SALINIZATION OWING TO EVAPORATION FROM BARE-SOIL SURFACES AND ITS INFLUENCES ON THE EVAPORATION

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 te mperature-, humidity- and/or wind-speed-controlled ambient atmosphere. For the dune sand, chemicals such as Na+, Ca2+, Cl- and SO42-, dissol ved mainly from CaSO4, Na2SO4, CaCo3 and NaCl in the sand particles, c aused marked salinization near the top surface, Slow dissolution of Na 2SO4 and CaSO4 influenced the development of concentration profiles fo r SO42- and Na+ markedly for months after the beginning of the experim ent, while the profile of Cl- was not affected directly, because disso lution 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 be came larger because of the increase in the gradient of the vapour dens ity caused by greater chemical enrichment near the top surface compare d with that at the evaporation surface. The vapour flux also became sm aller as the gradient of the vapour density decreased, owing to the ma rkedly enriched evaporation surface. In the experiment with glass bead s, filled with the NaCl solution, solute crystallization (4-10 mm thic k) 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 wat er vapour as the permeable cross-sectional area decreased. The resista nce to transfer increased three to ten times for the glass beads but o nly 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 .