Nature and mechanism of moisture migration in soils

Water migration in unsaturated soils is due to a complex mass transfer mechanism and a variety of water exchange driving forces. For geocryological problems the most interesting is the migration of bound and capillary water and vapour. Seepage (movement of free or gravitational water) in fine-grained materials is of minor significance and will not be discussed further. Water migration and vapour transfer in soils are related to the solution of many problems in earth science, engineering geology, pedology and geocryology (absorption and evaporation of water from the soil surface, its resorption by surrounding soil layers, capillary replenishment of soil, water migration towards the front of freezing and cooling, etc.). Thermodynamically, water and vapour migration in soil follow from the disequilibrium of the soil-water system caused by change in time and space of thermodynamic parameters (temperature, pressure, ion concentration, humidity, electrical, magnetic and gravitational potentials, etc.). It is usually impossible to measure directly the driving force of each mechanism separately. This is the reason to find a uniform (generalized) force comprising more or less fully all component forces. All this has resulted in an energy (thermodynamic) approach to mass transfer in colloidal and capillary porous bodies including the soil system.

All the water in soils, with the exception of free (gravitational) water, is held due to the free surface energy of the mineral soil skeleton Es.