Abstract: Usually considered that in extremely arid area the soil moisture comes from rainfall, atmosphere or condensed water, deep phreatic water evaporation does not exist. But the greenhouse method test shows that there exists phreatic water evaporation, and phreatic water is an important source of soil moisture in the extremely arid area. To further verify sources of soil water, two soil pits, were excavated with a scale of 200 cm×200 cm×200 cm, one pit’s connections were completely cutout with its bottom soil and surrounding soil(called as isolated pit), another pit’s connections was cutout with its surrounding soil, but was kept in with bottom soil(called as connected pit), two sets of HOBO-U23-1000 instrument were inserted into the two pits’ backfilling soil to monitor their temperature and humidity at the depths of 10, 30, 50 and 100 cm. After 27d, a 25mm rainfall simulation test was carried out for the two backfilled pits. Before the rainfall simulation test mentioned above, the connected pit’s soil relative humidity, absolute humidity, soil water content are significantly higher than that of the isolated pit. After a year during which the rainfall was without for long time, the test results found that the soil water content of the connected pit was higher than that of the isolated pit; however, under the condition of same meteorological environment, because the phreatic water evaporation spills over, the isolated pit’s soil humidity does not infinitely dropped down. The comparison experiment proves inversely that the deeply buried phreatic water’s evaporation exists in the extremely arid area’s.deeply buried soil.

Key words: isolation, the extremely arid areas, soil moisture, phreatic water, precipitation