Water holding capacity (WHC) is the ability of a soil to contain and to retain water. Dependent upon the factors, which determine hydraulic conductivity and permeability e.g., texture, organic matter, porosity, interconnectedness of pores and so on. The water is held in the spaces, or pores, between soil particles and in thin films surrounding those particles. It is calculated by mass of (water contained in the saturated soil / mass of the saturated soil)*100 or water holding capacity (%) = (m(water in saturated soil) / m(saturated soil)) * 100. Therefore it should be a fixed value during short time.
Water filled pore space (WFPS) is also known as degree of saturation; it is the volume of water present in the soil relative to total volume of pores. It is calculated as (percentage of volume water content / percentage of soil pore space) * 100, or WFPS (%) = (percentage of volume water content / percentage of soil pore space) * 100 = (mwater / mdry soil) * soil bulk density / (1 - soil bulk density / soil particle density) * 100. Normally soil particle density is assumed as 2.65 g/cm3.