Water potential is equal to

Water potential is defined as the difference between the chemical potential of water at any point in a system and that of pure water under standard conditions. The unit of measurement of water potential is the pascal, Pa (1 megapascal, Mpa=10 bars). It is represented by greek letter PSi (ψ). Water potential has two components solute potential which is always negative and pressure potential which is usually positive. Therefore, water potential ψ_{w} is expressed as ψ_{w}=ψ_{s} + ψ_{p.}