© 2005 Heron Publishing—Victoria, Canada
A unified nomenclature for quantification and description of water conducting properties of sapwood xylem based on Darcy’s
law
Douglas E. B. Reid (1, 2), Uldis Silins (2), Carl Mendoza (3) and Victor J. Lieffers (2)
1. Corresponding author (dereid@ualberta.ca) / 2. Centre for Enhanced Forest Management, Department of Renewable Resources, 4-42 Earth Sciences Building, University of Alberta,
Edmonton, AB T6G 2E3, Canada / 3. Department of Earth & Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB T6G 2E3, Canada / Received November 3, 2004; accepted November 20, 2004; published online June 1, 2005
Summary
The literature dealing with the water conducting properties of sapwood xylem in trees is inconsistent in terminology, symbols
and units. This has resulted from confusion in the use of either an analogy to Ohm’s law or Darcy’s law as the basis for nomenclature.
Ohm’s law describes movement of electricity through a conductor, whereas Darcy’s law describes movement of a fluid (liquid
or gas) through a porous medium. However, it is generally not realized that, in their full notation, these laws are mathematically
equivalent. Despite this, plant physiologists have failed to agree on a convention for nomenclature. As a result, the study
of water movement through sapwood xylem is confusing, especially for scientists entering the field. To improve clarity, we
suggest the adoption of a single nomenclature that can be used by all plant physiologists when describing water movement in
xylem. Darcy’s law is an explicit hydraulic relationship and the basis for established theories that describe three-dimensional
saturated and unsaturated flow in porous media. We suggest, therefore, that Darcy’s law is the more appropriate theoretical
framework on which to base nomenclature describing sapwood hydraulics. Our proposed nomenclature is summarized in a table
that describes conventional terms, with their formulae, dimensions, units and symbols; the table also lists the many synonyms
found in recent literature that describe the same concepts. Adoption of this proposal will require some changes in the use
of terminology, but a common rigorous nomenclature is needed for efficient and clear communication among scientists.
Keywords:
conductance, hydraulic capacity, hydraulic conductivity, permeability, water potential.
