Abstract: Wet sand layer moisture and its movement process is the important link for water cycle in desert areas. The mega-dunes in the Badain Jaran Sand Sea contain very huge wet sand layer. The result showed that there was a regional similarity characteristics for wet sand layer moisture with water content less than 3%. Adsorbed water and water vapor existed in the pore among sand grains were the two primary water types for wet sand layer, and their movement process and reciprocal transformation along the vertical section of mega-dunes might maintain the relatively poised state of wet sand layer moisture, and grain size distribution had important influence on adsorbed water and its water content. The formation of "thermal inversion layer" in the dune surface layer caused the movement of heat and air to the dune interior, which had an opposite air flow direction with the moisture evaporation of wet sand layer. This was why wet sand layer moisture could maintain in sunny days of summer. Affected by temperature gradient in summer, the water vapor and film water contained in wet sand layer moved slowly toward the bottom of the mega-dunes. Under the dual influence of temperature gradient and water potential gradient in winter, the water vapor and film water near phreatic surface in the bottom of mega-dune moved upward to the top of the mega-dunes and recharged the wet sand layer. The wet sand layer could get recharged water from atmospheric precipitation, water vapor, condensate water and underground water.

Key words: wet sand layer, grain size, moisture movement, mega-dune, Badain Jaran Sand Sea