Evaporative flux from a shallow watertable: The influence of a vapour–liquid phase transition

Evaporation from bare soil in arid and semi-arid regions is characterised by vapour transport in the upper part and liquid transport in the lower part of the soil profile. Under such conditions, both phases must be taken into account to describe the movement of water in and the evaporation from soil above a watertable. This requires location of the evaporation front, defined as the plane at which phase transformation occurs. In this study we develop a pseudo steady-state model to locate the evaporation front and then to predict the rate of evaporation under isothermal conditions. The model was tested using laboratory experiments and then developed to encompass different soil textures, salinities, watertable depths and evaporative demands. There was good agreement between the measured and simulated depths of the evaporation front and rates of evaporation. The lighter the soil texture, the deeper was the simulated evaporation front. Under salt-saturated conditions, the simulated evaporation front was deeper by a factor of 1.11, irrespective of soil type. The lower the evaporative demand, the shallower were the depth of the evaporation front and the depth of salinisation, even for deep groundwater. The depth of the evaporation front influences the rate of salinisation and the depth at which salt accumulates. It is therefore an important consideration in drainage design for irrigated systems in arid and semi-arid regions where salt control is often required. However, in practice, the assumption of isothermal conditions is a simplification because the extreme temperature gradient that exists close to the soil surface leads to a more complex situation.