Determination of dry soil layer and its soil heat flux distribution using actively heated distributed temperature sensing

A dry soil layer (DSL) of certain thickness is usually formed on the sandy soil surface during the soil water evaporation process, with the bottom boundary of DSL corresponding to the evaporating surface. As the water transfer mechanism in the DSL is different from that beneath the DSL, in the numerical analysis, it is essential to know such DSL in particular in terms of initial conditions. Moreover, the change in water transfer mechanism further leads to the change in energy transfer mechanism in the DSL, rendering the whole soil drying process more complex. To master the DSL distribution and its accompanying energy changes, a new method for determining the DSL and its soil heat flux (G) distribution based on the actively heated distributed temperature sensing (AH-DTS) technology is proposed. Through in-situ tests on the Loess Plateau in Yan'an, China, the DSL thickness and the G profiles in sandy soil are determined successfully. The results show that there is a DSL with a thickness of 0.25 m in the test site. The G in the DSL fluctuates with depth, and its value is significantly higher than that of the soil below the DSL, indicating that the G in the DSL provides energy for water evaporation at the evaporating surface. Note that the AH-DTS technology can realize multi-parameter measurement at the same time with high-resolution, which is effective and reliable for determining the DSL thickness and the G distribution. In practical applications, it is recommended that the DSL thickness be determined according to both soil temperature (T0) profile and volumetric water content (θ) profile. Moreover, it is suggested that a reasonable heating time-interval of at least two hours and the close contact between the AH-DTS sensor and the surrounding soil be guaranteed to ensure the accuracy of G profile determination.