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Pat et al.:
We have recently invested fairly heavily in the MoisturePoint technology.
"We" refers to the Oklahoma Mesonet, a statewide weather/soil monitoring
network that reports data every 15 minutes. Mesonet is jointly operated by
the University of Oklahoma and Oklahoma State University, and consists of
114 stations with an average spacing of about 32 km. We have installed a
single MoisturePoint Type "F" probe at a subset (60) of our 114 sites.
These "F" probes provide measurements for 5 different soil layers -- 0-15,
15-30, 30-45, 45-60, and 60-90 cm. As you describe, a "console" or "smart
box" is carried to the field and connected to the probe. We will be making
measurements fairly infrequently (whenever a Mesonet field technician visits
the site). However with the probes already in place, more intensive
monitoring schedules can be arranged for special experiments, field
campaigns, etc.
Since our installation phase is just now being completed, it's too early for
me to share conclusions, but here are some general observations to date. I
was not personally involved with the field work, but I think I can say that
we did not experience great difficulties in the installation process. As
you can imagine, with 60 sites we encountered quite a variety of soil types,
depths, etc. In fact, we selected our sites to provide not only a good
geographic distribution, but also a good textural distribution. Good
probe-soil contact is obviously a must, and the installation equipment and
instructions are useful in this regard. We also had the advantage of a
MoisturePoint company representative assisting in our first few installations.
Why are we doing what we're doing? First of all, as background, Mesonet has
been around for a few years, but the soil water measurements are a recent
addition. We have installed thermal dissipation sensors (Campbell
Scientific 229-L) at 4 depths at each of the same 60 sites. These sensors
are read by our dataloggers and are a part of our automated data stream.
For several reasons, I insisted that we add a second, non-automated
measurement technique as an independent check, and to hopefully provide an
opportunity for in-situ calibration of the 229-L sensors. Since these
thermal dissipation sensors measure soil water potential, to get to
volumetric water content, one must either use the water retention curve (as
much "art" as science?!?), or perform a soil-specific calibration. We are
going to ATTEMPT to do the latter, in the field and over a period of time as
we gather sufficient TDR data. One difficulty in this approach is that we
are comparing a layered measurement to what is essentially a point
measurement. But keep in mind that, although Mesonet is a wonderful
research tool, it is primarily an operational network (i.e., our error
tolerances may be a little higher than those doing careful research).
At 3 Mesonet sites close to our campus, we have been making MoisturePoint
measurements about 2-3 times per week for several months. And we have the
229-L data from the same sites. Data analysis is under way, but a quick
look at some graphs indicates a good correlation between the TDR data and
the "raw" 229-L data. ("Raw" data means the temperature rise in the sensor
after a specified interval of heating, prior to any conversion to soil water
potential or soil water content.) The trend appears to be fairly linear,
but I'm not ready to suggest a linear relationship between the two.
By the way, we would like to see one thing re-designed on the MoisturePoint
system. Instead of having the connecting cable permanently or
semi-permanently attached to the "smart box", you have to screw the coupling
on each time you head to the field (the box enclosure does not allow you to
leave the cable connected). This may not sound like a big deal, but the
connection seems to be a tight fit and a bit tedious to accomplish. The
connection at the probe end presents no problems.
One other thing -- some suggest that you should use texture-specific
calibrations for the MoisturePoint TDR. We plan to look at this, and would
welcome comments/suggestions from others.
Sorry to be so long winded, but you asked about something that I have a
LITTLE experience with. Happy Holidays!
--Ron Elliott
Biosystems & Agricultural Engineering
Oklahoma State University
Stillwater, Oklahoma
relliot@agen.okstate.edu