In Chapter 2, we described soils as being three-phase systems: gaseous, liquid, and solid. The water content of a soil is a significant determinant of its behaviour. Properties of soils, such as strength, compressibility and consolidation, thermal and hydraulic conductivity, plasticity and consistency, change measurably with corresponding changes in water content. Changes in water content occur all the time in the field. These changes can be large in surficial soils because of evaporation and also because of infiltration from rainfall. Changes in water content in subsurface soils occur because of water movement due to external and internal forces.