In extremely arid regions, deeply buried phreatic water evaporates during the daytime from March to November in the northern hemisphere. It has been found that the earth-air undergoes 'autonomous breathing' and 'passive breathing', respectively caused by the changes of temperature and atmospheric pressure. In this paper, the effects of these breathing modes on phreatic evaporation (PhE) were investigated as well as the responsible mechanisms. Quantitative estimates suggest that the direct contribution from autonomous breathing is only 0.55 gÁm −2 Áyr −1 . Passive breathing pumps water vapour upwards from the deeply buried phreatic water table. Film water on the soil continuously migrates in pulsation from deep layers to the upper layer. Na 2 SO 4 in the shallow soil absorbs moisture from the earth-air at night and decomposes during the day, forming water vapour, which is critical to the occurrence of PhE. The diurnal PhE process can be elucidated in detail by the bimodal variation in the atmospheric pressure. PhE occurs mainly from 10:00 to 17:00 during daytime from March to November, which correlates with passive breathing of the earth-air.The amplitude of atmospheric fluctuation determines the amount of earth-air that outflows, while temperature determines the water vapour concentration. In calculation, PhE is equal to the net absolute humidity (AH) times the amount of earth-air.There is 1.55 mmÁyear −1 of PhE caused by daily peak!valley differences, and about 2.97 mmÁyear −1 in estimation caused by numerous atmospheric fluctuations smaller than 2.84 hPa. The results coincide with the actual amount of PhE monitored of 4.52 mmÁyear −1 . Therefore, the amount of PhE is proportional to the range and frequency of fluctuation in external atmospheric pressure, and is also positively related to soil temperature, salt content, water content, porosity, and vadose zone thickness.

“Earth‐air” refers to air in the vadose zone. Studies have long suggested that earth‐air undergoes a passive breathing effect due to atmospheric pumping. The earth‐air can be modeled as an ideal gas in a closed system, so that the volume of earth‐air varies as the atmospheric pressure (AP) fluctuates. When AP rises, earth‐air is compressed and external air is drawn into the vadose zone. Conversely, when AP drops, earth‐air expands and moist earth‐air enters the atmosphere. In arid regions, there is a moisture gradient in soil due to dryness of the climate: the humidity is relatively low in the upper soil layer, and relatively high in the subsoil. Therefore, if earth‐air undergoes passive breathing, the humidity of earth‐air will fluctuate with AP. This means that passive breathing of the earth‐air should leave traces in the recorded moisture which exactly match AP fluctuations. In this work, clear evidence is presented for the existence of earth‐air passive breathing. This is accomplished by using an innovatively‐designed trap to monitor humidity in 30 cm gravelly sand and showing that these moisture/temperature records contain fingerprints which are characteristic of passive breathing. It is found that relative humidity (RH) in soil is negatively correlated with the atmospheric fluctuations. The correlation between 10‐min average AP and RH timeseries reaches −0.75 for the time‐periods of 20 days. This study provides a new understanding in land‐atmosphere heat/mass exchange from the perspective of vertical fluid dynamics.

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