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(1) Sand dunes—precipitation balance
Big and high sand dunes are storage of precipitation. Thickness of dry sand layers of dunes is only 30cm. Water of wet sand layers under the dry exists in the form of absorbed water，film water and capillary hanging water. Water permeability of sand layers is conducive to the infiltration of precipitation. It forms sources of replenishment of phreatic water below the critical depth, and emerges in the form of springs at the foot of sand dunes (Figure 4-15). Because huge dunes horizontally occupy rather large space per unit area, which means that the storage of precipitation is quite large, phreatic water of sand dunes is easy to overflow outside, and collects at lower places to form lakes (Tao, Wang, 1990).
Figure 4-15 Generalized geological section between sand dunes and intermontane lowlands (salt lake)
(According to Wang Tao, 1990) (2) Ancient lakes and modern rainfall forming surface water
During the early-middle Pleistocene epoch, the connected rivers and lakes formed water systems throughout the Alxa Plateau from the northwest to the southeast, and they existed in the Badain Jaran Desert also. At Nuoertu and Zhalate ancient lake strandlines are preserved (Xiaoping, Yang, 2001). As the climate gradually became dry and the lakes shrunk during the past 10,000 years, the present lakes along the border of the southeast Badain Jaran Desert may be remains of the ancient lakes. Shallow groundwater around lakes may be formed by local precipitation of 30 to 100 years (Xiaoping, Yang, 2001).
(3) Percolating water of rivers, lakes and precipitation running off through major faults and recharging in long distance
It is possible that the Badain Jaran region was originally an integrated great lake of 5,000 square kilometers. Recharge sources of the lake water come from the Gyaring Lake and the Ngoring Lake located in the Qinghai-Tibet Plateau (Jiansheng, Chen, 2006). The percolating lake water runs off through the Shigatse-MLang buried fault (Figure 4-16) to the Badain Jaran region. With the result of large-scale travertine in the lakes and caliche on sand dunes, groundwater goes through the Cambrian and Ordovician limestone faults, carbon dioxide dissolves minerals, then they are released from the springs in the deserts.
Figure 4-16 Water source recharge and discharge planimetric map of inland lakes in Badain Jaran Desert
(After Hongwei, Ding, etc., 2007)
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