The total soil water potential and its components are defined as energy per unit quantity of pure water. Therefore, their relevant units vary if reference is made to a unit mass, unit volume, or unit weight of water.
When referring to unit mass, the dimensions are L2/ T2, and in the SI system the potential units are J/kg. Although it is better to use the unit mass of water because it does not change with temperature and pressure, this definition of the potential energy is widely used only in thermodynamics. If one considers that, in the most practical applications, water can be supposed incompressible and its density is independent of potential, energy can be referred to the unit volume instead of unit mass. Hence, the dimensions are those of a pressure M-L-1 T-2 and in the SI system the units of potential are J/m3 = N/m2 (pascal, Pa). By expressing the potential as energy per unit weight of water, the units are J/N = m, and the relevant dimensions are those of a length L. This latter way to evaluate the water energy potential is more useful and effective.
When effects of the presence of solutes in the soil solution can be neglected, as applies to most cases, instead of using the symbol ^, it is customary in analogy to hydraulics to define the total soil water potential per unit weight, H (m), in terms of head units as
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