text: In soil mechanics and petroleum engineering the water saturation or degree of saturation, , is defined as
  where  is the porosity, in terms of the volume of void or pore space  and the total volume of the substance . Values of S can range from 0 (dry) to 1 (saturated). In reality, S never reaches 0 or 1 - these are idealizations for engineering use.
  The normalized water content, , (also called effective saturation or ) is a dimensionless value defined by van Genuchten as:
  where  is the volumetric water content;  is the residual water content, defined as the water content for which the gradient  becomes zero; and,  is the saturated water content, which is equivalent to porosity, .