Drying front dynamics during the falling rate stage of evaporation from homogeneous sandy soil profiles

抄録

In recent decades, arid and semi-arid areas have been suffering from high evaporation rates and scarcity in rainfall events. Besides the soil degradation and reduction in groundwater availability, those severe conditions are exacerbating desertification. The need for finding innovative solutions requires a thorough understanding of the surface-atmosphere boundary fluxes and the water movement through unsaturated soil profiles. Among those crucial fluxes is the evaporation flux, which is dominant in dry regions. Evaporation is a complex process where water moves through soil pores and gets lost into the atmosphere. It involves three stages that differ in their actual evaporation rates and water transport mechanisms. One of the main controlling factors of the process is the region separating the saturated and the unsaturated zones known as the drying front. During the constant rate stage (Stage 1), the capillary transport from the drying front to the soil surface maintains a high and constant evaporation rate. At a specific drying front depth, governed by the pore size distribution, a sudden drop in the evaporation rate occurs, marking the onset of the falling rate stage (Stage 2). During this stage, a continuous reduction in the evaporation rate develops due to the change in the transport mechanism. It is believed that a new vaporization plane forms below the soil surface, allowing water to transport by capillarity from the drying front and continue as vapor through an air-dry soil layer to the soil surface. Finally, when the evaporation rate converges to a low constant value, the residual stage (Stage 3) starts.

The drying front during Stage 1 was theoretically and experimentally elaborated in the literature. However, due to the complexity of the transport mechanisms during Stage 2, the drying front behavior is not clearly understood. This paper investigates the spatial and temporal development of the drying front during the falling rate stage for sandy soils. The drying front was traced experimentally through 1-D homogeneous drying column tests. The soil samples used for testing vary in their pore structure, where the influence of the related soil properties on the drying front dynamics from a micro-scale perspective is studied. This study is expected to serve as a fundamental step towards understanding and systemizing the determination of the drying front dynamics during the evaporation process.