3.2. Moisture and stable isotopes of sand profiles
Rainfall can percolate through sand layers only when the sand
moisture content is greater than its field moisture capacity. Fig. 6
shows the infiltration process for the saturated sand layers in the
Alxa Desert. The two curves were obtained by in-house laboratory
investigation with sand samples collected from the Badain Jaran
Desert and the Tengger Desert. They indicate that the moisture
content of the sand will decrease exponentially from the saturation
moisture capacity when there is no evaporation, i.e. the infiltrated
water decreases exponentially with time. Fig. 6 illustrates that the
saturation moisture capacity of the sand in the Alxa Desert is about
24e25%; the infiltration tends to stop when the moisture content
drops to about 4e5%, i.e. field moisture capacity.