Water, water, water… in the trees

August 27, 2012 by · Leave a Comment
Filed under: CPCRW, Field Work 

One of the easiest and coolest measurements I made this year is tree water content. I applied a tool that is normally used for soils (TDR or time domain reflectometry) to trees! This website has some excellent blah blah blah on the topic: http://www.sowacs.com/sensors/tdr.html

Basically, the readout I get from the instrument isn’t water content. I have to convert the data through calibration. How might we convert a waveform to water content? This is a really long process. We cut a birch tree down when it was at its wettest state (in early to mid May). We cut this tree into relatively uniform “cookies” about 6 inches long, and we installed probes into them (the exact way the probes were installed into the trees in the field). These probes are where we attach the reader cables for the TDR instrument. Every week or two, I made a TDR measurement on each wood cookie and I weighed it as well. The wood smelled terrible. This was done over several months as the blocks of wood dried out. When the wood was dry, I measured the volume of each wood chunk by immersing it in water (volume displacement). The weight loss of the wood over time was converted to volume of water lost. This allowed me to plot data from the TDR waveform vs. the volumetric water content over time, and voila! I am able to convert field TDR data to water content values.

The cables are attached to the probes or rods in the trees. The TDR instrument and a battery are in my backpack, and the TDR instrument talks to the laptop I’m holding. Once the cables are hooked up to the tree, I tell the TDR to make a reading.

This is a picture of the cables attached to the probes in the tree.

 

Autumn has arrived, and more water potential measurements

August 24, 2012 by · Leave a Comment
Filed under: CPCRW, Field Work 

It’s already August and autumn is setting in at the watershed. We can see the colors change at small and large scales.

But, the plants are still active, so I’m still out measuring them every 10 days or so. As I mentioned before, one measurement I make pretty regularly is plant moisture stress. This measurement tells us about the water demand of a plant. This measurement integrates a lot of information (seeĀ  http://pmsinstrument.com/importan.htm) for a good description. Smaller numbers (more negative numbers) means that the water demand or water stress of a plant is fairly high. The water stress of a plant shows diurnal and seasonal variation, and there is variation between species. I do these measurements on black spruce and on birch trees. The spruce tend to be more stressed than the birch trees.

Step 1: Get a branch down.

Step 2: Get the pressure chamber off the four wheeler and set it up.

Step 3: Cut a stem and put it in the holder so that the cut part is facing you, and then crank the holder onto the chamber.

Step 4: Turn on the compressed gas, get out the magnifying glass, hunch over it and wait for the stem to change color (so that it looks wet). When the color changes, the pressure in the chamber and the pressure in the plant have been equalized. Read the pressure on the gauge (15 Bars in this case).

If you see bubbles of water coming out of the stem, you waited too long to take the reading and you have to start over with another stem.