干旱区生态环境知识资源中心

To investigate the relationship between evaporation and salinization, the surfaces of three columns of uniform porous materials, desert dune sand, silica sand and glass beads, respectively, were exposed to a temperature-, humidity- and/or wind-speed-controlled ambient atmosphere.

For the dune sand, chemicals such as Na+, Ca2+, Cl- and SO42-, dissolved mainly from CaSO4, Na2SO4, CaCo3 and NaCl in the sand particles, caused marked salinization near the top surface, Slow dissolution of Na2SO4 and CaSO4 influenced the development of concentration profiles for SO42- and Na+ markedly for months after the beginning of the experiment, while the profile of Cl- was not affected directly, because dissolution of NaCl was rapid. Concentration profiles of Cl- for the glass beads and for the silica sand columns filled with a high concentration of NaCl solution of (10(4) mg l(-1) for Cl-), were analysed similarly.

Experimental results suggested that the vapour flux in a dry soil became larger because of the increase in the gradient of the vapour density caused by greater chemical enrichment near the top surface compared with that at the evaporation surface. The vapour flux also became smaller as the gradient of the vapour density decreased, owing to the markedly enriched evaporation surface.

In the experiment with glass beads, filled with the NaCl solution, solute crystallization (4-10 mm thick) was observed. For the dune sand, only when a turbulent airflow was applied did a crust (a few millimetres in thickness) form entirely on the top surface. Such deposition led to a reduction in the flux of water vapour as the permeable cross-sectional area decreased. The resistance to transfer increased three to ten times for the glass beads but only by 30% for the dune sand. The lower increase for the dune sand may be due to penetration of the applied airflow into cracks in the crust.