During the soil freezing period, soil salt accumulates on the surface and reaches the maximum content in February. Salt content decreases after thawing and rain. During the freezing and thawing periods, the soil's salt content shows greater changes than soil's water content, thereby indicating that the salt transfer process is complicated.