Soil plant atmosphere continuum
The transport of water along this pathway occurs in components, variously defined among scientific disciplines:
- Soil physics characterizes water in soil in terms of tension,
- Physiology of plants and animals characterizes water in organisms in terms of diffusion pressure deficit, and
- Meteorology uses vapour pressure or relative humidity to characterize atmospheric water.
SPAC integrates these components and is defined as a:
...concept recognising that the field with all its components (soil, plant, animals 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. 
This 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.
- Hydraulic redistribution; a parameter now being considered in the soil-plant-atmosphere modeling community
- John R. Philip (1966). Plant water relations: some physical aspects. Annu. Rev. Plant Physiol. 17, 245–268.
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