The yearly amount and characteristics of deep-buried phreatic evaporation in hyper-arid areas

Abstract. Water scarcity is the primary cause of land deterioration, so finding new available water resources is crucial to ecological restoration. We investigated a hyper-arid Gobi location in the Dunhuang Mogao Grottoes in this work wherein the burial depth of phreatic water is over 200 m. An air-conditioner was used in a closed greenhouse to condense and measure the yearly amount of phreatic evaporation (PE) from 2010 to 2015. The results show that the annual quantity of PE is 4.52 mm, and that the PE has sinusoidal characteristics. The average PE is 0.0183 mm d^-1 from March to November. Accordingly, by monitoring the annual changes in soil–air temperature and humidity to a depth of 5.0 m, we analyzed the water migration mechanism in the heterothermozone (subsurface zone of variable temperature). The results show that, from March to November, the temperature and absolute humidity (AH) increase. This is due to the flow of solar heat entering the soil – the soil subsequently releases moisture and the soil is in a state of increasing AH so that evaporation occurs. From November to March, the temperature decreases. Now, the soil absorbs water vapor and AH is in a state of decline. Thus, it is temperature alternation in the heterothermozone – due to solar heat transfer – that provides the main driving power for PE. When it drives water vapor to move downwards in the heterothermozone, a small part is reversed upwards and evaporates. Solar radiation intensity dominates the annual sinusoidal PE characteristics.