Effect of salt crust on the soil temperature of wet sandy soils

In arid regions, severe salinization can commonly result in the formation of a salt crust layer that covers the soil surface. The salt crust, composed of salt crystals, has significantly different physical properties compared to its matrix soil. It can alter the energy budget in soil, thus affecting soil temperature. However, the effect of salt crust on soil temperature remains unclear. To address this, we conducted an experiment comparing soil temperatures between salt-free and saline soils. The two soils were initially saturated with fresh water and a saline solution (∼25%, NaCl), respectively, with a fixed shallow groundwater table. Salt precipitation was induced by evaporation using a halogen lamp (600 W m^-2). Moreover, we measured the dynamic variation of evaporative water loss, salt cover fraction, and temperature (surface, 5 cm, and 15 cm). We developed a numerical model involving the variation of cover fraction and thickness of the salt crust, as well as the exothermic enthalpy of salt precipitation. The experimental results revealed that salt crust can significantly increase soil temperature. The variation of saline soil temperature was dominated by the evolution of the salt crust. The model accurately described soil temperature changes with the evolution of the salt crust and found that the higher temperature in saline soil than that in salt-free soil was attributed to lower heat loss due to greater evaporation resistance and lower surface emissivity by the salt crust, very small thermal resistance of the salt crust, the unique heat released during the crystallization of saline solution, and lower soil heat capacity. Higher temperatures in saline soil were also observed under various radiation intensities (200–1000 W m^-2). Our findings help improve understanding of the energy balance in saline soil and provide new insights into quantitatively describing soil temperature variation with evolving salt crust.