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The 'Soil-Plant-Atmosphere Continuum (SPAC)' is the pathway for water moving from soil through the plant to the atmosphere.
The transport of water along this pathway occurs in separate components, defined differently between three scientific disciplines of the environment:

★ soil physics characterizes soil water in terms of tension,

★ plant physiology characterizes plant water in terms of ''diffusion pressure deficit'', and

★ meteorology uses ''vapour pressure'' or ''relative humidity'' to characterize atmospheric water.
SPAC integrates the three separate components. It is defined as a:

...concept recognising that the field with all its components (soil, plant and the ambient atmosphere taken together) constitutes a physically integrated, dynamic system in which the various flow processes involving energy and matter occur simultaneously and independently like links in the chain. ^[1]

This definition characterises the state of water in different components of the SPAC as expressions of the energy level or water potential of each. Modelling of water transport between components relies on SPAC, as do studies of water potential gradients between segments.

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References

References

1. John R. Philip (1966). Plant water relations: some physical aspects. Annu. Rev. Plant Physiol. 17, 245–268.