Water potential in plants?
Ok let me try to tackle this one - still could be off - let me know. 

The book Chapter from UCDavis that is linked from your wikipedia page is a good reference I think. Overall the term 'water potential' will try to estimate an energy function that describes the behavior of water in a chemo-mechanical sense.

$\Psi_{total} = \Psi_{reference} + \Psi_{solute} + \Psi_{pressure} + \Psi_{gravitational} + \Psi_{humidity} + \Psi_{matrix}$

It looks a lot like a Hamiltonian expression of total energy that tries to see the water as a fluid.  The water potential is not the same as an energy equation for fluid dynamics which tries to understand the motion of a body of liquid and does not typically take into consideration chemical changes like solute in the behavior. 

So first we are looking at a set of behaviors that the fluid experiences.  some of them treat the fluid as a body ( pressure, gravitational) others look at the potential for evaporation or diffusion (solute and humidity to some degree).  wetting of surfaces and surface tension (matrix).  

The water potential is an equation broken down into a set of measurable or approximately measurable energy terms.  But like a lot of Energy equations, when  you are looking at a biological system, it is more often qualitative than a quantitated entity.  The general observation that surface tension decreases when temperature goes up, or when the solutes have higher concentration is used to make arguments of the mechanism of say physiology 

Generally speaking looking at the magnitudes of the various terms you can see how influential they are and then try to focus the hypothesis into a test that can make a distinction about the mechanism with specific measurements.  The actual overall measurement of a water potential or any total energy is almost always intractable.  The difference in energy is what you might be able to measure, but in the case of the water potential, the measurement will usually be an observation, with some uncertainty that water is moving from point A to B and not elsewhere.  

Take a look at this paper describing Drought Stress in Arabidopsis.  The water potential is cited as a critical element of drought response - and of course it should be.  But all this work here, with all the proteins acting to change the physiology of the cell, adjust the chemical potential (and therefore the osmotic potential) of the cell membrane, its hard to imagine how measuring a root system at all points would be possible. 


  Physical properties of the cell wall play a crucial role in the response of plants to water deficit (Bacon, 1999). Transcriptome analysis of pDr showed the repression of many expansin genes (Bray, 2004), while mild osmotic stress revealed the induction of expansin genes (Skirycz et al., 2010). Cell expansion in response to drought was characterized in the maize (Zea mays) root system as an adaptation to low water potential (Wu and Cosgrove, 2000).