The daily evaporation characteristics of deeply buried phreatic water in an extremely arid region

• The condensed water in the greenhouse comes from underground water.
• Daily evaporation is only a small part of the water fluxes in the upper soil layers.
• The daily evaporation speed can be expressed as a sinusoidal curve.

SummaryMeasurements of the daily evaporation characteristics of deeply buried phreatic water in an extremely arid area are reported. The results are used to analyze the mechanism responsible for water movement in the groundwater–soil–plant–atmosphere continuum. A closed PVC greenhouse was set up on Gobi land at the top of the Mogao Grottoes where phreatic water is more than 200 m deep. An air-conditioning unit and an automatic weighing scale were placed inside the greenhouse to condense and monitor phreatic evaporation and soil water changes in this extremely arid region. Soil temperature and humidity at various depths (0–40 cm) and other meteorological factors were also recorded on a sub-hourly basis. The relationship between evaporated water and soil water movement was analyzed by observing changes in soil weight, the condensate from the air-conditioning unit, and air moisture. The results show that phreatic water evaporation occurs from this deeply buried source in this extremely arid zone. The daily characteristics are consistent with the variation in the Sun’s radiation intensity (i.e. both show a sinusoidal behavior). In the daytime, most of the soil water does not evaporate but moves to cooler sub-layers. In the afternoon, the shallow soil layer absorbs moisture as the temperature decreases. At night, an abundance of water vapor moves upwards from the sub-layers and supplements the evaporated and downward-moving moisture of the superstratum in the daytime, but there is no evaporation. The stable, upwardly migrating vapor and film water is supported by geothermy and comes from phreatic water, the daily evaporation characteristics of which changes according to soil temperature when it reaches the ground.