Impacts of an Internal Finer-Textured Layer on Soil Evaporation and Salt Distribution

Previous investigations have demonstrated that an internal finer-textured soil layer will reduce the vertical water flow rate and increase water and solute storage in the vadose zone. However, most of them have been conducted under conditions of infiltration or drainage with low salinity and focused on the salinity across the fine-textured layer. Very little is known about the effects of an internal fine-textured layer on evaporation with an added effect of salt precipitation, and also the salt distribution within the fine-textured layer. In this study, three columns were packed with sand and an internal silt loam layer with thicknesses of 5, 10, and 15 cm; a fourth column had no added finer-textured layer. The four columns were all exposed to 100 g/L NaCl solution and maintained at a constant bottom pressure head. The water storage in the upper sand layer was reduced sharply by the presence of a fine-grained layer. The resulting dryer vadose zone reduced the evaporation rate by about 70% in the layered columns compared to the sand column. Salt precipitated at the evaporating surface as a crust on the ground surface in the sand column and as a quasi-crust subsurface in the multilayered columns; salt also accumulated at the top of the silt loam layer due to evaporative loss via vapor from it. The closer the top of the fine-grained layer was to the surface, the more salt accumulated at the top of it. Accordingly, when considering the impacts of a fine-textured soil layer, its location relative to the root zone is very important with respect to the impacts of any salt accumulation.