Evaporation and Water Redistribution in Layered Unsaturated Soil Profiles

The mechanisms controlling surface evaporation and water redistribution in layered, unsaturated soil profiles are of particular interest for the design of soil covers for landfills and mine waste (e.g., tailings) where layering of texturally contrasting soils is common. Evaporation and water redistribution were observed in laboratory tests of three layered soil columns (sand overlying silt, silt overlying sand, and multiple sand and silt layers), with two homogeneous soil columns (sand or silt) serving as controls. Changes in water storage were monitored along with changes in potential and actual evaporation with time. Near continuous monitoring of temperature along the columns was also conducted. The hydraulic properties of the sand and silt were measured but also verified through simulations of the homogeneous sand and silt columns. These optimized parameters were then used to simulate the layered columns. Layered columns with silt at the surface had more evaporation than homogeneous soil columns of either sand or silt. Changes in water storage in the columns with sand at the surface occurred primarily within the surficial sand layer without drawing water from deeper in the column. Columns with silt at surface drew water for evaporation from sand layers deeper within the column. Numerical modeling of the columns was able to reproduce the observed changes in evaporation rates and water content for all layered soil columns. The laboratory and simulation studies illustrated the strong influence that textural layering may have on water dynamics in soil profiles under evaporative conditions.