Capillary tube Analogy
Soil can be regarded as a set of interconnected capillary tubes which are able to hold water at a given suction(negative pressure; matric potential) depending upon the radius of the tube. The pressure potential due to a capillary of radius r is equivalent to the weight of (i.e. "force of gravity" on)the water column of length h.
h=(2ycos)/r
where is the surface tension constant cos is generally assumed=1
where expressed in mm,h=15/r. it is convenient to express soil water potentials in terms of the height of rise"h" which would be observed in a capillary tube having a radius resulting in the potential expressed("equivalent radius").