Four sand dune profiles in the Alxa Desert were investigated in
2009 (Fig. 7a). The results showed that a dry sand layer approximately
20 cm in depth always existed below the surface when there
had been no precipitation for a long time. The moisture measurements
of these profiles suggested that the percolated water from
precipitation was mostly adsorbed onto the surface of the sand
particles, with an infiltration depth of 50e100 cm, and that the
adsorbed water then evaporated. Another six sand dune profiles
were surveyed during the period of August 20e26, 2011 (Fig. 7b).
Two heavy rainfall events occurred in the desert in August 15e18,
2011, with a total rainfall of 30e50 mm (52.2 mm in the Minqin
Station; 31.4 mm in the Zhongwei Station; 35.9 mm in the climate
station of the Badain Jaran Desert, Fig. 1), which is nearly half of the
mean annual precipitation. The moisture content (mass water
content) measurements for these profiles showed that the wetting
fronts approached a depth of 30e50 cm in average, with a moisture
content of 5e8%, decreasing to less than 2% below 50 cm (Fig. 2b;
dry sand layers still existed at greater depths after a rainstorm in
August 2011 in the Tengger Desert), i.e. the rainfall did not percolate
deeper than 50 cm. The infiltration process in Fig. 6 demonstrates
that the lower limit for downward penetration was 4e5%.
Accordingly, it is unlikely that the limited local precipitation
(approximately 100 mm/yr) can percolate through the mega-dunes
and recharge the phreatic water in the Alxa Desert.