Soil consisting of grains, water and air is an example of a multiphase material (or mixture). The basic concepts for multiphase materials, such as volume fraction, partial quantities and interaction forces are introduced. The Darcy's equation and balance equations for mixtures are introduced.

On their basis, the consolidation theory is presented. The equations describing steady and unsteady groundwater flow are derived. Transport within groundwater by means of convection, diffusion and dispersion is explained. The main principles describing unsaturated soil are presented: capillary and osmotic suction, the function of filters is explained. The soil–water characteristic curve is also introduced. The effective stresses in unsaturated soil are discussed.

Soil water occurs in the unsaturated zone and is the only source of water and nutrients for most agricultural crops. Since nutrients, including fertilizers, are dissolved in soil water, it is the only source from which plants can extract them. The objective of irrigation is to maintain enough soil water in the soil. This chapter discusses some rudimentary aspects of this water.