The volumetric content of water or moisture (VWC) is a numerical measure, of the soil moisture.
Any volume of soil will be occupied by a solid fraction, (either organic as plant material and mineral as sand particles). Another hollow fraction consisting of small and large pores, capable of being filled by air and water.
In the absence of water, all the pores are occupied by air (permanent wilting situation) and the water added to the soil fills the small pores first (field capacity situation) and then the large pores (soil saturation situation) . The difference between field capacity and permanent wilting point will give us the water available for the plant.
Soils of different textures like sands and clays, have saturation or field capacity at different whater percentages. It is common to find on USGA greens saturation percentages over 40% volumetric, field capacities around 25% volumetric and according to the species of grass C3 or C4 (with less or greater tolerance to dry situations), the grass will reach the point of permanent wilting below 10% volumetric.
It is intuitive to see how the different irrigation cycles coincide with the field capacity of the green (26% volumetric). See graphic attached.
There are numerous sensors to measure the volumetric content of water or soil moisture. The TDR of the Spectrum brand to the FDR of the Decagon brand, through the coaxial impedance sensors of Stevens Waters, or the new POGO or Environmesh systems of Tiloom .