Both the amount and the availability of water in soil is important to plant roots and soil-dwelling organisms. To describe the amount of water in the soil we use the term water content. To describe the availability we talk of water potential. In thermodynamics, the water content would be referred to as the extensive variable and the water potential as the intensive variable. Both are needed to correctly describe the state of water in soil and plants.
In addition to describing the state of water in the soil, it may also be necessary to know how fast water will move in the soil. For this, we need to know the hydraulic conductivity. Other important soil parameters are the total pore space, the drained upper limit for soil water, and the lower limit of available water in a soil. Since these properties vary widely among soils, it would be helpful to establish correlations between these very useful parameters and easily measured properties such as soil texture and bulk density. This paper will present the information needed for simple models of soil water processes.
Measuring the stomatal conductance of a leaf should be a pretty straightforward problem. The conductance is just the flux density of water vapor divided by the concentration difference between the leaf and its surroundings. Common approaches to this problem involve either flowing air of known vapor concentration over the leaf and measuring how much water vapor is picked up, or sealing a cup of known capacity to the leaf surface and measuring how quickly the vapor concentration in the cup increases. Both of these, though simple in concept, require quite a bit of expensive equipment to pull off. We wanted a simpler approach. We put a humidity sensor in a small tube, the end of which could be pressed against the leaf. As vapor diffused through the tube the humidity in the tube increased. The conductance of the tube is easily calculated. It is the diffusivity for water vapor divided by the tube length. The leaf conductance could be computed from the tube length, the humidity in the tube and the ambient humidity. That worked, but it turned out that ambient humidity variations introduced too much error, so we later added a second humidity sensor toward the distill end of the tube. Our approach was very simple, and works well, but it wasn’t a new idea.
Cross section of METER’s Leaf Porometer
I read of a similar device in a conference proceedings (I don’t recall the name of the conference) in 1977 when I was on sabbatical at University of Nottingham in England. The device wasn’t for leaves. It was developed by a medical researcher to assess severity of burn injuries, and for use on neonatal infants. The skin of a non-sweating human is pretty impermeable to water. A typical conductance is around 5 mmol m-2 s-1. This is about half the value for a leaf with stomates closed, and about two orders of magnitude lower than leaves with open stomates. Burned skin, however, is much more permeable, and the permeability is related to the severity of the burn. A device that could measure the permeability of skin would therefore give information on the severity of the burn. The researcher built an apparatus, similar to our porometer, with two closely spaced humidity sensors in a diffusion tube. As I recall, it was somewhat successful, but I’m not aware of it ever having been commercialized or used much after that. The application for infants is also interesting. Full-term babies have low skin conductances. I haven’t seen measurements, but assume they are similar to adult conductances. The skin of premature infants, though, has a much higher conductance. I don’t know typical conductance values, but do know that, without intervention, the conductance can be so high that evaporative water loss from the baby will reduce body temperature to dangerously low levels, even in an incubator. I don’t know if later work has been done to measure skin conductance, but it is interesting that the first applications of the technology we now use in our porometer was for measuring conductance of the human epidermis, not the epidermis of leaves.